All Free Physics MCQs with Answers

101. Resonance occurs when

• A. the driving frequency equals the natural frequency of the system
• B. the driving frequency is much higher than the natural frequency
• C. the amplitude is very small
• D. there is no damping present at all

Explanation: At resonance energy is transferred most efficiently to the oscillator and the amplitude builds to a large value, limited only by damping. It explains why a radio can be tuned to one station, why a wine glass can be shattered by a sung note, and why marching troops break step on a bridge. Damping reduces the peak amplitude but does not abolish the effect.

102. The Doppler effect explains why the pitch of an approaching ambulance siren sounds

• A. lower than the true pitch
• B. higher than the true pitch, because the waves reach the observer more frequently
• C. unchanged
• D. louder but of the same pitch

Explanation: Motion towards the observer crowds the wavefronts together, shortening the observed wavelength and raising the frequency, and the pitch drops abruptly as the source passes and begins to recede. The speed of sound in the air is unchanged throughout; only the observed frequency shifts. The same effect on light gives the red shift used to measure the recession of galaxies.

103. Only transverse waves can be polarised because polarisation restricts

• A. the speed of the wave
• B. the frequency of the wave
• C. the plane in which the vibrations occur
• D. the amplitude of the wave

Explanation: A transverse wave vibrates in any plane perpendicular to its direction of travel, so a filter can select one of those planes, whereas a longitudinal wave vibrates only along the direction of travel and there is nothing to select. This is why sound cannot be polarised but light can. The fact that light can be polarised is direct evidence that it is a transverse wave.

104. Electric current is defined as

• A. the charge flowing per unit time past a point in the circuit
• B. the energy delivered per unit charge
• C. the resistance per unit length
• D. the potential difference per unit resistance only

Explanation: One ampere is one coulomb per second, so current measures rate of flow of charge. Energy per unit charge is potential difference, measured in volts, and confusing the two is the commonest error in this chapter. Conventional current is taken as the direction positive charge would move, which is opposite to the actual drift of electrons in a metal.

105. Ohm's law states that, at constant temperature, the current through a conductor is

• A. inversely proportional to the potential difference across it
• B. directly proportional to the potential difference across it
• C. independent of the potential difference
• D. proportional to the square of the potential difference

Explanation: A graph of current against voltage for an ohmic conductor is a straight line through the origin whose gradient is the reciprocal of the resistance. The condition of constant temperature matters, since a filament lamp heats up as the current rises and its graph curves. Semiconductor diodes are non ohmic for a different reason, conducting in one direction only.

106. The resistance of a uniform wire is

• A. proportional to its length and to its cross sectional area
• B. proportional to its length and inversely proportional to its cross sectional area
• C. independent of its dimensions
• D. inversely proportional to its length

Explanation: R equals rho L over A, so a longer wire offers more resistance and a thicker one less, which is why thick cables are used for heavy currents. The constant rho is the resistivity, a property of the material rather than of the sample. Doubling the diameter quarters the resistance, because area depends on the square of the radius.

107. A wire is stretched so that its length doubles while its volume stays constant. Its resistance becomes

• A. double
• B. half
• C. four times
• D. unchanged

Explanation: Constant volume means doubling the length halves the cross sectional area, and since resistance is proportional to length over area, the effect is two multiplied by two, giving four times the original. Answering double accounts for the length change only. This is a favourite examination trick precisely because the area change is easy to overlook.

108. The SI unit of resistivity is

• A. ohm
• B. ohm metre
• C. ohm per metre
• D. volt per ampere

Explanation: Rearranging R equals rho L over A gives rho equal to RA over L, whose units are ohm times metre squared divided by metre, that is ohm metre. The plain ohm, equivalently the volt per ampere, is the unit of resistance itself. Resistivity is a material property, so copper has the same resistivity whether it is a thin wire or a thick bar.

109. As the temperature of a metal conductor rises, its resistance

• A. increases, because the lattice ions vibrate more and impede the electrons
• B. decreases, because the electrons move faster
• C. stays constant
• D. falls to zero

Explanation: More vigorous lattice vibrations scatter the drifting electrons more often, shortening the mean free path and raising resistance, which is why a filament lamp's resistance is much higher when lit. Semiconductors behave in the opposite way, since heating releases many more charge carriers and resistance drops sharply. That contrast is the basis of the thermistor.

110. The temperature coefficient of resistivity for a semiconductor such as silicon is

• A. positive
• B. zero
• C. negative
• D. always exactly one

Explanation: Heating a semiconductor promotes far more electrons across the energy gap than it adds scattering, so the number of charge carriers rises steeply and resistivity falls, giving a negative coefficient. Metals have a positive coefficient because their carrier number is already fixed and only scattering changes. A thermistor exploits the negative case to measure temperature.