Free Alternating Current MCQs with Answers

11. Electromagnetic waves consist of

• 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 any medium, which is why light crosses empty space. Being transverse, electromagnetic waves can be polarised. All of them travel at 3 times 10 to the 8 metres per second in a vacuum.

12. Which of the following electromagnetic waves has the longest wavelength?

• 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 ultraviolet to X rays and gamma rays at the other extreme. Since all travel at the same speed, the longest wavelength necessarily has the lowest frequency. Photon energy is proportional to frequency, which is why gamma rays are the most damaging.

13. The speed of all electromagnetic waves in a vacuum is

• 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. In a material medium the speed falls and does depend slightly on frequency, which is the dispersion that splits white light in a prism. The value 340 m per second is the speed of sound in air.

14. 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.

15. Alternating current cannot be measured with a moving coil galvanometer directly because

• 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. Measuring AC therefore requires either a rectifier ahead of the meter or an instrument such as a hot wire ammeter that responds to the heating effect, which does not depend on direction. This is why hot wire meters read rms values directly.

16. The peak value of an alternating current whose rms value is 10 A is about

• 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. Dividing instead gives 7.07 A, which would be the rms if 10 A were the peak. Keeping track of which value a question quotes is the whole difficulty in these problems.

17. 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.

18. Microwaves are used in a microwave oven because their frequency

• 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 than only at the surface. Microwaves are non ionising, since their photon energy is far too low to remove electrons, so they do not make food radioactive. Metal containers are unsuitable because they reflect the waves and can cause arcing.

19. 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.

20. Ultraviolet radiation is more damaging to skin than visible light because ultraviolet photons

• 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 biological molecules, which is why it causes sunburn and raises skin cancer risk. Visible photons simply cannot deliver that much energy however intense the beam. All electromagnetic waves travel at the same speed, so speed is not the difference.