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Circuits and electromagnetic induction
7 MDCAT Physics questions with answers and explanations.
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A cell of emf 12 V and internal resistance 1 ohm is connected to a 5 ohm resistor. What current flows?
- 1 2 A
- 2 2.4 A
- 3 12 A
- 4 6 A
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A Wheatstone bridge is balanced when:
- 1 P x Q = R x S
- 2 P / Q = R / S
- 3 P + Q = R + S
- 4 P = S
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Correct answer: P / Q = R / S
At balance, the ratio of resistances in one arm equals the ratio in the other, and no current flows through the galvanometer.
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Which law gives the direction of an induced current?
- 1 Ohm's law
- 2 Lenz's law
- 3 Coulomb's law
- 4 Ampere's law
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Correct answer: Lenz's law
Lenz's law says the induced current opposes the change in flux that produces it.
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A transformer has 100 turns in the primary and 500 turns in the secondary. If the input is 20 V, what is the output voltage?
- 1 4 V
- 2 20 V
- 3 100 V
- 4 500 V
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Correct answer: 100 V
Vs / Vp = Ns / Np, so Vs = 20 x 500 / 100 = 100 V.
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Eddy currents are:
- 1 Currents in a vacuum
- 2 Loops of current induced in a conductor by a changing magnetic flux
- 3 Currents in a battery
- 4 Currents that flow only in insulators
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Correct answer: Loops of current induced in a conductor by a changing magnetic flux
A changing magnetic flux induces circulating currents inside a solid conductor. These cause heating.
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The SI unit of resistivity is the:
- 1 Ohm
- 2 Ohm metre
- 3 Siemens
- 4 Ohm per metre
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Correct answer: Ohm metre
Resistivity = R x A / L, whose unit is ohm metre.
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How much power is lost as heat when a current of 2 A flows through a wire of resistance 5 ohm?
- 1 10 W
- 2 20 W
- 3 2.5 W
- 4 40 W