Electromagnetic Induction - General Physics - Solved Exam, Exams of Physics

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2008 Question 8 (i) What is electromagnetic induction? Electromagnetic Induction occurs when an emf is induced in a coil due to a changing magnetic flux. (ii) State the laws of electromagnetic induction. Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux. Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it. (iii)Explain why the amplitude of the swings decreases rapidly. An emf is induced in the copper because is its experiencing a changing magnetic field. This produces a current. This current has a magnetic field associated with it which opposes the motion of the magnet. (iv) What is the main energy conversion that takes place as the magnet slows down? Kinetic (or potential) to electrical (or heat). (v) time = dist/velocity = 5 cm / 500 cm s-1^ = 0.01 s (vi) What is the magnetic flux cutting the loop when it is completely in the magnetic field? Φ = BA = (8)(.05 × .05) = 0.02 webers. (vii) What is the average emf induced in the loop as it enters the magnetic field? Induced emf = (Final Flux –Initial Flux) / Time Taken = (0 - 0.02)/0. = 2 Volts

2007 Question 12 (c) (i) State Faraday’s law of electromagnetic induction. Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux. (ii) Describe an experiment to demonstrate Faraday’s law. Move the magnet in and out of the coil slowly and note a slight deflection. Move the magnet quickly and note a greater deflection. (iii)What is the effect on the current flowing in the circuit? Current is reduced (iv) Justify your answer An emf induced in coil which induces a current which opposes the initial current.

2006 Question 11 (a) How does resonance occur in an acoustic guitar? Energy is transferred from the strings to the hollow body and both vibrate at the same frequency. (b) What is the relationship between frequency and tension for a stretched string? Frequency is proportional to the square root of tension. (c) A stretched string of length 80 cm has a fundamental frequency of vibration of 400 Hz. What is the speed of the sound wave in the stretched string? v = f λ  v = 400(1.6) = 640 m s- (d) Why must the strings in the electric guitar be made of steel? Because only metal strings can be magnitised. (e) Define magnetic flux. Magnetic flux is the product of magnetic flux density multiplied by area. (f) Why does the current produced in a coil of the electric guitar vary? Because the induced emf varies due to the amplitude of the vibrating string. (g) What is the effect on the sound produced when the number of turns in a coil is increased? A louder sound is produced. (h) A coil has 5000 turns. What is the emf induced in the coil when the magnetic flux cutting the coil changes by 8 × 10–4^ Wb in 0.1 s? E= −N(dφ / dt) E = 5000(8 × 10-4^ /0.1) = 40 V

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2005 Question 12 (b) (i) Define magnetic flux. Magnetic flux is defined as the product of magnetic flux density multiplied by area. (ii) State Faraday’s law of electromagnetic induction. The size of the induced emf is proportional to the rate of change of flux. (iii)What is the magnetic flux cutting the coil? A = (0.05)^2 = 0. φ ( = BA ) = (4)(0.0025) φ = 0.01 Wb (iv) Calculate the magnitude of the average e.m.f. induced in the coil while it is being rotated. E = N( Δφ / Δ t) Δφ /Δ t = (0.01 – 0 )/0.2 = 0. E = 200(0.05)  E = 10 V

2004 Question 12 (c) (i) What is electromagnetic induction? Electromagnetic Induction occurs when an emf is induced in a coil due to a changing magnetic flux. (ii) Describe an experiment to demonstrate electromagnetic induction. Set up as shown. Move the magnet in and out of the coil and note the deflection in the galvanometer. (iii)Explain why. Current flows in the ring in such a direction as to oppose the change which caused it. Therefore the ring follows the magnet. (iv) What would happen if the magnet were moved towards the ring? The ring would be repelled.

2003 Question 12 (d) (i) State the laws of electromagnetic induction. Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux. Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it. (ii) Describe the current flowing in the circuit. Alternating current. (iii)If the switch at A is open, the magnet will take longer to come to rest. Explain why. There is no longer a full circuit, so even though there is an induced potential difference there is no (induced) current, therefore no induced magnetic field in the coil therefore no opposing force.

2002 Question 12 (c) (i) What is meant by electromagnetic induction? Electromagnetic Induction occurs when an emf is induced in a coil due to a changing magnetic flux. (ii) State Lenz’s law of electromagnetic induction. Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it. (iii)Explain why the current was reduced when an iron core was inserted in the coil. There would normally be a back emf in the coil due to the alternating current being supplied. When the core was inserted it increased the magnetic flux which in turn increased the self-induction (back emf) and this reduced the overall voltage and therefore the overall current. (iv) Give an application of the principle shown by this experiment. Dimmer switch, smooth d.c., tuning radios, braking trains, damping in balances, induction coil