Electromagnetic induction In this section you will learn

Electromagnetic induction In this section you will learn: The concept of electromagnetic induction What effects the size and direction of the induced current/emf Experiment to demonstrate the electromagnetic induction. Concept of magnetic flux and unit Understand use Faraday's laws of electromagnetic induction. Understand use Lenz's law How electricity is generated using generators. Experiment to demonstrate Lenz's law Understand a. c. current and voltage. Calculate r. m. s I and V values for a. c. Understand mutual and self induction Effect of inductors ón a. c. Uses of inductors. a. c. And capacitance Transformers, step up and step down and uses.

Electromagnetic induction We have seen that if you have a current flowing in a magnetic field, a force will act on it to try to move it. What might happen if you move a wire in a magnetic field? A current is induced in a conductor which moves inside a magnetic field. © TPS 2010

Electromagnetic induction If you rotate the magnet in the coil or move it in or out of the coil or move the coil around the magnet an Emf is induced. Rem: show phet simulation

Electromagnetic induction As long as there is a changing magnetic field an emf is induced. Whenever the magnetic field passing through a coil changes and emf is induced. This is called electromagnetic induction Show using datastudio with a coil, voltage and current sensor. The size of the emf depends on: The strength of the magnetic field The number of loops (turns) in the coil The speed of the coil or magnet. The direction of the emf depends ón: If the magnet is approaching the coil or moving away from it. If a north or south pole goes in first.

Electromagnetic induction Magnetic Flux This a measure of the strength of the magnetic field. The greater the magnetic flux the greater the induced emf. It depends on: The area of the magnetic field at right angles to the wire/coil. The magnetic flux density B (strength of the magnetic field/unit area Magnetic flux = magnetic flux density x area = BA (usually measured in Wb) 1 Wb = 1 T m 2

Electromagnetic induction Faraday’s law of electromagnetic induction The emf (E) induced in a conductor, due to a changing magnetic field, is proportional to the rate of change of flux E = (d /dt). Rotating Magnet Doubling the speed of rotation will double the emf i. e. Faraday’s law is shown to be true. For leaving certificate physics E = change in or E = final flux – initial flux Time time © TPS 2010

Electromagnetic induction Clearly if you have a coil of N turns, = BAN In fact for a coil of N turns (loops) Change in flux E= N Time taken You must be able to discribe and experiment to demonstrate Faraday's law of electromagnetic induction

Electromagnetic induction Lenz’s law The induced emf, E, is created such as to oppose the motion that created it. E = - dϕ dt Induced emf Opposes the motion that created it Rate of cutting of flux

Electromagnetic induction You must be able to discribe and experiment to demonstrate Lenz's law (show using Al ring, copper wire, strong magnet) Notice the S pole in the coil to oppose the motion of the magnet towards it. (How? ) Notice the N pole of the coil to oppose the motion of the magnetic away from the coil. (How? )

Alternating Current Mains electricity is high voltage and its direction changes 100 times every second (alternating current). This makes its behaviour very different to the electricity that we get from a cell or battery. irect his is d c). T ent (d curr Voltage Time is is Th ating ust altern (ac) j rent cur ains! the m like © TPS 2010

Alternating Current T Voltage Time The arrow marks the time taken for one complete oscillation. This is the time period. In Ireland Europe it is 0. 02 s (1/50 th of a second). The frequency is the number of complete cycles per second. That is 50 in Ireland. We say the mains frequency is 50 Hz. © TPS 2010

Alternating Current The European Commission has mandated that all EU countries harmonise their mains voltages. The regulations state that the voltage should be in the range of 230 V plus 8% and minus 10% I. e. between 207. 0 V and 243. 8 V. © TPS 2010

Alternating Current This value, 230 V, is called the RMS value – the root mean square value. It is the DC equivalent that would cause the same heating effect. We can obtain the peak voltage by multiplying the RMS voltage by √ 2. Peak voltage Vp Time © TPS 2010

Alternating Current R. M. S values of a. c. When we give a value of say 5 A to an a. c current we mean it has the same heating effect as 5 A d. c. It has a maximum value of greater 5 A in each direction. This 5 A value is called the rms value. (Note we take rms (root meán square) values as way averaging the a. c I and V as an ordinary average would give zero. ) The following are the rms equations for I, V and P Irms = I 0 / 2 or Io = Irms x 2 Vrms = V 0 / 2 or Vo = Vrms x 2 2 P = Irms X Vrms or P = Irms R I 0 and V 0 are the max values of I and V respectively

Alternating Current Getting the electricity to your home is not an easy business. A great deal of energy could be lost in the cables that go from the power station to the local distribution point. We know that V = IR and that P=IV so we can see that P = I 2 R - this is an important result. Suppose a long wire has a resistance of 2 and we pass a current of 10 A through it. P = I 2 R = 102 x 2 = 200 W that means that 200 W of power are wasted in the wire What if we put 10 times the current through it: 100 A? P = I 2 R = 1002 x 2 = 20000 W that means that 20 k. W of power are wasted in the wire 10 times the current and 100 times the wasted power. For this very reason electricity companies must transmit their high powers © TPS 2010 at low currents.

Alternating Current Fortunately it is possible to do this by first stepping up the voltage (this lowers the current) and then transmitting it. It can finally be stepped down when it is close to the place that it will be used in. High voltage, low current, low power loss Power Station Step up transformer Low voltage, high current, less dangerous for consumer Step down transformer Consumer © TPS 2010

Induction Consider two coils which are placed close to each other. If an alternating current is passed through one coil, it will produce an alternating magnetic field around that coil. © TPS 2010

Induction Notice how the changing magnetic field due to one coil, cuts the other coil. It is just as if somebody is moving a magnet next to the coil on the right. © TPS 2010

Induction The result is that an alternating emf is induced in the coil on the right. Alternating emf It is just as if somebody is moving a magnet next to the coil on the right. © TPS 2010

Induction If a changing magnetic field in one coil induces an emf in a nearby coil this is called mutual induction. Alternating emf The voltages across the two coils vary in direct proportion to the ratio of the number turns (N) on the coils. I. e. © TPS 2010

Induction The size of the induced emf is increased by Having the coils closer together Winding the coils on the same soft iron core. Increasing the number of turns of both or either of the coils. Demostrate mutual inductance using two coils.

Induction Self-inductance. When an a. c current passes through a coil it induces an emf due to the changing magnetic field of the current. This induced emf oposes the varying emf that causes it. (called back emf) The size of the back emf can be increased by winding the coil on a soft iron core.

Induction A. C. and inductors A coil has a greater resistance to a. c than d. c. When a d. c flows through a coil its resistance is simply its ohmic resistance using V = I R When a. c flows through a coil because of the constantly changing magnetic field the resistance is the ohmic resistance plus the resistance due to the back emf which opposes the current causing it. The greater the self inductance the more resistance a coil offers to a. c.

Induction Uses of Inductors. Smooth out variations in d. c power supply units. Tuning circuits for radios. Dimmer switches used in stage lighting. Capacitors and a. c Capacitors do not allow d. c to flow. (once charged) Capacitors do allow a. c to flow This is because the capacitor charges and discharges as the current changes direction

Induction

Induction If a changing magnetic field in one coil induces an emf in a nearby coil this is called mutual induction. Alternating emf The voltages across the two coils vary in direct proportion to the ratio of the number turns (N) on the coils. I. e. © TPS 2010

Induction Transformers This is the principle of a transformer – a device for changing voltages and currents. If the voltages increase, the currents decrease and vice versa. If the voltages increase, it is called a step-up transformer. If the voltages decrease, it is called a step-down transformer. If it was 100% efficient, then P 1, the power input in the first coil (the primary) would be equal to P 2, the power output in the second coil. Since P=IV we can see that Ip. Vi = Is. V 0 and © TPS 2010

Induction However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings. The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core. Primar y Seconda ry The input The output © TPS 2010

Induction However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings. The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core. Primar y Seconda ry The input The continuous nature of the core makes sure that as much of the field from the primary, passes through the secondary. The output © TPS 2010

Induction However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings. The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core. Soft iron is used as it has no residual magnetism. Primar y Seconda ry The input Work is not wasted by overcoming residual magnetism in the soft iron. The output © TPS 2010

Induction However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings. The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core. The laminations of the core mean that it is made of thin sheets of soft iron, insulated from each other. Primar y Seconda ry The input This stops eddy currents flowing in the core which would be a waste of energy, producing unwanted heat in the core. The output © TPS 2010

Induction Uses of Transformers Electricity in power stations is generated a high voltages 20 -30 k. Vand then stepped up to 220 - 400 k. V to reduce loses in transmission of the power in the grid. This has to be reduced down to 220 V for the domestic use. Computers, televisions and all manor of electronic devices need transformers to supply the necessary voltage for the device. © TPS 2010