All Free Physics MCQs with Answers

171. 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.

172. The frequency of the alternating mains supply in Pakistan is

• 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. A frequency of 50 Hz means the current reverses direction 100 times each second, since there are two reversals per cycle. The figure 220 belongs to the voltage rather than the frequency.

173. 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.

174. 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.

175. 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.

176. 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.

177. 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.

178. 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.

179. 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.

180. 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.