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Electricity and magnetism
7 ECAT Physics questions with answers and explanations.
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Two 4 microfarad capacitors are connected in series. What is the equivalent capacitance?
- 1 8 microfarad
- 2 4 microfarad
- 3 2 microfarad
- 4 16 microfarad
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Correct answer: 2 microfarad
In series, 1/C = 1/4 + 1/4 = 1/2, so C = 2 microfarad.
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How much energy is stored in a 2 microfarad capacitor charged to 10 V?
- 1 100 microjoule
- 2 20 microjoule
- 3 200 microjoule
- 4 1 millijoule
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Correct answer: 100 microjoule
Energy = 1/2 C V^2 = 0.5 x 2 x 10^-6 x 100 = 100 microjoule.
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If the distance between two point charges is doubled, the force between them becomes:
- 1 Half
- 2 Double
- 3 One quarter
- 4 Four times
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Correct answer: One quarter
Coulomb's law follows an inverse square law, so doubling the distance divides the force by 4.
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A wire of length 0.4 m carries 2 A at right angles to a magnetic field of 0.5 T. What force acts on it?
- 1 0.4 N
- 2 0.2 N
- 3 4 N
- 4 1.6 N
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At resonance in a series RLC circuit, the impedance is:
- 1 Maximum
- 2 Minimum and equal to R
- 3 Zero
- 4 Equal to the reactance of the inductor
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Correct answer: Minimum and equal to R
At resonance the inductive and capacitive reactances cancel, leaving only the resistance, so the current is largest.
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What is the time constant of an RC circuit with R = 1 kilohm and C = 1 microfarad?
- 1 1 s
- 2 1 microsecond
- 3 1 ms
- 4 1000 s
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Correct answer: 1 ms
Time constant = RC = 1000 x 10^-6 = 10^-3 s, which is 1 ms.
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The electric field of a point charge varies with distance r as:
- 1 1 / r
- 2 1 / r^2
- 3 r
- 4 r^2
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Correct answer: 1 / r^2
E = kq / r^2, so it falls with the square of the distance.