Free AC Through Resistor, Capacitor and Inductor MCQs with Answers

1. In a purely resistive AC circuit, the current and the voltage are

• A. in phase with each other
• B. 90 degrees out of phase, with current leading
• C. 90 degrees out of phase, with current lagging
• D. 180 degrees out of phase

Explanation: A resistor obeys Ohm's law at every instant, so the current peaks exactly when the voltage peaks and both pass through zero together. Phase differences appear only when capacitance or inductance is present. Because they are in phase, all the power in a resistive circuit is dissipated as heat.

2. In a purely capacitive AC circuit, the current

• A. lags the voltage by 90 degrees
• B. leads the voltage by 90 degrees
• C. is in phase with the voltage
• D. is zero

Explanation: A capacitor must charge before a voltage can appear across it, so the current is greatest when the voltage is momentarily zero and falls to zero when the voltage is at its peak, putting the current a quarter cycle ahead. The mnemonic ICE captures this: in a capacitor, I comes before E. An inductor does the reverse, with the current lagging.

3. The capacitive reactance of a capacitor

• A. increases as the frequency increases
• B. decreases as the frequency increases
• C. is independent of frequency
• D. is the same as resistance

Explanation: Reactance is one over 2 pi f C, so a capacitor passes high frequencies readily and blocks direct current entirely, since at zero frequency the reactance is infinite. This is why capacitors are used to couple audio signals while blocking any DC bias. An inductor behaves in the opposite way, opposing high frequencies most.

4. The inductive reactance of a coil is given by

• A. 2 pi f L
• B. 1 divided by 2 pi f L
• C. L divided by 2 pi f
• D. 2 pi f divided by L

Explanation: Reactance rises in proportion to frequency, so a coil that passes direct current almost freely offers a large opposition at high frequency. That is why an inductor is called a choke and is used to smooth or to block radio frequency signals. It has units of ohms but dissipates no power, since the current and voltage are 90 degrees out of phase.

5. The average power dissipated in a purely inductive or purely capacitive AC circuit over a complete cycle is

• A. maximum
• B. zero, because energy is stored and returned each quarter cycle
• C. equal to VI
• D. half of VI

Explanation: With a 90 degree phase difference the power is positive for one quarter cycle and equally negative for the next, so the net energy transfer over a full cycle is zero: the component stores energy in its field and returns it to the source. Only resistance actually dissipates energy as heat. This is why the power factor, cos of the phase angle, is zero for a pure reactance.

6. In a series RLC circuit at resonance

• A. the impedance is maximum and the current minimum
• B. the impedance is minimum and equal to the resistance, so the current is maximum
• C. the current is zero
• D. the capacitor and inductor can be removed without effect

Explanation: At resonance the inductive and capacitive reactances are equal and cancel, leaving only the resistance to oppose the current, so the current peaks sharply. The resonant frequency is 1 over 2 pi times the square root of LC. Tuning a radio means adjusting a capacitor until the resonant frequency matches the station wanted.

7. The power factor of an AC circuit is defined as

• A. the ratio of peak to rms current
• B. the cosine of the phase angle between current and voltage
• C. the ratio of frequency to reactance
• D. the product of voltage and current

Explanation: True power is V times I times cos theta, so the power factor measures how much of the apparent power is actually being converted to useful work. It is 1 for a purely resistive circuit and zero for a pure reactance. Industrial users correct a poor power factor with capacitors, because a low value means large currents and heavy cable losses for the same useful power.

8. A choke is preferred to a resistor for controlling current in an AC circuit because a choke

• A. is cheaper
• B. limits the current without dissipating energy as heat
• C. increases the voltage
• D. converts AC to DC

Explanation: An inductor opposes the current through reactance rather than resistance, and because the current and voltage are a quarter cycle out of phase it consumes no net power, returning stored energy to the supply each cycle. A resistor doing the same job would waste that energy as heat. This is why fluorescent tubes are run through a choke rather than a series resistor.

9. In an AC circuit the quantity that plays the same role as resistance does in a DC circuit is

• A. reactance only
• B. impedance
• C. capacitance
• D. inductance

Explanation: Impedance combines resistance and net reactance and is what relates the rms voltage to the rms current, being measured in ohms. For a series RLC circuit it is the square root of R squared plus the square of the difference of the reactances. Reactance alone accounts for only the capacitive or inductive part.

10. In a step up transformer, if the voltage is doubled then for an ideal transformer the current in the secondary is

• A. doubled
• B. halved
• C. unchanged
• D. quadrupled

Explanation: Power is conserved in an ideal transformer, so if the voltage doubles the current must halve to keep the product the same. This is why high voltage transmission lines carry surprisingly small currents. Any real transformer loses a few per cent, so the output power is slightly less than the input.