All Free Physics MCQs with Answers

381. Terminal voltage of a cell equals its EMF only when:

• A. No current flows
• B. Current is maximum
• C. Internal resistance is infinite
• D. Load resistance is zero

Explanation: The lost volts inside the cell are the current multiplied by the internal resistance, so only when the current is zero, that is on open circuit, does the terminal voltage rise to the full emf. This is why a high resistance voltmeter across an unloaded cell reads its emf. Short circuiting the cell does the opposite, making the current maximum and the terminal voltage nearly zero.

382. A loop moves toward a stationary magnet. The induced current will:

• A. Enhance the motion
• B. Try to stop the approach
• C. Become zero
• D. Become alternating

Explanation: By Lenz's law the induced current sets up a field that opposes the change producing it, so the loop is repelled and work must be done to keep pushing it towards the magnet. That work is the source of the electrical energy generated, which makes Lenz's law a statement of energy conservation. A current that enhanced the motion would create energy from nothing.

383. The energy of a simple harmonic oscillation depends upon the

• A. Frequency
• B. Time period
• C. Wavelength
• D. Amplitude

Explanation: This is the option marked correct in the official UHS answer key, and the total energy of a simple harmonic oscillator is proportional to the square of the amplitude, so doubling the amplitude quadruples the energy. Strictly the energy also depends on the square of the frequency, since the expression is half m omega squared A squared, but amplitude is the variable the question is pointing at. The energy stays constant through the cycle, merely exchanging between kinetic and potential forms.

384. The wave that has the highest frequency and penetrating power is

• A. X-rays
• B. Ultraviolet rays
• C. Gamma rays
• D. Microwaves

Explanation: Gamma rays sit at the extreme short wavelength end of the electromagnetic spectrum, so they carry the highest frequency and photon energy and need several centimetres of lead to attenuate them. X rays are next and overlap gamma rays in energy, the difference being origin: gamma from the nucleus, X rays from electron transitions or deceleration. Microwaves are at the low energy end.

385. According to first law of thermodynamics, when heat flows into a system and no work is done the internal energy of the system must

• A. Increase
• B. Decrease
• C. Remain constant
• D. Become zero

Explanation: The first law states that the change in internal energy equals the heat added plus the work done on the system, so with no work term every joule of heat entering raises the internal energy and the temperature rises. This is the constant volume case, which is why the molar specific heat at constant volume is smaller than at constant pressure. If the gas were allowed to expand, part of the heat would be spent doing work instead.

386. If the length of the conductor is made 4 times its original length, its resistance becomes

• A. Quarter
• B. Half
• C. Zero
• D. 4 times

Explanation: Resistance is proportional to length for a given cross section and material, so quadrupling the length quadruples the resistance. Note that if the wire were stretched to four times its length at constant volume, the area would fall to a quarter and the resistance would rise sixteenfold, which is the harder variant of this question. Here only the length is stated to change.

387. The unit of temperature coefficient of resistivity is

• A. 1/C
• B. 1/K
• C. 1/A
• D. 1/s

Explanation: The coefficient is the fractional change in resistivity per unit rise in temperature, so the resistivity units cancel and only reciprocal temperature remains, giving per kelvin. Because a kelvin and a degree Celsius are the same size, per degree Celsius is numerically identical, which is why the first option is such a close distractor. Reciprocal ampere and reciprocal second belong to quite different quantities.

388. The work done by the gravitational force on an object as it moves from a reference level to a higher point is:

• A. Always positive
• B. Always negative
• C. Zero
• D. Depends on the path taken

Explanation: Gravity acts downwards while the displacement is upwards, so the force has a component opposite to the motion and the work it does is negative, which is exactly why the potential energy of the object increases. Coming back down, gravity does positive work. Because gravity is a conservative force, the work depends only on the change in height and not on the path.

389. If two speakers emit sound at same frequency and phase, maximum loudness occurs when path difference equals:

• A. half a wavelength
• B. one whole wavelength
• C. a quarter of a wavelength
• D. three quarters of a wavelength

Explanation: A path difference of a whole number of wavelengths brings the two waves back into phase so their crests coincide, giving constructive interference and maximum loudness. An odd number of half wavelengths puts them in antiphase and produces a quiet point instead. The Greek symbol for wavelength did not print cleanly in the original paper.

390. When an object attains terminal velocity, its acceleration is:

• A. 9.8 m/s2 upward
• B. Zero
• C. 1 m/s2
• D. 9.8 m/s2 downward

Explanation: At terminal velocity the upward drag and upthrust exactly balance the weight, so the resultant force is zero and by Newton's second law the acceleration is zero, even though the body continues to fall at a steady speed. What has vanished is the resultant force, not any individual one. This is why a raindrop reaches the ground at a survivable speed.