Our story so far Charges behave

Our story so far…. . • Charges behave logically, physics works – Forces between them – Fields created – Potential as a measure of work to move • Capacitors – a device to store charge – So that the charge can do work later for us • We studied these charges when they were static, not moving

Chapter 18 Electric Currents= charges on the move. • Think of the difference between – chapter 4 (dynamics) • SF=ma – And chapter 9 (static equilibrium) • SF=0

Water analogy for electricity • Flow • Pressure • restriction

Units of Chapter 18 • The Electric Battery • Electric Current • Ohm’s Law: Resistance and Resistors • Resistivity • Electric Power

Units of Chapter 18 • Power in Household Circuits • Alternating Current • Microscopic View of Electric Current • Superconductivity • Electrical Conduction in the Human Nervous System

A little history • Electricity was static, and a curiosity until 1800. – Parlor game shocks, animal muscle twitches • 1748: “battery” of charged glass plates (Franklin) • Late 18 th c. Galvani studied nerves – Electricity is created by differing metals and a salty liquid • 1800: Volta made first “Voltaic Pile” based on this chemistry of metals and salty liquids • 1800’s: many improvements in battery • Mid to late 1800’s: inventions in mechanical generation of electricity

Then came the animal dissection

A little history • Electricity was static, and a curiosity until 1800. – Parlor game shocks, animal muscle twitches • 1748: “battery” of charged glass plates (Franklin) • Late 18 th c. Galvani studied nerves – Electricity is created by differing metals and a salty liquid • 1800: Volta made first “Voltaic Pile” based on this chemistry of metals and salty liquids • 1800’s: many improvements in battery • Mid to late 1800’s: inventions in mechanical generation of electricity

18. 1 The Electric Battery, allowed constant source of charges Volta formalized that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte. This is a simple electric cell.

18. 1 The Electric Battery A battery transforms chemical energy into electrical energy. Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them. This potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved. Rechargable batteries use outside current to reassemble the terminals

18. 1 The Electric Battery Several cells connected together make a battery, although now we refer to a single cell as a battery as well. Carbonzinc cell (old design)

Alkaline cell (current design) The construction (as opposed to chemistry) of alkaline cells differs significantly from ordinary carbon -zinc cells. Alkaline cells are effectively turned inside-out. The shell of the alkaline battery is nothing more than that–a protective shell. The anode of the cell is a gelled mixture of powered zinc combined with the electrolyte (itself a mixture of potassium hydroxide–and water), and the combination is linked to the negative terminal of the cell by a brass spike running up the middle of the cell. The cathode, a mixture of carbon and manganese dioxide, surrounds the anode and electrolyte. http: //www. batterybatch. com/articles/the-inside-of-battery-aa. html

18. 2 Electric Current Electric current is the rate of flow of charge through a conductor: (18 -1) Unit of electric current: the ampere, A. 1 A = 1 C/s. Electrons flow: why? Explain using field and potential

18. 2 Electric Current A complete circuit, or circuit, is one where current can flow all the way around. What happens to the flow if the circuit is cut? So now we have a field in the conductor.

Buy a multimeter a. k. a. digital volt/ohm meter • Must have DC/AC volts m. V to 200 V, DC amps m. A-10 A, Ohms m to M range • Buy one at Home Depot, Menards, Radio Shack, hardware store, online • Pay $30 -$40 • We will start to use them next week, so have yours ready. • You will use and cherish this the rest of your life…. .

18. 2 Electric Current In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal. Only one of these circuits will work:

18. 2 Electric Current By convention, current is defined as flowing from + to -. Electrons actually flow in the opposite direction, but not all electrical currents consist of electrons. Darn that Ben Franklin! what?

13 Oct: 18. 3 Ohm’s Law: Resistance and Resistors Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends:

18. 3 Ohm’s Law: Resistance and Resistors The ratio of voltage to current is called the resistance: (18 -2 a) (18 -2 b) Analogy of water in a tube: in the water system, what is I, V, and R?

18. 3 Ohm’s Law: Resistance and Resistors In many conductors, the resistance is independent of the voltage; this relationship is called Ohm’s law. Materials that do not follow Ohm’s law are called nonohmic. Unit of resistance: the ohm, Ω. 1 Ω = 1 V/A.

18. 3 Ohm’s Law: Resistance and Resistors Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision. Q: Why did Mr. Ohm marry Mrs. Ohm? A: Because he couldn't resistor!!

18. 3 Ohm’s Law: Resistance and Resistors

18. 3 Ohm’s Law: Resistance and Resistors Some clarifications: • Batteries maintain a (nearly) constant potential difference; the current varies. • Resistance is a property of a material or device. • Current does have a direction or a +/- sign. • Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end. • But potential does get changed (up or down) at each circuit element • Assume wires offer no resistance unless stated.

18. 4 Resistivity The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area: (18 -3) The constant ρ, the resistivity, is characteristic of the material. Can the resistance for a given resistor change?

18. 4 Resistivity

18. 4 Resistivity For any given material, the resistivity increases with temperature: (18 -4) Semiconductors are complex materials, and may have resistivities that decrease with temperature. We will not study semiconductors now.

16 oct: Practice ohm’s law & resistance • If you touch 120 V, suppose you conduct 2. 0 A, and now you are dead. What is your R? • Lightbulb: 120 V, R = _____. What is I? • Nichrome wire, R=_____, battery, what is I? – What happens to R and I as it heats to red hot (1000 K)? • Your electric company says: "We would be delighted if you send in your bill. However, if you don't, you will be. “

18. 5 Electric Power, as in kinematics, is the energy transformed by a device per unit time: (18 -5)

Practice with V, I, R, and P • Use: analog V and I meters, high power resistors, AA batteries (of 1. 2 -1. 4 V), wires, VOM • Measure: V of battery, short circuit current. – Calculate R of wires, note that V of battery will drop • Measure: I through Resistor and see if V=IR is satisfied

Quiz chpt 17: 3 charges of 25 m. C each. Draw a potential elevation plot + - -

Electrical power is used for…. • • Heat Light Mechanical work Sound Magnetism Chemical conversions Life: Frankenstein Death: electrocution and accidents

Electrical “power circuits” vs. “logic circuits” • Power circuits: the output or function depends on energy being delivered to a device. • Logic circuits: the output or function depends on circuits that perform a calculation or sort data • Eg. Lightbulb, heater, calculator, watch, cell phone, television, pacemaker

US household electricity • • • Volts? Amps? Resistance? Power? Energy? A 10 A heater uses ____W A 100 W lightbulb uses ____A A “ 100 W” LED bulb uses ____W and _____A

Using the VOM/DVM/Multimeter • Measure potential: – plug V and common, dial V • Measure resistance: – Plug and common, dial – do not have potential on when measuring • Measure current: – Make sure there is a load to reduce the current – PLUG I and common, dial A first, then m. A – If no readings at all, probably blew the fuse

Practicum in lab, work in pairs and compare both VOMs • Clip resistor from a circuit • Predict on paper what is I using battery (this is to avoid your maximum current) • Given resistor and battery, use Ohm’s law to find R. • Compare to measured R using VOM • Compare to indicated R on resistor • Which is most accurate? • Measure potential: – • Measure resistance: – – • plug V and common, dial V Plug and common, dial do not have potential on when measuring Measure current: – – Make sure there is a load to reduce the current PLUG I and common, dial A first, then m. A How to set the VOM

How about your fridge? • What is the wattage? • What is the amps? • Why do we see these stickers on our new appliances?

18. 5 Electric Power The unit of power is the watt, W. For ohmic devices, we can make the substitutions: (18 -6 a) (18 -6 b)

18. 5 Electric Power What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. We have been measuring energy in joules, but the electric company measures it in kilowatthours, k. Wh. Power costs about 10 c/k. W*hr. How much will it cost for one k. W?

18 oct: 18. 6 Power in Household Circuits The wires used in homes to carry electricity have very low resistance. However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire. To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.

Calculatation with nichrome wire • 12. ? ? V robot battery • Ammeter (amp meter) • Nichrome wire • Measure current vs. length • Measure current at burnout • Look up thickness of wire from guage # • calculate P and R (for two lengths), P at burnout, and ρ for nichrome wire from demo • This calculation is due tomorrow. Hand it in.

Demo and calculation • • Use WATTs UP meter and light bulb V time W • • • A W hr What is power? What is energy? What do you pay the power company for?

18. 6 The fuse Fuses are one-use items – if they blow, the fuse is destroyed and must be replaced. Q: Why did the lights go out? A: Because they liked each other!

18. 6 The circuit breaker Circuit breakers, which are now the standard in homes, are switches that will open if the current is too high; they can then be reset.

22 oct: Solving power problems • A “power problem” is where power, current, potential (voltage), and resistance are all used • Find what is fixed: resistance of an element • Find what is consistent: current • Solve for unknowns as you can.

Power transmission problem for homework • Extension cord 15 m long with 1000 W (nominal) heater at the end. It has 0. 50 resistance. – How much current in wire? – What is V at end? – How much power lost in cord?

18. 7 Alternating Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC). Q. Why do fluorescent lights hum? A. Because they can't remember the words.

The Battle of Currents: Edison vs. Tesla • 1879: Edison patents electric light • 1880’s: Edison patents system for distribution of DC current, low voltage throughout – Generators had to be in neighborhood • Tesla had AC system with high voltage distribution and low voltage home use via transformers • By ~1900, AC won the battle • Do NOT watch this Epic Rap Battle

AC vs. DC • Why do we use DC for electronics? – What are electronics? • Why do we use AC for power electricity? – What is power electricity? • How did AC/DC get their name? • The 1978 release of Powerage marked the debut of bassist Cliff Williams

Watts and Watts of sound • A major rock band, such as AC/DC, will use 25, 000 W during a concert. • Assume this all goes into sound production from the stage speakers. • Assume you are standing 50 m away. • How much power is received by your ear drums? Eardrum diameter is 7. mm.

Solar power: DC to AC • • • http: //tenksolar. com/ http: //egauge 14394. egaug. es/index. html Why is solar power DC? How many joules produced today? Estimate power efficiency of panels – 96 panels @ 2. 6 m 2 each • http: //tenksolar. com/wpcontent/uploads/PV-System-10. 16. 14. pdf • Solar intensity goes down to ½x in Dec vs. July

18. 7 Alternating Current The voltage varies sinusoidally with time: as does the current: (18 -7)

18. 7 Alternating Current Multiplying the current and the voltage gives the power:

18. 7 Alternating Current Usually we are interested in the average power:

18. 7 Alternating Current The current and voltage both have average values of zero, so we square them, take the average, then take the square root, yielding the root mean square (rms) value. (18 -8 a) (18 -8 b)

VOM quiz (all answers 3 sig fig) • • • Measure R of resistor. Find V of battery Predict I through resistor Measure I through resistor Place paper, resistor, and battery on back table together

Safety rules for Physics at ISM • 120 V can kill NO handling any source of >20 V AC without my approval and me watching. NO handling any source of >50 V DC without my approval and me watching. This includes the Wimshurst and high voltage static source.

: Multimeter practicum • Measure your resistance – Hand to hand – Across a hand • • • V of a battery V of the wall outlet R of a resistor I of a lightbulb circuit I of a weak battery Calculate your current at 120 V. It would be higher!

30 Oct 2015 practicum 1. 2. 3. 4. 5. 6. Measure R of cold lightbulb (or nichrome) Measure V of battery Predict I based on above Measure I with lightbulb connected Measure V with lightbulb connected Calculate R of hot lightbulb (or nichrome) 7. Calculate P of hot lightbulb (or nichrome) 8. Calculate Rhot/Rcold

18. 8 Microscopic View of Electric Current Electrons in a conductor have large (106 m/s), random speeds just due to their temperature. They are very mobile, and thus they shake. When a potential difference is applied, the electrons also acquire an average drift velocity, which is generally considerably smaller than thermal velocity.

18. 8 Microscopic View of Electric Current This drift speed is related to the current in the wire, and also to the number of electrons per unit volume. (18 -10) Do ex. 18 -14. So if drift speed is so slow, why is effect so fast when I turn on a light?

Current is caused by fields • Classical view: • An e is floating and shaking around • The e nearby suddenly responds to its neighbor’s field and moves a bit • This causes a shift in field for the next electron • The shift in field moves at speed of light • The net flux of e’s depends on current I

• What did the lawyer who studied at Ohm’s Law School say when defending the physics teacher? – “Currently, my client has no potential to offer resistance to the negative charge, and will conduct himself positively. ”

18. 9 Superconductivity In general, resistivity decreases as temperature decreases. Some materials, however, have resistivity that falls abruptly to zero at a very low temperature, called the critical temperature, TC.

18. 9 Superconductivity Experiments have shown that currents, once started, can flow through these materials for years without decreasing even without a potential difference. Critical temperatures are low; for many years no material was found to be superconducting above 23 K. And magnets in presence of current behave very interestingly: http: //www. youtube. com/watch? feature=player_e mbedded&v=Ws 6 AAh. Tw 7 RA

26 oct: 18. 10 Electrical Conduction in the Human Nervous System The human nervous system depends on the flow of electric charge. The basic elements of the nervous system are cells called neurons. Neurons have a main cell body, small attachments called dendrites, and a long tail called the axon.

18. 10 Electrical Conduction in the Human Nervous System Signals are received by the dendrites, propagated along the axon, and transmitted through a connection called a synapse.

18. 10 Electrical Conduction in the Human Nervous System This process depends on there being a dipole layer of charge on the cell membrane, and different concentrations of ions inside and outside the cell.

18. 10 Electrical Conduction in the Human Nervous System This applies to most cells in the body. Neurons can respond to a stimulus and conduct an electrical signal. This signal is in the form of an action potential.

Summary of Chapter 18 • A battery is a source of constant potential difference. • Electric current is the rate of flow of electric charge. • Conventional current is in the direction that positive charge would flow. • Resistance is the ratio of voltage to current: Does Ohm’s Law apply to all materials?

Summary of Chapter 18 • Ohmic materials have constant resistance, independent of voltage. • Resistance is determined by shape and material: • ρ is the resistivity.

Summary of Chapter 18 • Power in an electric circuit: • Direct current is constant • Alternating current varies sinusoidally

Summary of Chapter 18 • The average (rms) current and voltage: • Relation between drift speed and current:

Another look at power source Vs Transmission wire Rw user Ru The circuit actually looks like this, and we’ll learn to solve these next chapter

Advice on power problems • At each step, clearly identify what segment/component you are solving for • Label each segment (generator, line, factory) • Use subscripts for each (PF, VL, etc) • Recall that usually I is constant across all, so don’t need subscript • Keep each segment clear • Look at quiz for example

: Analyze my phone battery? • 1960 m. Ah, 3. 7 V • How many Coulombs? • If I can get 6 hrs talk time, then I = ? • What Power from the battery? • Most cell phones broadcast a signal at 0. 6 – 3 W. Does this make sense?

In class analysis • Use loggerpro current meter and potential probes • Find R of a lightbulb • V = IR therefore R = V/I

Lab for Ohm’s Law • See assignment sheet in Files • Show to use the Vernier probes in class • Prelab is due Thursday, so bring questions to me on Tuesday

Lab on ohms law V Voltmeter to loggerpro R A Ammeter to loggerpro V powersupply

Quiz 27 oct: harder • A very long extension cord is used for Halloween lights in your backyard. It is plugged into the house (120 V) and the lights are turned on at the other end, and with your VOM, you measure 107 V at the far end, and you measure 3. 3 A. – How much power are the lights drawing? – How much power is the extension cord drawing? – What is the resistance of the extension cord?

The classic problems chapter 18 • • • Simple ohm’s law resistor, potential Power generator-house problem Energy and charge in a battery Using the VOM Using plot of V vs. I to get R, as in lab

Quiz 22 oct R = 30. 0 k. W switch V = 25 m. V 1. The switch is open: A. What is current through the resistor? B. What is resistance of resistor? 2. The switch is closed: A. What is current through the battery? B. What is power dissipated by the resistor? C. After 20. minutes, how much energy in Joules is drained from the battery?

• • • • Some useful equations: F = k Q 1 Q 2/r 2 Equations on E = F/q the exam V = W/q V = k. Q/r 6 short answer DPE = q. V 10 MX Q = C V 4 problems C = ke 0 A/D U = ½ Q 2/C = ½ QV R = r L/A r. T = ro[1 + a (T-To)] e 0 = 1/(4 pk) = 8. 85 x 10 -12 C 2/N. m 2 k = 1/(4 pe 0) = 9. 0 x 109 N. m 2/ C 2 r for copper = 1. 68 x 10 -8 W m P = IV

Review for exams • • Main concepts Classic problems Questions? Weak points – Definition of electric current, use it! – Graphs can help us analyze V=IR, r = r(T) – Definition of P = energy / time – Power transmission problem: separate the 3 elements (supply, transmission, user)

Power problem • This extension cord has 0. 4 resistance. • I have a vacuum pump that draws 6. 0 amps at 120 V • What is resistance of the motor? • What is power of motor? • I connect the pump to the extension cord, and want to know the new power the motor will deliver.

Quiz for 28 Oct Edison is installing a local powerplant in NY City. He wants 50. V and 1500 W delivered to a house, and he knows the wires to the house have R = 0. 90 . a. What voltage should he produce at the generator side of the wire? b. What power will be lost over the wire?

Class dynamic, path forward For reference: What do I talk about in my letters of recommendation? 1. Attitude…do you show you care? Do you show that you enjoy the process? Are you positive and optimistic? 2. Responsibility… do you deliver when you say you will? 3. Willingness to try …. Do you willingly jump in even if you don’t know how to do it? Do you say yes to challenges? 4. Creativity …. Do you think beyond regurgitation of what you’ve been told? 5. Leadership…. Are you willing to be the first to answer, to form a group? Do you take some ownership of the group? 6. Achievement . . do you produce good results

What specifically? • You should ask, “Dr. Fisher, what specifically should we be doing that we don’t do now? ” • ……… 1. Ask more questions, any questions! 2. Offer to answer questions in class 3. Disagree with a point!