20.5. Electromagnetic induction
A subsection of Physics, 9702, through 20. Magnetic fields
Listing 10 of 56 questions
Use A magnet is suspended vertically from a fixed point by means of a spring, as shown in . spring coil magnet R One end of the magnet hangs inside a coil of wire. The coil is connected in series with a resistor R. The magnet is displaced vertically a small distance D and then released. shows the variation with time t of the vertical displacement d of the magnet from its equilibrium position. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 +D –D d t /s Examiner’s Use State and explain, by reference to electromagnetic induction, the nature of the oscillations of the magnet. Calculate the angular frequency ω0 of the oscillations. ω0 = …………….……………… rad s–1 The resistance of the resistor R is increased. The magnet is again displaced a vertical distance D and released. On , sketch the variation with time t of the displacement d of the magnet. Examiner’s Use The resistor R in is replaced by a variable-frequency signal generator of constant r.m.s. output voltage. The angular frequency ω of the generator is gradually increased from about 0.7ω0 to about 1.3ω0, where ω0 is the angular frequency calculated in . On the axes of , sketch a graph to show the variation with ω of the amplitude A of the oscillations of the magnet. A 0.7 0 1.0 0 1.3 0 ω ω ω ω State the name of the phenomenon illustrated in the graph of . Briefly describe one situation where the phenomenon named in is useful and one situation where it should be avoided. useful: avoid:
9702_s07_qp_4
THEORY
2007
Paper 4, Variant 0
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A transformer is illustrated in . load secondary coil primary coil laminated iron core Explain why the coils are wound on a core made of iron. Suggest why thermal energy is generated in the core. State Faraday’s law of electromagnetic induction. Use Faraday’s law to explain why the potential difference across the load and the e.m.f. of the supply are not in phase. Electrical energy is usually transmitted using alternating current. Suggest why the transmission is achieved using high voltages, alternating current.
9702_s11_qp_41
THEORY
2011
Paper 4, Variant 1
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Two coils P and Q are placed close to one another, as shown in . power supply coil P coil Q V The current in coil P is constant. An iron rod is inserted into coil P. Explain why, during the time that the rod is moving, there is a reading on the voltmeter connected to coil Q. The current in coil P is now varied as shown in . current W2 WLPH W1 On , show the variation with time of the reading of the voltmeter connected to coil Q for time t = 0 to time t = t2. voltmeter reading time W1 W2
9702_s16_qp_42
THEORY
2016
Paper 4, Variant 2
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An ideal transformer is illustrated in . secondary coil soft-iron core primary coil output input State Faraday’s law of electromagnetic induction. Use the law to explain why a transformer will not operate using a direct current input. State Lenz’s law. Use Lenz’s law to explain why the input potential difference and the output e.m.f. are not in phase. Electrical energy is usually transmitted using alternating high voltages. Suggest one advantage, for the transmission of electrical energy, of using alternating voltage, high voltage.
9702_w09_qp_42
THEORY
2009
Paper 4, Variant 2
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An incomplete diagram for the magnetic flux pattern due to a current-carrying solenoid is illustrated in . direction of current On , draw arrows on the field lines to show the direction of the magnetic field. State the feature of that indicates that the magnetic field strength at each end of the solenoid is less than that at the centre. A Hall probe is placed near one end of the solenoid in , as shown in . Y Hall probe to circuit for Hall probe X The Hall probe is rotated about the axis XY. State and explain why the magnitude of the Hall voltage varies. State Faraday’s law of electromagnetic induction. The Hall probe in is replaced by a small coil of wire connected to a sensitive voltmeter. State three different ways in which an e.m.f. may be induced in the coil. 1. 2. 3.
9702_w13_qp_41
THEORY
2013
Paper 4, Variant 1
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Questions Discovered
56