Topic 12: Electromagnetic induction

16/07/ Topic 12: Electromagnetic induction Describe the inducing of an emf by relative motion between a conductor and a magnetic field.

16/07/2015 4

5

6 Electromagnetic Induction N The direction of the induced current is reversed if… 1)The wire is moved in the opposite direction 2)The field is reversed The size of the induced current can be increased by: 1)Increasing the speed of movement 2)Increasing the magnet strength

16/07/ Electromagnetic induction The direction of the induced current is reversed if… 1)The magnet is moved in the opposite direction 2)The other pole is inserted first The size of the induced current can be increased by: 1)Increasing the speed of movement 2)Increasing the magnet strength 3)Increasing the number of turns on the coil

16/07/ The north pole of a permanent bar magnet is pushed along the axis of a coil as shown below. The pointer of the sensitive voltmeter connected to the coil moves to the right and gives a maximum reading of 8 units. The experiment is repeated but on this occasion, the south pole of the magnet enters the coil at twice the previous speed. Which of the following gives the maximum deflection of the pointer of the voltmeter? A.8 units to the right B.8 units to the left C.16 units to the right D.16 units to the left(1)

Faraday’s Law 16/07/2015 9

10 Derivation of Emf By conservation of energy Derive the formula for the emf induced in a straight conductor moving in a magnetic field. Students should be able to derive the expression induced emf = Blv without using Faraday’s law.

16/07/ Induced E.m.f.

Magnetic flux 16/07/ Define magnetic flux and magnetic flux linkage.

Magnetic flux 16/07/

16/07/

16/07/ Magnetic Flux As we said, magnetic field strength is also called magnetic flux density. The “magnetic flux” is the amount of flux that passes through a given area: Magnetic flux = magnetic flux density x area Φ = BA (in Weber, Wb) in T in m 2 For a coil of N turns the total magnetic flux is NΦ Φ Faraday ( ) Faraday’s law: The induced EMF for a magnet in a coil is directly proportional to the rate of change of flux linkage: EMF = NΔΦ ΔT

16/07/ Magnetic flux Define magnetic flux and magnetic flux linkage.

16/07/

16/07/ Induced e.m.f NΦ is known as the magnetic flux linkage Φ the magnetic flux through one coil Faraday ( ) Describe the production of an induced emf by a time-changing magnetic flux.

16/07/ Cutting Magnetic Fields Consider a conductor of length l moving at speed v through a magnetic field at 90 0 : From Faraday’s Law: Not on syllabus Induced emf = -N ΔΦ Δ t But Φ =BA and B is constant, so: Induced emf = -NB A t This is a single wire, so N=1, and A = lvt, therefore: Induced emf = -Blv

Calculating the EMF 16/07/

16/07/

16/07/

Lenz’s Law 16/07/

16/07/

16/07/

16/07/ Hyperlink

16/07/ Lenz’s Law Consider a magnet in a solenoid: N S The current induced by the magnet induces a north pole that repels the magnet again. Lenz’s Law: Lenz ( ) Any current driven by an induced emf opposes the change that caused it. In other words, emf = -NΦ/t

16/07/ Lenz’s Law Click to play

16/07/ Right hand rule

16/07/ Lenz’s law question Explain why it is harder to turn a bicycle dynamo when it is connected to a light bulb than when it is not connected to anything.

16/07/ When a simple d.c. electric motor is connected to a battery, the current which flows during the first few seconds varies (approximately) as shown by the graph below. Explain the shape of this graph.

16/07/ Questions Hamper page 211 Q’s 39,40,41 IB review pack Q’s 1,2,4,5

12.2 Alternating current 16/07/

16/07/

16/07/ Generator Hyperlink Describe the emf induced in a coil rotating within a uniform magnetic field. Explain the operation of a basic alternating current (ac) generator. Students should understand, without any derivation, that the induced emf is sinusoidal if the rotation is at constant speed.

16/07/ Effect of changing the speed of rotation of the coil on the induced emf Slow rotationFaster rotation Describe the effect on the induced emf of changing the generator frequency. Students will be expected to compare the output from generators operating at different frequencies by sketching appropriate graphs.

16/07/ Generator question A rectangular coil, 2cm by 3cm, rotates in a uniform magnetic field of flux density 0.15T. The axis around which the coil rotates is at 90° to the flux lines. The coil has 250 turns and its rotational frequency is 50s - 1.a)Calculate the maximum emf induced in the coil. What is the position of the coil (relative to the flux lines) when the induced emf has its maximum value?b)What is the magnitude of the induced emf at a time 5×10 -3 s after it has passed through its maximum value.c)Calculate the magnitude of the induced emf at a time 2.5×10 -3 s after it has passed through its maximum value.

16/07/ Revision of DC and AC DC stands for “Direct Current” – the current only flows in one direction: AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) 1/50 th s 240V V V Time T Discuss what is meant by the root mean squared (rms) value of an alternating current or voltage. Students should know that the rms value of an alternating current (or voltage) is that value of the direct current (or voltage) that dissipates power in a resistor at the same rate. The rms value is also known as the rating.

16/07/

16/07/ Rms and peak values What value do you use for an a.c. current? Discuss what is meant by the root mean squared (rms) value of an alternating current or voltage.

16/07/

16/07/

16/07/

16/07/ Rms calculations This question is about rms currents and voltages. For the circuit shown above calculate athe rms value of the current, I b)the maximum value of the current c)the mean power dissipated in R 1 d)the rms value of the voltage across R 2.

16/07/ Coupled Inductors Describe the production of an induced emf by a time- changing magnetic flux.

16/07/

The national grid Explain the use of high-voltage step-up and step-down transformers in the transmission of electrical power. 16/07/

16/07/

16/07/

16/07/

16/07/ Transformers Time Current through primary Magnetic flux through core EMF induced in secondary

16/07/ Hyperlink

16/07/ Hyperlink

16/07/ Transformers Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil. Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage: Voltage across primary (V p ) No. of turns on secondary (N s )Voltage across secondary (V s ) No. of turns on primary (N p )

16/07/ Describe the operation of an ideal transformer. Solve problems on the operation of ideal transformers.

16/07/

16/07/ Transformer questions a)Explain the operation of a step-up transformer. b)A transformer has 250 turns on its primary coil and 4000 turns on its secondary coil. It is connected to a 220V supply. Under normal operating conditions the current flowing through the secondary coil is 25mA and the transformer is 90% efficient (assume that the main source of inefficiency is the resistance of the secondary coil). Calculate i)the secondary voltage when on open circuit (that is, when the secondary coil is not connected to anything). ii)the current flowing through the primary coil when the current flowing through the secondary coil is 25mA.

16/07/

16/07/ Some transformer questions Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 12V24V100?? 400V200V20?? 25,000V50,000V1,000?? 23V230V150??

16/07/

16/07/ Energy losses in a transformer Energy can be lost as: (a) heat in the coils because of the resistance of the wire; (b) incomplete transfer of magnetic field; (c) heating of the core due to induced currents in it. This is reduced by making the core out of insulated soft iron in laminated strips. If this were not done the cores of large transformers would get so hot that they would melt. Outline the reasons for power losses in transmission lines and real transformers.

Transmission of electricity 16/07/

16/07/

16/07/

16/07/

16/07/ The transmission of electricity To minimise losses through heating, then the current should be as low as possible i.e. make the voltage as high as possible.

16/07/ Example of power loss Explain the use of high-voltage stepup and step-down transformers in the transmission of electrical power. Students should be aware that, for economic reasons, there is no ideal value of voltage for electrical transmission.

Questions Hamper page 218 Q’s 46,47. IB pack Q’s 7,8,9. 16/07/

16/07/ Discuss some of the possible risks involved in living and working near high-voltage power lines. Students should be aware that current experimental evidence suggests that low ‑ frequency fields do not harm genetic material. Students should appreciate that the risks attached to the inducing of current in the body are not fully understood. These risks are likely to be dependent on current (density), frequency and length of exposure.

16/07/ Health risks Power cables carry a.c. currents These produce e-m fields These can induce currents in the body These might do harm Risk increases with Current Frequency Time of exposure. Suggest how extra-low- frequency electromagnetic fields, such as those created by electrical appliances and power lines, induce currents within a human body.