Free Alternating Current MCQs with Answers
592 Alternating Current MCQs from Physics, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
Alternating current varies periodically, so phase relationships become essential when current passes through a resistor, capacitor or inductor. The chapter distinguishes resistance from capacitive and inductive reactance, explains phase lead and lag, and connects changing electric and magnetic fields with the production and properties of electromagnetic waves.
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Read the Alternating Current notesFree MDCAT chapter notes with key terms592 questions · page 1 of 30
- A. equal to its peak value
- B. the peak value divided by the square root of 2
- C. the peak value multiplied by 2
- D. always zero
Explanation: The rms value is about 0.707 of the peak and is defined as the steady direct current that would dissipate the same power in a given…
Correct answer: the peak value divided by the square root of 2- A. 440 V
- B. 220 V
- C. 311 V
- D. 155 V
Explanation: Dividing 311 by the square root of 2, that is by about 1.414, gives roughly 220 V, the standard supply voltage in Pakistan.
Correct answer: 220 V- 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.
Correct answer: in phase with each other- 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…
Correct answer: leads the voltage by 90 degrees- 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…
Correct answer: decreases as the frequency increases- 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…
Correct answer: 2 pi f L- 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…
Correct answer: zero, because energy is stored and returned each quarter cycle- 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…
Correct answer: the impedance is minimum and equal to the resistance, so the current is maximum- 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…
Correct answer: the cosine of the phase angle between current and voltage- A. 50 Hz
- B. 60 Hz
- C. 100 Hz
- D. 220 Hz
Explanation: Pakistan, like most of Asia and Europe, uses 50 Hz at 220 V, while North America uses 60 Hz at 110 V.
Correct answer: 50 Hz- A. vibrating particles of a medium
- B. oscillating electric and magnetic fields at right angles to each other and to the direction of travel
- C. a stream of charged particles
- D. longitudinal compressions and rarefactions
Explanation: A changing electric field generates a magnetic field and vice versa, so the two sustain each other and the wave propagates without needing…
Correct answer: oscillating electric and magnetic fields at right angles to each other and to the direction of travel- A. Gamma rays
- B. X rays
- C. Visible light
- D. Radio waves
Explanation: The spectrum runs from radio waves at the long wavelength, low frequency, low energy end through microwaves, infrared, visible light and…
Correct answer: Radio waves- A. 3 times 10 to the 8 m per second, whatever their frequency
- B. different for each frequency
- C. 340 m per second
- D. dependent on the intensity
Explanation: In a vacuum every electromagnetic wave from radio to gamma travels at the same speed c, so wavelength and frequency vary inversely.
Correct answer: 3 times 10 to the 8 m per second, whatever their frequency- 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…
Correct answer: limits the current without dissipating energy as heat- A. the current is too large
- B. the deflection reverses every half cycle and the pointer cannot follow, so it averages to zero
- C. the galvanometer has no coil
- D. AC does not flow through a galvanometer
Explanation: The torque reverses with the current, and at 50 Hz the inertia of the pointer prevents it from following, so it simply sits at zero.
Correct answer: the deflection reverses every half cycle and the pointer cannot follow, so it averages to zero- A. 7.07 A
- B. 14.14 A
- C. 10 A
- D. 20 A
Explanation: The peak is the rms multiplied by the square root of 2, so 10 times 1.414 gives 14.14 A.
Correct answer: 14.14 A- 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.
Correct answer: impedance- A. is absorbed strongly by water molecules, which then rotate and heat the food
- B. ionises the food
- C. reflects off water
- D. changes the chemical composition of the food
Explanation: The oscillating field makes polar water molecules flip back and forth, and the friction between them heats the food from within rather…
Correct answer: is absorbed strongly by water molecules, which then rotate and heat the food- 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.
Correct answer: halved- A. travel faster
- B. have a longer wavelength
- C. carry more energy, enough to break chemical bonds in DNA
- D. are more numerous in sunlight
Explanation: Photon energy is proportional to frequency, so the shorter wavelength ultraviolet carries enough energy per photon to damage the bonds in…
Correct answer: carry more energy, enough to break chemical bonds in DNAAlternating Current MCQs: common questions
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