32 Electrostatics involves electric charges, the forces between them, and their behavior in materials.
32 Electrostatics FOCUSED LEARNING TARGET • Given different materials , I will be able to explain conductivity by induction and polarization. • Given different elements , I will be able to review , how a material is charge caused by the difference in the number of protons and electrons. • Given different materials, I will be able to compare insulators, conductors and semiconductors in terms of conductivity.
32 Electrostatics HOMEWORK 1. CHAPTER 34 COPY AND ANSWER QS 1 -25 COPY SUMMARY P 545 -546 DUE ON FEBRUARY 28 2. CHAPTER 35 COPY AND ANSWER QS 1 -15 COPY SUMMARY P 559 -560 DUE ON MARCH 7
32 Electrostatics 32. 7 Charge Polarization Rub an inflated balloon on your hair and it becomes charged. Place the balloon against the wall and it sticks. The charge on the balloon induces an opposite surface charge on the wall. The charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign.
32 Electrostatics Balloon VS Hair Balloon VS Bits of Paper
32 Electrostatics 32. 6 Charging by Induction If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface.
32 Electrostatics 32. 6 Charging by Induction Charging by induction can be illustrated using two insulated metal spheres. Uncharged insulated metal spheres touching each other, in effect, form a single noncharged conductor.
32 Electrostatics 32. 6 Charging by Induction • When a negatively charged rod is held near one sphere, electrons in the metal are repelled by the rod. • 21. Excess negative charge has moved to the other sphere, leaving the first sphere with an excess positive charge. • The charge on the spheres has been redistributed, or induced.
32 Electrostatics 32. 6 Charging by Induction • When the spheres are separated and the rod removed, the spheres are charged equally and oppositely. • They have been charged by induction, which is the charging of an object without direct contact.
32 Electrostatics 32. 6 Charging by Induction Charging by induction occurs during thunderstorms. The negatively charged bottoms of clouds induce a positive charge on the surface of Earth below. Most lightning is an electrical discharge between oppositely charged parts of clouds. The kind of lightning we are most familiar with is the electrical discharge between clouds and oppositely charged ground below.
32 Electrostatics 32. 6 Charging by Induction If a rod is placed above a building and connected to the ground, the point of the rod collects electrons from the air. This prevents a buildup of positive charge by induction. The primary purpose of the lightning rod is to prevent a lightning discharge from occurring. If lightning does strike, it may be attracted to the rod and shortcircuited to the ground, sparing the building.
32 Electrostatics 32. 6 Charging by Induction What happens when a charged object is placed near a conducting surface?
32 Electrostatics 32. 7 Charge Polarization Charge polarization can occur in insulators that are near a charged object.
32 Electrostatics 32. 7 Charge Polarization Charging by induction is not restricted to conductors. Charge polarization can occur in insulators that are near a charged object. When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material. Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves.
32 Electrostatics 32. 7 Charge Polarization One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side. The atom or molecule is said to be electrically polarized.
32 Electrostatics 32. 7 Charge Polarization a. When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange.
32 Electrostatics 32. 7 Charge Polarization a. When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange. b. All the atoms or molecules near the surface of the insulator become electrically polarized.
32 Electrostatics 32. 7 Charge Polarization Examples of Charge Polarization explains why electrically neutral bits of paper are attracted to a charged object, such as a charged comb. Molecules are polarized in the paper, with the oppositely charged sides of molecules closest to the charged object.
32 Electrostatics 32. 7 Charge Polarization The bits of paper experience a net attraction. Sometimes they will cling to the charged object and suddenly fly off. Charging by contact has occurred; the paper bits have acquired the same sign of charge as the charged object and are then repelled.
32 Electrostatics 32. 7 Charge Polarization A charged comb attracts an uncharged piece of paper because the force of attraction for the closer charge is greater than the force of repulsion for the farther charge.
32 Electrostatics 32. 7 Charge Polarization Rub an inflated balloon on your hair and it becomes charged. Place the balloon against the wall and it sticks. The charge on the balloon induces an opposite surface charge on the wall. The charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign.
32 Electrostatics 1. Electrostatics, or electricity at rest, involves electric charges, the forces between them, and their behavior in materials. An understanding of electricity requires a step-by-step approach, for one concept is the building block for the next.
32 Electrostatics Focused Learning Target • Given different elements , I will be able to review , how a material is charge caused by the difference in the number of protons and electrons. • Given charged materials, I will be able to calculate the force of attraction and repulsion using Coulomb’s Law
32 Electrostatics 32. 1 Electrical Forces and Charges 2. The fundamental rule at the base of all electrical phenomena is that like charges repel and opposite charges attract.
32 Electrostatics 32. 1 Electrical Forces and Charges The Atom 3. Electrical forces arise from particles in atoms. The protons in the nucleus attract the electrons and hold them in orbit. Electrons are attracted to protons, but electrons repel other electrons.
32 Electrostatics 32. 1 Electrical Forces and Charges The fundamental electrical property to which the mutual attractions or repulsions between electrons or protons is attributed is called charge. By convention, 4. electrons are negatively charged and 5. protons positively charged. 6. Neutrons have no charge, and are neither attracted nor repelled by charged particles.
32 Electrostatics 32. 1 Electrical Forces and Charges The helium nucleus is composed of two protons and two neutrons. The positively charged protons attract two negative electrons.
32 Electrostatics 32. 1 Electrical Forces and Charges Attraction and Repulsion
32 Electrostatics 32. 1 Electrical Forces and Charges The fundamental rule of all electrical phenomena is that like charges repel and opposite charges attract.
32 Electrostatics 32. 1 Electrical Forces and Charges What is the fundamental rule at the base of all electrical phenomena?
32 Electrostatics 32. 2 Conservation of Charge 7. An object that has unequal numbers of electrons and protons is electrically charged.
32 Electrostatics 32. 2 Conservation of Charge Electrons and protons have electric charge. In a neutral atom, there as many electrons as protons, so there is no net charge.
32 Electrostatics 32. 2 Conservation of Charge If an electron is removed from an atom, the atom is no longer neutral. It has one more positive charge than negative charge. A charged atom is called an ion. • 8. A positive ion has a net positive charge; it has lost one or more electrons. • 9. A negative ion has a net negative charge; it has gained one or more extra electrons.
32 Electrostatics 32. 2 Conservation of Charge Electrically Charged Objects Matter is made of atoms, and atoms are made of electrons and protons. 10. numbers of electrons = the number protons no net electric charge. 11. But if there is an imbalance in the numbers, the object is then electrically charged. An imbalance comes about by adding or removing electrons.
32 Electrostatics 32. 2 Conservation of Charge The innermost electrons in an atom are bound very tightly to the oppositely charged atomic nucleus. 12. The outermost electrons of many atoms are bound very loosely (valence electrons )and can be easily dislodged. How much energy is required to tear an electron away from an atom varies for different substances.
32 Electrostatics 32. 2 Conservation of Charge When electrons are transferred from the fur to the rod, the rod becomes negatively charged.
32 Electrostatics 32. 3 Coulomb’s Law 13. Coulomb’s law states that for charged particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them.
32 Electrostatics 32. 3 Coulomb’s Law Recall from Newton’s law of gravitation that the gravitational force between two objects of mass m 1 and mass m 2 is proportional to the product of the masses and inversely proportional to the square of the distance d between them:
32 Electrostatics 32. 3 Coulomb’s Law Force, Charges, and Distance The electrical force between any two objects obeys a similar inverse-square relationship with distance. The relationship among electrical force, charges, and distance—Coulomb’s law—was discovered by the French physicist Charles Coulomb in the eighteenth century.
32 Electrostatics 32. 3 Coulomb’s Law For charged objects, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. 14. Where: d is the distance between the charged particles. q 1 represents the quantity of charge of one particle. q 2 is the quantity of charge of the other particle. k is the proportionality constant.
32 Electrostatics 32. 3 Coulomb’s Law The SI unit of charge is the coulomb, abbreviated C. 15. A charge of 1 C is the charge of 6. 24 × 1018 electrons. A coulomb represents the amount of charge that passes through a common 100 -W light bulb in about one second.
32 Electrostatics 32. 3 Coulomb’s Law The Electrical Proportionality Constant The proportionality constant k in Coulomb’s law is similar to G in Newton’s law of gravitation. 16. k = 9, 000, 000 N·m 2/C 2 or 9. 0 × 109 N·m 2/C 2 If a pair of charges of 1 C each were 1 m apart, the force of repulsion between the two charges would be 9 billion newtons. That would be more than 10 times the weight of a battleship!
32 Electrostatics 32. 3 Coulomb’s Law Newton’s law of gravitation for masses is similar to Coulomb’s law for electric charges. Whereas the gravitational force of attraction between a pair of onekilogram masses is extremely small, the electrical force between a pair of one-coulomb charges is extremely large. The greatest difference between gravitation and electrical forces is that gravity only attracts but electrical forces may attract or repel.
32 Electrostatics 32. 3 Coulomb’s Law Electrical Forces in Atoms Because most objects have almost exactly equal numbers of electrons and protons, electrical forces usually balance out. Between Earth and the moon, for example, there is no measurable electrical force. In general, the weak gravitational force, which only attracts, is the predominant force between astronomical bodies.
32 Electrostatics Focused Learning Target • Given different materials, I will be able to compare insulators, conductors and semiconductors in terms of conductivity.
32 Electrostatics 32. 4 Conductors and Insulators Outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but are free to roam in the material. 17. Materials through which electric charge can flow are called conductors. Metals are good conductors for the motion of electric charges because their electrons are “loose. ”
32 Electrostatics 32. 4 Conductors and Insulators Electrons in other materials—rubber and glass, for example—are tightly bound and remain with particular atoms. They are not free to wander about to other atoms in the material. 18. These materials, known as insulators, are poor conductors of electricity.
32 Electrostatics 32. 4 Conductors and Insulators A substance is classified as a conductor or an insulator based on how tightly the atoms of the substance hold their electrons. The conductivity of a metal can be more than a million trillion times greater than the conductivity of an insulator such as glass. In power lines, charge flows much more easily through hundreds of kilometers of metal wire than through the few centimeters of insulating material that separates the wire from the supporting tower.
32 Electrostatics 32. 4 Conductors and Insulators 19. Semiconductors are materials that can be made to behave sometimes as insulators and sometimes as conductors. Atoms in a semiconductor hold their electrons until given small energy boosts. 20. Examples : This occurs in photovoltaic cells that convert solar energy into electrical energy. Thin layers of semiconducting materials sandwiched together make up transistors.
32 Electrostatics 32. 4 Conductors and Insulators What is the difference between a good conductor and a good insulator?
32 Electrostatics Focused Learning Target • Given an electrical circuit devices , I will be able define , explain and calculate voltage, current and resistance.
32 Electrostatics Voltage produces a flow of charge, or current, within a conductor. The flow is restrained by the resistance it encounters. The rate at which energy is transferred by electric current is power.
32 Electrostatics 34. 1 Flow of Charge When the ends of an electric conductor are at different electric potentials, charge flows from one end to the other.
32 Electrostatics 34. 1 Flow of Charge Heat flows through a conductor when a temperature difference exists. Heat flows from higher temperature to lower temperature. When temperature is at equilibrium, the flow of heat ceases.
32 Electrostatics 34. 1 Flow of Charge flows in a similar way. Charge flows when there is a 21. Voltage is potential difference, or difference in potential (voltage), between the ends of a conductor. The flow continues until both ends reach the same potential. When there is no potential difference, there is no longer a flow of charge through the conductor. 22. Electric Current - a sustained flow of charge in a conductor, one end must remain at a higher potential than the other. The situation is analogous to the flow of water.
32 Electrostatics 34. 1 Flow of Charge a. Water flows from higher pressure to lower pressure. The flow will cease when the difference in pressure ceases.
32 Electrostatics 34. 1 Flow of Charge a. b. Water flows from higher pressure to lower pressure. The flow will cease when the difference in pressure ceases. Water continues to flow because a difference in pressure is maintained with the pump. The same is true of electric current.
32 Electrostatics 34. 1 Flow of Charge What happens when the ends of a conductor are at different electrical potentials?
32 Electrostatics 34. 2 Electric Current A current-carrying wire has a net electric charge of zero.
32 Electrostatics 34. 2 Electric Current 22. Electric current is the flow of electric charge. In solid conductors, electrons carry the charge through the circuit because they are free to move throughout the atomic network. These electrons are called conduction electrons. Protons are bound inside atomic nuclei, locked in fixed positions. In fluids, such as the electrolyte in a car battery, positive and negative ions as well as electrons may flow.
32 Electrostatics 34. 2 Electric Current Measuring Current 23. Electric current is measured in amperes, symbol A. An ampere is the flow of 1 coulomb of charge per second. When the flow of charge past any cross section is 1 coulomb (6. 24 billion electrons) per second, the current is 1 ampere.
32 Electrostatics 34. 3 Voltage Sources Voltage sources such as batteries and generators supply energy that allows charges to move steadily.
32 Electrostatics 34. 3 Voltage Sources Charges do not flow unless there is a potential difference. Something that provides a potential difference is known as a voltage source. Batteries and generators are capable of maintaining a continuous flow of electrons.
32 Electrostatics 34. 3 Voltage Sources 24. Steady Voltage Sources In a battery, a chemical reaction releases electrical energy. Generators—such as the alternators in automobiles—convert mechanical energy to electrical energy. The electrical potential energy produced is available at the terminals of the battery or generator.
32 Electrostatics What is a Battery? Four cells are in parallel. • Battery is a group of: • Cells that can be charged with an electric current to store electrical energy • Cells that can produce an electric current by converting stored chemical energy to electrical energy. Four cells are in series. A battery is a device that stores electrical energy. 3/16/2018 65 65
32 Electrostatics The Battery is a Storage Device for Electrical Energy There are two types of batteries: • A primary battery is not rechargeable. It is made to be used until run down. It is then thrown away. The energy stored in a primary battery is available immediately after the battery is manufactured. • Secondary batteries must be charged before use; they are usually assembled with active materials in the discharged state. Rechargeable batteries or secondary cells can be recharged by applying electric current, which reverses the chemical reactions that occur during its use. 3/16/2018 AWIM 66 66
32 Electrostatics Alessandro Volta (1745 -1827) Please don’t touch the batteries! Italian scientist; made the first working battery in 1800, more than 200 years ago. 3/16/2018 67 67
32 Electrostatics 34. 3 Voltage Sources The potential energy per coulomb of charge available to electrons moving between terminals is the voltage. The voltage provides the “electric pressure” to move electrons between the terminals in a circuit.
32 Electrostatics 34. 3 Voltage Sources Power utilities use electric generators to provide the 120 volts delivered to home outlets. The alternating potential difference between the two holes in the outlet averages 120 volts. When the prongs of a plug are inserted into the outlet, an average electric “pressure” of 120 volts is placed across the circuit. This means that 120 joules of energy is supplied to each coulomb of charge that is made to flow in the circuit.
32 Electrostatics Q&A 1 Q: What does a battery do? A: A battery produces electrical energy by means of a chemical reaction. Q: What is the difference between a battery and a cell? A: The definitions vary. A battery may be a group of cells connected together, or it may be the name applied to a single cell, as in “flashlight battery”. A cell is always a single electrochemical unit. Q: What is a chemical reaction? A: A chemical reaction is a process that involves changes in the structure and energy content of atoms, molecules, or ions. A chemical reaction forms new chemical substances. 3/16/2018 AWIM 70 70
32 Electrostatics Q&A 2 Q: What is an electrochemical reaction? A: An electrochemical reaction is a chemical reaction that can be used in a battery (or fuel cell) to produce electric power. Q: How are equations written for electrochemical reactions? A: 2 H 2 + O 2 = H 2 O hydrogen + oxygen water (This is a fuel cell equation. ) Zn + 2 HCl = Zn. Cl 2 + H 2 zinc + hydrogen chloride zinc chloride + hydrogen (This equation applies to the organic battery cell experiment) 3/16/2018 AWIM 71 71
32 Electrostatics Q&A 3 • Q: What is an ion? A: An ion is an atom or a part of a molecule that carries either a positive (+) or negative (-) electric charge, resulting from the gain or loss of one or more electrons. • Q: What do ions do in a battery? A: Ions carry the electric current that flows inside a battery between the positive and negative electrodes. Ions are produced (or consumed). • Q: What are electrodes? A: A battery’s two electrodes, the anode and cathode, are the two parts of a battery where the battery’s chemical reactions take place. The electrodes extend to the outside of the battery, where they form the battery terminals used to connect the battery to its external circuit. 3/16/2018 72
32 Electrostatics 3/16/2018 Cutaway of a Lead Acid Battery 73 73
32 Electrostatics Lead Acid Batteries: History, Chemistry and Application The lead acid battery was invented in 1859 by the French physicist Gaston Planté. It was refined by others so that it became the first commercially available rechargeable battery in the world market. The battery chemical equation: • Pb(s) + Pb. O 2(s) + 2 H 2 SO 4(aq) → 2 Pb. SO 4(s) + 2 H 2 O(l) • Cell potential = 2. 1 volts For more than a century automobiles have been powered by the internal combustion engine (gasoline) and have used lead acid batteries to start the engine. These batteries have the ability to provide large currents to the starting mechanisms. A good starter will normally draw approximately: – 130 to 150 amps for a 4 -cylinder engine, – 175 amps for a V 6, – 200 to 225 amps for a V 8. 3/16/2018 74 74
32 Electrostatics Pros and Cons of Lead Acid Batteries Pros (Advantages): Cons (Disadvantages): • Inexpensive and simple to manufacture; low cost per watthour • Low self-discharge; lowest among rechargeable batteries • High specific power, capable of high discharge currents (for example, to start a car) • Good low temperature and high temperature performance • Poor weight-to-energy ratio (heavy!) • Slow to charge; fully saturated charge takes 14 hours. • Must be stored in charged condition to prevent sulfation (formation of sulfite crystals which shorten battery life). • Limited cycle life; repeated deep-cycling (within 30% of zero charge); possible causes: leaving headlights on overnight or an alternator not recharging battery. 3/16/2018 75 75
32 Electrostatics A Battery’s Essential Components • Essential components are: – a, . Anode is the negative electrode where the electrons are released to the external circuit. – B. Cathode is the positive electrode that acquires electrons from the external circuit. (A cathode is an electrode through which electric current flows out of a polarized electrical device. The direction of electric current is, by convention, opposite to the direction of electron flow-thus electrons are considered to flow toward the cathode electrode while current flows away from it. ) – Electrolyte is the medium that provides the ion transport mechanism between the cathode and anode. – A Cation is a positively charged ion, i. e. , one that would be attracted to the cathode in electrolysis. It has fewer electrons than protons. The opposite is Anion. 3/16/2018 AWIM 76 76
32 Electrostatics Two Representations of a Single Cell Detail of alkaline cell Simplistic representation + - C A A T ANION N H CATION O O D D ELECTROLYTE E E 3/16/2018 AWIM 77 77
32 Electrostatics How Electrical Energy is Released • A battery converts stored chemical energy into useful electrical energy once an external connection is made between its positive and negative terminals. . • Then a chemical reaction is initiated that generates electrons at the anode to supply the current of the battery to the external circuit. • Not all batteries can be recharged. Single Electrochemical Cell 3/16/2018 AWIM + - C A A T ANION N H CATION O O D D ELECTROLYTE E E 78 78
32 Electrostatics The Potato as an Organic Battery Experiment No. 1 3/16/2018 AWIM 79 79
32 Electrostatics What is an Organic Battery Cell? • Organic cell is a living organism that contains an acidic electrolyte. • When a proper pair of electrodes is submerged in its electrolyte it produces electricity by chemical reaction. • We’ll use lemon and potato to generate electricity. • Lemon has an electrolyte called citric acid. • Potato has an electrolyte made of Sodium (Na), Potassium (K) and Chloride (Cl) ions (phosphoric acid). • Zinc and Nickel electrodes have free electrons with a potential voltage of -0. 76 V and -0. 25 V respectively. (See the next slide) • Copper electrode is positive at +0. 34 V that attracts negative (Zn) or (Ni) ions during the chemical reaction in an electrolyte. 3/16/2018 80 80
32 Electrostatics Standard Electrode Potentials in Aqueous Solution at 25°C. Standard Potential K+(aq) + e K(s) Ca 2+(aq) + 2 e Ca(s) Na+(aq) + e Na(s) Mg 2+(aq) + 2 e Mg(s) Al 3+(aq) + 3 e Al(s) 2 H 2 O (l) + 2 e H 2(g) + 2 OH (aq) Zn 2+ (aq) + 2 e Zn(s) Cr 3+ (aq) + 3 e Cr(s) Fe 2+ (aq) + 2 e Fe(s) Cd 2+ (aq) + 2 e Cd(s) Ni 2+ (aq) + 2 e Ni(s) Sn 2+ (aq) + 2 e Sn(s) Pb 2+ (aq) + 2 e Pb(s) Fe 3+ (aq) + 3 e Fe(s) Li+(aq) + e Li(s) (Volts) -3. 04 -2. 92 -2. 76 -2. 71 -2. 38 -1. 66 -0. 83 -0. 76 -0. 74 -0. 41 -0. 40 -0. 23 -0. 14 -0. 13 Standard Potential - 2 H+ (aq) + 2 e H 2(g) Sn 4+ (aq) + 2 e Sn 2+(aq) Cu 2+ (aq) + e Cu+(aq) - Cl. O 4 (aq) + H 2 O (l) + 2 e Cl. O 3 (aq) + 2 OH (aq) Ag. Cl (s) + e Ag (s) + Cl (aq) Cu 2+ (aq) + 2 e Cu (s) - Cl. O 3 (aq) + H 2 O (l) + 2 e Cl. O 2 (aq) + 2 OH (aq) - IO (aq) + H 2 O (l) + 2 e I (aq) + 2 OH (aq) Cu+ (aq) + e Cu(s ) I 2(s) + 2 e 2 I (aq) - Cl. O 2 (aq) + H 2 O(l) + 2 e Cl. O (aq) + 2 OH (aq) Fe 3+ (aq) + e Fe 2+(aq) Hg 22+ (aq) + 2 e 2 Hg (l) Ag+ (aq) + e Ag (s) (Volts) 0. 00 0. 15 0. 16 0. 17 0. 22 0. 34 0. 35 0. 49 0. 52 0. 54 0. 59 0. 77 0. 80 -0. 04 (aq)=aqueous or water solution; (s)=solid phase; (l)=liquid phase Aluminum (Al), Zinc (Zn), Nickel (Ni), and Copper (Cu) will be used in experiment Table of Standard Electrode Potentials taken from hyperphysics. phy-astr. gsu. edu/hbase/tables/8110/19/13 Electrode Potential determines the output voltage of an electro-chemical cell. 81 3/16/2018
32 Electrostatics Electrode Potential determines the output voltage of an electro-chemical cell • Each chemical element is characterized by its excess (-) or lack of (+) free electrons. • Electrodes have a potential to release FREE electrons (-) to those that don’t have any FREE electrons (+) Ex 1: Zinc, Zn Ex 2: Nickel, Ni = -0. 23 V Ex 3: Copper, Cu • = -0. 76 V = +0. 34 V Voltage difference between Cu and Zn electrodes in a lemon electrolyte can be calculated ahead of an experiment. Voltage difference between Cu to Zn electrodes = +0. 34 V – (-0. 76 V) = 1. 1 V (Important note: A cathode is an electrode through which electric current flows out of (and electrons into) a polarized electrical device. The direction of electric current is, by convention, opposite to the direction of electron flow. Therefore the electrons flow into (and current out of) the cathode electrode Cu and out of (and current into) anode electrode Zn. ) 3/16/2018 AWIM 82 82
32 Electrostatics What is Needed for This Experiment? 1. Electrolyte (natural fruit: lemon and potato) 1. The lemon contains citric acid which is the electrolyte of the battery. 2. The potato contains phosphoric acid which is the electrolyte. 2. Two electrodes of dissimilar material (copper and zinc, or copper and nickel) 1. Insert into the fruit (one inch apart, half an inch deep) 3. Voltmeter to measure voltage 4. Two Pairs of Copper Wires (with alligator clips to connect electrodes with a voltmeter), Red and Black 5. A Team of two Students (to make connections and write down the measurements) 6. Report and Discuss Results Caution: Do not eat or taste any lemon that has been contaminated by electrodes! 3/16/2018 83 83
32 Electrostatics Digital Multimeter Training Display Area DC Voltage Ranges Current Measurement Ranges Voltage Ranges for Experiment : 20 V or 2000 m. V = 2 V Positive + (Red) Lead goes here Resistance Ranges Negative – (Black) Lead goes here 3/16/2018 84 84
32 Electrostatics Organic Battery Cell Experiment Test No. 1 • Purpose: – Measure the voltage across Copper and Zinc electrodes placed in a fruit one inch apart and ½ inch deep! [Calculated voltage difference between Cu to Zn electrodes = +0. 34 V – (-0. 76 V) = 1. 1 V] • • • Connections (see the next photo): – Use red alligator clips to connect the Copper electrode with voltmeter (+); – Use black alligator clips to connect the Zinc electrode with voltmeter (-); – Set the voltmeter in the 2000 m. VDC range (calculated voltage is 2. 2 V. ) and turn it ON; Measurement: – Read the voltage and make a note of how many volts Record observations – Is the voltage stable? Presence of any bubbles at electrodes? 3/16/2018 85 85
32 Electrostatics An Organic CELL Generates Electricity, Test No. 1 Make connections as shown and measure the voltage. Voltmeter set on 2000 m. VDC scale Zinc electrode (Black) Copper electrode (RED) Measured Voltage: 938 m. V 3/16/2018 86
32 Electrostatics Organic Battery Cell Experiment Test No. 2 • Purpose: – Measure the voltage across Copper and Zinc electrodes placed in a fruit one inch apart and ½ inch deep in two lemons! [Calculated voltage difference between two pairs of Cu to Zn electrodes = 2 x (+0. 34 V – (-0. 76 V)) = 2. 2 V] • Connections (see the next photo): – – Use red alligator clips to connect the Copper electrode on Lemon 1 with voltmeter (+); Use black alligator clips on Lemon 1 to connect the Copper electrode on Lemon 2; Use black alligator clips on Lemon 2 to connect the Zinc electrode with voltmeter (-); Set the voltmeter in the 20 VDC range (calculated voltage is 2. 2 V. ) and turn it ON; • Measurement: – Read the voltage and record it; • Record observations – Is the voltage stable? Presence of any bubbles at electrodes? 3/16/2018 87 87
32 Electrostatics Two Organic Cells connected in SERIES generate twice the voltage of a single cell, Test No. 2 Voltmeter Set on 20 VDC SCALE +1. 78 Volt Copper electrode Zinc electrode 3/16/2018 Copper electrode 88
32 Electrostatics Organic Battery Cell Experiment Test No. 3 • Purpose: – Measure the voltage across Copper and Nickel electrodes placed in a fruit one inch apart and ½ inch deep! [Calculated voltage difference between Cu to Ni electrodes = +0. 34 V – (-0. 23 V) =. 57 V per pg. 19] • • • Connections (see the next photo): – Use red alligator clips to connect the Copper electrode with voltmeter (+); – Use black alligator clips to connect the Nickel electrode with voltmeter (-); – Set the voltmeter in the 2000 m. VDC range (calculated voltage is. 57 V) and turn it ON; Measurement: – Read the voltage and make a note of how many volts Record observations – Is the voltage stable? Presence of any bubbles at electrodes? 3/16/2018 89 89
32 Electrostatics Test No. 3 An Organic Battery CELL Generates Lower Voltage with Nickel Electrode Make connections as shown and measure the voltage. Voltmeter set on 2000 m. VDC scale Nickel electrode (Black) Copper electrode (RED) Measured Voltage: 425 m. V 3/16/2018 90
32 Electrostatics Student Name: ___________ Experiment No. 1 Organic Battery Cell Data Sheet with Estimated vs. Actual Test Results Expected Actual Voltage Voltage Test No. 1 Lemon with Copper to Zinc 1. 10 V _____ Test No. 2 Two lemon halves in series with Copper and Zinc 2. 20 V ______ Test No. 3 Lemon with Copper to Nickel 0. 57 V ______ Explain why is there a difference in voltage between No. 1 and 2 tests? ________________________________________ Why is the voltage lower in Test No. 3? _____________________________________________________ Explain whether another fruit like a potato would generate significantly different voltage than the lemon used in tests? If not, explain why? ____________________ 3/16/2018 ____________________________________ 91 91 _____
32 Electrostatics 25 - 33 New Terminology Primary batteries are made to be used one time and then discarded. The energy stored is available immediately after manufacturing. Secondary batteries or Rechargeable batteries can be recharged by applying electric current, which reverses the chemical reactions that occur during its use. A chemical reaction is a process that involves changes in the structure and energy content of atoms, molecules, or ions. A chemical reaction forms new chemical substances. An electrochemical reaction is a chemical reaction that can be used in a battery (or fuel cell) to produce electric power. Electrodes are the two parts of a battery where the battery’s chemical reactions take place. half-cell where chemical reactions take place. The electrodes extend to the outside of the battery, where they connect to its external circuit. Anode is the negative electrode where the electrons are released out of (and current into) the external circuit. Cathode is the positive electrode that acquires electrons into (and current out of) the external circuit. Electrolyte is the medium that provides the ion transport mechanism between the cathode and anode. 3/16/2018 92 An ion is an atom or molecule in which a total number of electrons is not
32 Electrostatics Homework, Experiment No. 1 Organic Battery Cell Due Date: October 31, 2013 Name: ---------------1. The Digital Multimeter can be used to measure electric current A, voltage V and Ohms? ------------------T------F 2. We will use a Multimeter to measure current in an electric circuit. The Multimeter dial is turned to DCA range. The red test lead is plugged into the terminal “V O m. A” and the black into the terminal “COM”. In this configuration, the device can also be called an Ammeter. Is Ammeter spelled correctly, since it measures current in amps, short of Amperes? -----T------F 3. Alkaline batteries must be charged initially by an electrical current before they are packaged and sold? ---------T------F 4. The Chevy Volt plug-in hybrid car and the Tesla electric car both use secondary batteries? -----------------T------F 5. A neutral atom that takes on or loses electron is called an ion? -----------------------------------T------F 6. All primary batteries use the same chemical reaction? -----------------------------------------T------F 7. An electrochemical reaction is a chemical reaction that can be used in a battery to produce electric power? ------T------F 8. A Lead-acid battery cannot produce current over 130 Amps? ------------------------------------T------F 9. Scientist Andre-Marie Ampere invented the lead acid battery? -----------------------------------T------F 10. In the Test 1 Organic Battery Cell experiment the estimated voltage difference between copper, Cu and zinc, Zn electrodes was 1. 1 Volt. Using aluminum instead of zinc electrode the estimated voltage difference between Cu and Al electrodes is higher? ------------------------------------------------------------------------T-----F 11. By choosing Lithium, Li electrode instead of Zinc the expected voltage difference between Cu and Li is lower? ----T-----F 12. If we repeated the Test No 2 with two lemon halves in parallel instead of in series the estimated voltage difference between the Cu and Zn electrodes would have been 1. 1 V? --------------------------------------------T-----F 13. In an organic battery cell the fruit acid has a great influence on a voltage that this battery can generate? --------T-----F 14. The citric acid found in a lemon is an electrolyte that provides the ion transport mechanism between the cathode and anode? ------------------------------------------------------------------------T-----F 15. An organic cell is not commonly used because of a fear that the fruit might become contaminated? -----------T-----F 3/16/2018 93
32 Electrostatics 34. 3 Voltage Sources Distinguishing Between Current and Voltage There is often some confusion between charge flowing through a circuit and voltage being impressed across a circuit.
32 Electrostatics 34. 3 Voltage Sources Consider a long pipe filled with water. • Water will flow through the pipe if there is a difference in pressure across the pipe or between its ends. • Water flows from high pressure to low pressure. Similarly, charges flow through a circuit because of an applied voltage across the circuit. • You don’t say that voltage flows through a circuit. • Voltage doesn’t go anywhere, for it is the charges that move. • Voltage causes current.
32 Electrostatics 34. 3 Voltage Sources What are two voltage sources used to provide the energy that allows charges to move steadily?
32 Electrostatics 34. 4 Electric Resistance 34. The resistance of a wire depends on the conductivity of the material used in the wire (that is, how well it conducts) and also on the thickness and length of the wire.
32 Electrostatics 34. 4 Electric Resistance The amount of charge that flows in a circuit depends on the voltage provided by the voltage source. 35. The current also depends on the resistance that the conductor offers to the flow of charge—the electric resistance. This is similar to the rate of water flow in a pipe, which depends on the pressure difference and on the resistance of the pipe.
32 Electrostatics 34. 4 Electric Resistance For a given pressure, more water passes through a large pipe than a small one. Similarly, for a given voltage, more electric current passes through a largediameter wire than a smalldiameter one.
32 Electrostatics Focused Learning Target • Given an electric circuit , I will be able to calculate the voltage , current and resistance using Ohm’s Law
32 Electrostatics 34. 4 Electric Resistance A simple hydraulic circuit is analogous to an electric circuit.
32 Electrostatics 34. 4 Electric Resistance The resistance of a wire depends on the conductivity of the material in the wire and on the thickness and length of the wire. • Thick wires have less resistance than thin wires. • Longer wires have more resistance than short wires. • Electric resistance also depends on temperature. For most conductors, increased temperature means increased resistance.
32 Electrostatics 34. 4 Electric Resistance The resistance of some materials becomes zero at very low temperatures, a phenomenon known as superconductivity. Certain metals acquire superconductivity (zero resistance to the flow of charge) at temperatures near absolute zero. Superconductivity at “high” temperatures (above 100 K) has been found in a variety of nonmetallic compounds. In a superconductor, the electrons flow indefinitely.
32 Electrostatics 34. 4 Electric Resistance What factors affect the resistance of a wire?
32 Electrostatics 34. 5 36. Ohm’s Law Ohm’s law states that the current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit.
32 Electrostatics 34. 5 Ohm’s Law Electric resistance is measured in units called ohms. Georg Simon Ohm, a German physicist, tested wires in circuits to see what effect the resistance of the wire had on the current. 37. The relationship among voltage, current, and resistance is called Ohm’s law.
32 Electrostatics 34. 5 Ohm’s Law For a given circuit of constant resistance, current and voltage are proportional. Twice the current flows through a circuit for twice the voltage across the circuit. The greater the voltage, the greater the current. If the resistance is doubled for a circuit, the current will be half what it would be otherwise.
32 Electrostatics 34. 5 Ohm’s Law The relationship among the units of measurement is: A potential difference of 1 volt impressed across a circuit that has a resistance of 1 ohm will produce a current of 1 ampere. If a voltage of 12 volts is impressed across the same circuit, the current will be 12 amperes.
32 Electrostatics 34. 5 Ohm’s Law The resistance of a typical lamp cord is much less than 1 ohm, while a typical light bulb has a resistance of about 100 ohms. An iron or electric toaster has a resistance of 15 to 20 ohms. The low resistance permits a large current, which produces considerable heat.
32 Electrostatics 34. 5 Ohm’s Law 38. Current inside electric devices is regulated by circuit elements called resistors. The stripes on these resistors are color coded to indicate the resistance in ohms.
32 Electrostatics 34. 5 Ohm’s Law think! Questions 1 -2 p 536 -537 39 – 42 How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?
32 Electrostatics CW: Ohm’s Law #16 -#17 p 560
32 Electrostatics 34. 5 Ohm’s Law think! How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it? Answer:
32 Electrostatics 34. 5 Ohm’s Law What does Ohm’s law state?
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock The damaging effects of electric shock are the result of current passing through the body.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock From Ohm’s law, we can see that current depends on the voltage applied, and also on the electric resistance of the human body.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock The Body’s Resistance Your body’s resistance ranges from about 100 ohms if soaked with salt water to about 500, 000 ohms if your skin is very dry. Touch the electrodes of a battery with dry fingers and your resistance to the flow of charge would be about 100, 000 ohms. You would not feel 12 volts, and 24 volts would just barely tingle. With moist skin, however, 24 volts could be quite uncomfortable.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Many people are killed each year by current from common 120 -volt electric circuits. Touch a faulty 120 -volt light fixture while standing on the ground and there is a 120 -volt “pressure” between you and the ground. The soles of your shoes normally provide a very large resistance, so the current would probably not be enough to do serious harm.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock If you are standing barefoot in a wet bathtub, the resistance between you and the ground is very small. Your overall resistance is so low that the 120 -volt potential difference may produce a harmful current through your body. Drops of water that collect around the on/off switches of devices such as a hair dryer can conduct current to the user.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Although distilled water is a good insulator, the ions in ordinary water greatly reduce the electric resistance. There is also usually a layer of salt on your skin, which when wet lowers your skin resistance to a few hundred ohms or less. Handling electric devices while taking a bath is extremely dangerous.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Handling a wet hair dryer can be like sticking your fingers into a live socket.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock High-Voltage Wires You probably have seen birds perched on high-voltage wires. Every part of the bird’s body is at the same high potential as the wire, and it feels no ill effects. For the bird to receive a shock, there must be a difference in potential between one part of its body and another part. Most of the current will then pass along the path of least electric resistance connecting these two points.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Suppose you fall from a bridge and manage to grab onto a highvoltage power line, halting your fall. If you touch nothing else of different potential, you will receive no shock, even if the wire is thousands of volts above ground potential. No charge will flow from one hand to the other because there is no appreciable difference in electric potential between your hands.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Ground Wires Mild shocks occur when the surfaces of appliances are at an electric potential different from other nearby devices. If you touch surfaces of different potentials, you become a pathway for current. To prevent this, electric appliances are connected to a ground wire, through the round third prong of a three-wire electric plug.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock All ground wires in all plugs are connected together through the wiring system of the house. The two flat prongs are for the current-carrying double wire. If the live wire accidentally comes in contact with the metal surface of an appliance, the current will be directed to ground rather than shocking you if you handle it.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock Health Effects One effect of electric shock is to overheat tissues in the body or to disrupt normal nerve functions. It can upset the nerve center that controls breathing.
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock think! If the resistance of your body were 100, 000 ohms, what would be the current in your body when you touched the terminals of a 12 -volt battery?
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock think! If the resistance of your body were 100, 000 ohms, what would be the current in your body when you touched the terminals of a 12 -volt battery? Answer:
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock think! If your skin were very moist, so that your resistance was only 1000 ohms, and you touched the terminals of a 24 -volt battery, how much current would you draw?
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock think! If your skin were very moist, so that your resistance was only 1000 ohms, and you touched the terminals of a 24 -volt battery, how much current would you draw? Answer: You would draw or 0. 024 A, a dangerous amount of current!
32 Electrostatics 34. 6 Ohm’s Law and Electric Shock What causes the damaging effects of electric shock?
32 Electrostatics CW : Ohm’s Law p 546 #29 – 33
32 Electrostatics Focused Learning Target • Given different voltage sources, I will be able to compare Direct Current and Alternating Current voltage sources. • Given an electric circuit , I will be able to calculate the Power = Voltage X Current P = IV P = I 2 R P = V 2/ R
32 Electrostatics 34. 7 Direct Current and Alternating Current Electric current may be DC or AC.
32 Electrostatics 34. 7 Direct Current and Alternating Current 43. By DC, we mean direct current, which refers to a flow of charge that always flows in one direction. • A battery produces direct current in a circuit because the terminals of the battery always have the same sign of charge. • Electrons always move through the circuit from the negative terminal toward the positive terminal. • Even if the current moves in unsteady pulses, so long as it moves in one direction only, it is DC.
32 Electrostatics 34. 7 Direct Current and Alternating Current 44. Alternating current (AC), as the name implies, is electric current that repeatedly reverses direction. • Electrons in the circuit move first in one direction and then in the opposite direction. • They alternate back and forth about relatively fixed positions. • This is accomplished by alternating the polarity of voltage at the generator or other voltage source.
32 Electrostatics 34. 7 Direct Current and Alternating Current Voltage Standards Voltage of AC in North America is normally 120 volts. In the early days of electricity, higher voltages burned out the filaments of electric light bulbs. Power plants in the United States prior to 1900 adopted 110 volts (or 115 or 120 volts) as standard.
32 Electrostatics 34. 7 Direct Current and Alternating Current By the time electricity became popular in Europe, light bulbs were available that would not burn out so fast at higher voltages. Power transmission is more efficient at higher voltages, so Europe adopted 220 volts as their standard. The United States stayed with 110 volts (today, officially 120 volts) because of the installed base of 110 -volt equipment.
32 Electrostatics 34. 7 Direct Current and Alternating Current Three-Wire Service Although lamps in an American home operate on 110– 120 volts, electric stoves and other appliances operate on 220– 240 volts. Most electric service in the United States is three-wire: • one wire at 120 volts positive • one wire at zero volts (neutral) • one wire at a negative 120 volts
32 Electrostatics 34. 7 Direct Current and Alternating Current In AC, the positive and negative alternate at 60 hertz. A wire that is positive at one instant is negative 1/120 of a second later. Most home appliances are connected between the neutral wire and either of the other two wires, producing 120 volts. When the plus-120 is connected to the minus-120, it produces a 240 volt difference—just right for electric stoves, air conditioners, and clothes dryers.
32 Electrostatics 34. 7 Direct Current and Alternating Current The popularity of AC arises from the fact that electrical energy in the form of AC can be transmitted great distances. Easy voltage step-ups result in lower heat losses in the wires. The primary use of electric current, whether DC or AC, is to transfer energy from one place to another.
32 Electrostatics 34. 7 Direct Current and Alternating Current What are the two types of electric current?
32 Electrostatics 34. 8 Converting AC to DC With an AC-DC converter, you can operate a batteryrun device on AC instead of batteries.
32 Electrostatics 34. 8 Converting AC to DC The current in your home is AC. The current in a battery-operated device, such as a laptop computer or cell phone, is DC. With an AC-DC converter, you can operate a battery-run device on AC instead of batteries.
32 Electrostatics 34. 11 Electric Power Electric power is equal to the product of current and voltage.
32 Electrostatics 34. 11 Electric Power Unless it is in a superconductor, a charge moving in a circuit expends energy. This may result in heating the circuit or in turning a motor. 45. Electric power is the rate at which electrical energy is converted into another form such as mechanical energy, heat, or light.
32 Electrostatics 34. 11 Electric Power 46. Electric power is equal to the product of current and voltage. electric power = current × voltage If the voltage is expressed in volts and the current in amperes, then the power is expressed in watts. 1 watt = (1 ampere) × (1 volt)
32 Electrostatics 34. 11 Electric Power The power and voltage on the light bulb read “ 60 W 120 V. ” The current that would flow through the bulb is: I = P/V = (60 W)/(120 V) = 0. 5 A.
32 Electrostatics 34. 11 Electric Power A lamp rated at 120 watts operated on a 120 -volt line will draw a current of 1 ampere: 120 watts = (1 ampere) × (120 volts). A 60 -watt lamp draws 0. 5 ampere on a 120 volt line.
32 Electrostatics 34. 11 Electric Power 47. A kilowatt is 1000 watts, and a kilowatt-hour represents the amount of energy consumed in 1 hour at the rate of 1 kilowatt. Where electrical energy costs 10 cents per kilowatt-hour, a 100 -watt light bulb burns for 10 hours for 10 cents. A toaster or iron, which draws more current and therefore more power, costs several times as much to operate for the same time.
32 Electrostatics 34. 11 48 -49 think! Question 1 -2 p 544 Electric Power How much power is used by a calculator that operates on 8 volts and 0. 1 ampere? If it is used for one hour, how much energy does it use?
32 Electrostatics 34. 11 Electric Power think! How much power is used by a calculator that operates on 8 volts and 0. 1 ampere? If it is used for one hour, how much energy does it use? Answer: Power = current × voltage = (0. 1 A) × (8 V) = 0. 8 W. Energy = power × time = (0. 8 W) × (1 h) = 0. 8 watt-hour, or 0. 0008 kilowatt-hour.
32 Electrostatics 34. 11 Electric Power think! Will a 1200 -watt hair dryer operate on a 120 -volt line if the current is limited to 15 amperes by a safety fuse? Can two hair dryers operate on this line?
32 Electrostatics 34. 11 Electric Power think! Will a 1200 -watt hair dryer operate on a 120 -volt line if the current is limited to 15 amperes by a safety fuse? Can two hair dryers operate on this line? Answer: One 1200 -W hair dryer can be operated because the circuit can provide (15 A) × (120 V) = 1800 W. But there is inadequate power to operate two hair dryers of combined power 2400 W. In terms of current, (1200 W)/(120 V) = 10 A; so the hair dryer will operate when connected to the circuit. But two hair dryers will require 20 A and will blow the 15 -A fuse.
32 Electrostatics CW : Electric Power • #51 p 547
32 Electrostatics 34. 11 Electric Power How can you express electric power in terms of current and voltage?
32 Electrostatics Assessment Questions 1. If a neutral atom has 22 protons in its nucleus, the number of surrounding electrons is a. less than 22. b. 22. c. more than 22. d. unknown.
32 Electrostatics Assessment Questions 1. If a neutral atom has 22 protons in its nucleus, the number of surrounding electrons is a. less than 22. b. 22. c. more than 22. d. unknown. Answer: B
32 Electrostatics Assessment Questions 2. When we say charge is conserved, we mean that charge can a. be saved, like money in a bank. b. only be transferred from one place to another. c. take equivalent forms. d. be created or destroyed, as in nuclear reactions.
32 Electrostatics Assessment Questions 2. When we say charge is conserved, we mean that charge can a. be saved, like money in a bank. b. only be transferred from one place to another. c. take equivalent forms. d. be created or destroyed, as in nuclear reactions. Answer: B
32 Electrostatics Assessment Questions 3. A difference between Newton’s law of gravity and Coulomb’s law is that only one of these a. is a fundamental physical law. b. uses a proportionality constant. c. invokes the inverse-square law. d. involves repulsive as well as attractive forces.
32 Electrostatics Assessment Questions 3. A difference between Newton’s law of gravity and Coulomb’s law is that only one of these a. is a fundamental physical law. b. uses a proportionality constant. c. invokes the inverse-square law. d. involves repulsive as well as attractive forces. Answer: D
32 Electrostatics Assessment Questions 4. Which is the predominant carrier of charge in copper wire? a. protons b. electrons c. ions d. neutrons
32 Electrostatics Assessment Questions 4. Which is the predominant carrier of charge in copper wire? a. protons b. electrons c. ions d. neutrons Answer: B
32 Electrostatics Assessment Questions 5. When you scuff electrons off a rug with your shoes, your shoes are then a. negatively charged. b. positively charged. c. ionic. d. electrically neutral.
32 Electrostatics Assessment Questions 5. When you scuff electrons off a rug with your shoes, your shoes are then a. negatively charged. b. positively charged. c. ionic. d. electrically neutral. Answer: A
32 Electrostatics Assessment Questions 6. When a cloud that is negatively charged on its bottom and positively charged on its top moves over the ground below, the ground acquires a. a negative charge. b. a positive charge. c. no charge since the cloud is electrically neutral. d. an electrically grounded state.
32 Electrostatics Assessment Questions 6. When a cloud that is negatively charged on its bottom and positively charged on its top moves over the ground below, the ground acquires a. a negative charge. b. a positive charge. c. no charge since the cloud is electrically neutral. d. an electrically grounded state. Answer: B
32 Electrostatics Assessment Questions 7. When a negatively charged balloon is placed against a non-conducting wall, positive charges in the wall are a. attracted to the balloon. b. repelled from the balloon. c. too bound to negative charges in the wall to have any effect. d. neutralized.
32 Electrostatics Assessment Questions 7. When a negatively charged balloon is placed against a non-conducting wall, positive charges in the wall are a. attracted to the balloon. b. repelled from the balloon. c. too bound to negative charges in the wall to have any effect. d. neutralized. Answer: A
32 Electrostatics Assessment Questions 1. Electric charge will flow in an electric circuit when a. electrical resistance is low enough. b. a potential difference exists. c. the circuit is grounded. d. electrical devices in the circuit are not defective.
32 Electrostatics Assessment Questions 1. Electric charge will flow in an electric circuit when a. electrical resistance is low enough. b. a potential difference exists. c. the circuit is grounded. d. electrical devices in the circuit are not defective. Answer: B
32 Electrostatics Assessment Questions 2. The electric current in a copper wire is normally composed of a. electrons. b. protons. c. ions. d. amperes.
32 Electrostatics Assessment Questions 2. The electric current in a copper wire is normally composed of a. electrons. b. protons. c. ions. d. amperes. Answer: A
32 Electrostatics Assessment Questions 3. Which statement is correct? a. Voltage flows in a circuit. b. Charge flows in a circuit. c. A battery is the source of electrons in a circuit. d. A generator is the source of electrons in a circuit.
32 Electrostatics Assessment Questions 3. Which statement is correct? a. Voltage flows in a circuit. b. Charge flows in a circuit. c. A battery is the source of electrons in a circuit. d. A generator is the source of electrons in a circuit. Answer: B
32 Electrostatics Assessment Questions 4. Which of the following type of copper wire would you expect to have the least electric resistance? a. a thick long wire b. a thick short wire c. a thin long wire d. a thin short wire
32 Electrostatics Assessment Questions 4. Which of the following type of copper wire would you expect to have the least electric resistance? a. a thick long wire b. a thick short wire c. a thin long wire d. a thin short wire Answer: D
32 Electrostatics Assessment Questions 5. When you double the voltage in a simple electric circuit, you double the a. current. b. resistance. c. ohms. d. resistors.
32 Electrostatics Assessment Questions 5. When you double the voltage in a simple electric circuit, you double the a. current. b. resistance. c. ohms. d. resistors. Answer: A
32 Electrostatics Assessment Questions 6. To receive an electric shock there must be a. current in one direction. b. moisture in an electrical device being used. c. high voltage and low body resistance. d. a difference in potential across part or all of your body.
32 Electrostatics Assessment Questions 6. To receive an electric shock there must be a. current in one direction. b. moisture in an electrical device being used. c. high voltage and low body resistance. d. a difference in potential across part or all of your body. Answer: D
32 Electrostatics Assessment Questions 7. The difference between DC and AC in electrical circuits is that in DC a. charges flow steadily in one direction only. b. charges flow in one direction. c. charges steadily flow to and fro. d. charges flow to and fro.
32 Electrostatics Assessment Questions 7. The difference between DC and AC in electrical circuits is that in DC a. charges flow steadily in one direction only. b. charges flow in one direction. c. charges steadily flow to and fro. d. charges flow to and fro. Answer: B
32 Electrostatics Assessment Questions 8. To convert AC to a fairly steady DC, which devices are used? a. diodes and batteries b. capacitors and diodes c. capacitors and batteries d. resistors and batteries
32 Electrostatics Assessment Questions 8. To convert AC to a fairly steady DC, which devices are used? a. diodes and batteries b. capacitors and diodes c. capacitors and batteries d. resistors and batteries Answer: B
32 Electrostatics Assessment Questions 9. What is it that travels at about the speed of light in an electric circuit? a. charges b. current c. electric field d. voltage
32 Electrostatics Assessment Questions 9. What is it that travels at about the speed of light in an electric circuit? a. charges b. current c. electric field d. voltage Answer: C
32 Electrostatics Assessment Questions 10. When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons a. must be supplied by you, just as water must be supplied for a water pipe. b. are already in the wire. c. may fall out, which is why wires are insulated. d. enter it from the electric outlet.
32 Electrostatics Assessment Questions 10. When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons a. must be supplied by you, just as water must be supplied for a water pipe. b. are already in the wire. c. may fall out, which is why wires are insulated. d. enter it from the electric outlet. Answer: B
32 Electrostatics Assessment Questions 11. If you double both the current and the voltage in a circuit, the power a. remains unchanged if resistance remains constant. b. halves. c. doubles. d. quadruples.
32 Electrostatics Assessment Questions 11. If you double both the current and the voltage in a circuit, the power a. remains unchanged if resistance remains constant. b. halves. c. doubles. d. quadruples. Answer: D
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