Topic 12: Electromagnetic induction. Topic 12: Electromagnetic induction. Topic 12: Electromagnetic induction. Describe the inducing of an emf by relative motion between a conductor and a magnetic field. Electromagnetic Induction. N. The direction of the induced current is reversed if…

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Topic 12: Electromagnetic induction

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

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

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)

Derivation of Emf 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 = Blvwithout using Faraday’s law. By conservation of energy

Magnetic Flux Magnetic flux = magnetic flux density x area Φ = BA (in Weber, Wb) in T in m2 The induced EMF for a magnet in a coil is directly proportional to the rate of change of flux linkage: EMF = NΔΦ ΔT Φ Faraday (1791-1867) 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: For a coil of N turns the total magnetic flux is NΦ Faraday’s law:

Magnetic flux Define magnetic flux and magnetic flux linkage.

Induced e.m.f Faraday (1791-1867) Describe the production of an induced emf by a time-changing magnetic flux. Φ the magnetic flux through one coil NΦ is known as the magnetic flux linkage

Cutting Magnetic Fields Induced emf = -N ΔΦ Δt Induced emf = -NB A t Consider a conductor of length l moving at speed v through a magnetic field at 900: From Faraday’s Law: Not on syllabus But Φ=BA and B is constant, so: This is a single wire, so N=1, and A = lvt, therefore: Induced emf = -Blv

Lenz’s Law S N Lenz (1804-1865) Consider a magnet in a solenoid: The current induced by the magnet induces a north pole that repels the magnet again. Lenz’s Law: Any current driven by an induced emf opposes the change that caused it. In other words, emf = -NΦ/t

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.

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

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

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 5×10-3s after it has passed through its maximum value.c)Calculate the magnitude of the induced emf at a time 2.5×10-3s after it has passed through its maximum value.

Revision of DC and AC V DC stands for “Direct Current” – the current only flows in one direction: Discuss what is meant by the root mean squared (rms) value of an alternating current or voltage. Time 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. 1/50th s 240V AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) T V

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.