All Free Physics MCQs with Answers

371. The reciprocal of the resistivity of a material is called its:

• A. Impedance
• B. Conductivity
• C. Admittance
• D. Reactance

Explanation: Conductivity measures how readily a material carries current and is the reciprocal of resistivity, with units of siemens per metre. Admittance is the reciprocal of impedance and reactance is the opposition offered by a capacitor or inductor, both of which belong to circuit analysis rather than to material properties. Copper has a high conductivity and rubber a very low one.

372. The Balmer series of hydrogen spectrum appears in the

• A. Infrared region
• B. Ultraviolet region
• C. X-ray region
• D. Visible region

Explanation: Balmer transitions all end on the second energy level, and those energy gaps correspond to visible photons, which is why these are the hydrogen lines seen through a laboratory spectroscope. The Lyman series ends on the ground state and lies in the ultraviolet because its gaps are larger, while Paschen and the higher series fall in the infrared. Identifying the level a series ends on tells you its region immediately.

373. A ball is thrown into the air with certain velocity v making an angle with horizontal. If air resistance is neglected, then at maximum height its velocity is:

• A. Equal to initial velocity
• B. Half of initial velocity
• C. Equal to zero
• D. Minimum but not zero

Explanation: At the top the vertical component has fallen to zero, but the horizontal component is unchanged throughout the flight because no horizontal force acts, so the ball still moves horizontally at v cos theta. The speed is therefore at its minimum for the flight yet not zero, which is the distinction being tested. Only a ball thrown straight up would momentarily stop.

374. A cannon is placed on a smooth surface. When it fires a shell, the cannon moves backward. This recoil occurs due to:

• A. Law of conservation of energy
• B. Backward thrust of the gases
• C. Newton's third law of motion
• D. Newton's first law of motion

Explanation: The cannon pushes the shell forward and the shell pushes the cannon back with an equal and opposite force, which is the third law, and the same situation expressed as conservation of momentum gives the recoil velocity. Because the cannon is far more massive than the shell, its backward speed is correspondingly small. Energy is conserved too, but it does not by itself explain the direction of the motion.

375. If the surface charge density of an infinite sheet increases by 25%, the electric field intensity:

• A. Increases by 25%
• B. Increases by 50%
• C. Decreases by 25%
• D. Remains the same

Explanation: The field of an infinite charged sheet is the surface charge density divided by twice the permittivity, so it is directly proportional to that density and rises by the same 25 per cent. Notably the field does not depend on distance from the sheet at all, which is what makes the infinite sheet such a useful idealisation. A point charge behaves quite differently, falling off as the inverse square.

376. An incompressible fluid flows through a pipe that becomes narrower in one section. The fluid speed increases in that region to maintain:

• A. Constant pressure
• B. Constant energy
• C. Constant mass flow rate
• D. Constant volume

Explanation: The same mass must pass every cross section each second, so where the area falls the speed must rise in proportion, which is the equation of continuity and is simply conservation of mass applied to a fluid. Pressure certainly does not stay constant; it falls in the constriction, which is Bernoulli's principle. This is why a thumb over a hose makes the water squirt further.

377. In a pipe of varying cross-section, as fluid enters the narrower region it exhibits:

• A. High velocity, high pressure
• B. High velocity, low pressure
• C. Low velocity, high pressure
• D. Low velocity, low pressure

Explanation: Continuity forces the speed up in the constriction, and since the total energy per unit volume is fixed, the gain in kinetic energy must be paid for by a drop in pressure energy. This pairing of fast flow with low pressure is the heart of Bernoulli's principle. A Venturi meter measures flow rate by reading exactly this pressure difference.

378. The curved shape of an airplane wing causes air to move faster over the top surface. This leads to:

• A. Greater pressure on the top
• B. Lower pressure on the top
• C. Equal pressure on both sides
• D. Zero pressure above the wing

Explanation: Faster flow means lower pressure by Bernoulli's principle, so the pressure above the wing falls below that underneath and the difference multiplied by the wing area provides the lift. The wing also deflects air downwards, and by Newton's third law that contributes lift as well, so both descriptions account for the same force. The pressure above never reaches zero.

379. The protons in the nucleus of an atom DO NOT exit due to

• A. Electromagnetic force
• B. Gravitational force
• C. Weak nuclear force
• D. Strong nuclear force

Explanation: The strong nuclear force acts over only a few femtometres but far exceeds the electrostatic repulsion between protons at that range, so it is what holds the nucleus together. The electromagnetic force pushes the protons apart and would blow the nucleus open on its own. Gravity is utterly negligible at nuclear scale and the weak force governs beta decay rather than binding.

380. Mechanical waves cannot travel through outer space because they:

• A. Have low speed in vacuum
• B. Disperse in space due to long wavelength
• C. Lose frequency in the absence of air
• D. Lose transmission without interacting particles

Explanation: A mechanical wave propagates by particles of a medium pushing on their neighbours, so with no particles there is nothing to carry the disturbance and sound simply does not exist in a vacuum. Electromagnetic waves need no medium, which is why light crosses space while sound cannot. The frequency of a wave is set by its source and is not lost.