Physics Questions and Answers

Answer: B

The mass number is the sum of the number of protons and neutrons present in the nucleus.

The mass number A counts the nucleons: A = Z + N, where Z is the number of protons (the atomic number) and N the number of neutrons. For carbon-12, 6 protons and 6 neutrons give A = 12.

The mass number is never less than the atomic number, because it includes the protons, and for hydrogen-1 the two are equal. It is a whole number, not a fraction, since it counts particles. It is not 'always more than the atomic weight': the atomic weight is an average over isotopes and can lie above or below the mass number of a given isotope (chlorine-35 has mass number 35, below chlorine's atomic weight of 35.45).

Answer: B

They are non-contact forces.

Magnetic, electrostatic and gravitational forces all act through a field across a distance, without the bodies touching. A magnet pulls a nail before it touches it, a charged comb lifts bits of paper, and the Earth pulls a falling ball.

Contact forces need touching surfaces, as in friction, a push, the normal reaction or tension in a string. 'Pushing forces' and 'Frictional forces' are both examples of contact forces, so none of the three fits the list in the question.

Answer: B

Night vision apparatus uses infrared rays.

All warm bodies, people and animals included, give off infrared radiation. Thermal night vision devices detect this infrared and build an image from it, and image-intensifier devices are also sensitive to near-infrared light, which is why many carry an infrared illuminator. Infrared needs no visible light, so it works in darkness.

Ultraviolet rays are not emitted by warm bodies in useful amounts and are absorbed by most lenses. X-rays are used to see inside bodies, not in the dark. Beta rays are streams of electrons from radioactive decay, not waves used for vision.

Answer: C

Its kinetic energy is greater than zero.

Kinetic energy is \(\frac{1}{2}mv^{2}\). A moving particle has a non-zero speed, so \(v^{2}\) is positive and, with positive mass, its kinetic energy is positive.

Kinetic energy can never be less than zero, because both mass and the square of speed are positive. It is zero only for a body at rest, which a moving particle is not. A freely moving particle need not have kinetic energy equal to its potential energy; the two are equal only at particular points, so that option is not true in general.

Answer: A

A current-carrying good conductor is electrically neutral.

In a metal wire carrying a current, free electrons drift along the wire, but for every electron that enters one end another leaves the other end. The number of electrons inside stays equal to the positive charge of the metal ions, so the conductor as a whole carries no net charge.

That rules out 'positively charged' and 'negatively charged', and so also 'Either (B) or (C)'. A current is a flow of charge, not a build-up of charge.

Answer: A

Both statements are correct and Statement II is the correct explanation for Statement I.

Shimla is about 2,200 m above sea level, where the air pressure is noticeably lower than in Delhi. Water boils when its vapour pressure equals the surrounding pressure, so at lower pressure it boils at a lower temperature, a few degrees below 100 °C.

Food in boiling water therefore cooks at a lower temperature and takes longer. That is exactly what Statement II supplies, which is why pressure cookers are so widely used in hill stations.

Answer: A

Only the beam balance gives the same reading on the Moon.

A beam balance compares the unknown mass with standard masses. Gravity acts equally on both pans, so the weaker lunar gravity cancels out and the balance still reads the object's mass correctly.

A spring balance measures weight, the force of gravity on the object, by how much the spring stretches. On the Moon, where gravity is about one-sixth of the Earth's, it would read about one-sixth as much. So the spring balance, and the options that include it, give a different reading.

Answer: C

Concave mirrors are used in 1, 2 and 4 only.

A torch and a vehicle headlight place the bulb at the focus of a concave reflector, so the reflected light leaves as a strong parallel beam. A shaving mirror is concave because a face held closer than the focus gives an upright, magnified image.

Rear-view wing mirrors are convex: they always give an upright, diminished image and a much wider field of view of the traffic behind. So options containing item 3 are wrong.

Answer: D

Velocity, amplitude and wavelength change, so the answer is 1, 2 and 4 only.

The frequency of a wave is set by its source and does not change when it crosses into a new medium. The speed does change, because it depends on the medium. Since \(v = f\lambda\), the wavelength changes in the same proportion as the speed.

The amplitude also changes, because part of the wave's energy is reflected at the boundary and the transmitted wave carries less. Options including frequency (3) are therefore wrong.

Answer: C

The water level remains the same.

By Archimedes' principle, floating ice displaces a weight of water equal to its own weight. When the ice melts it becomes water of exactly that weight, and so it fills exactly the volume it was already displacing.

\[ \begin{aligned} m_{\text{ice}} &= \rho_{w} V_{\text{displaced}} \\ V_{\text{melt}} &= m_{\text{ice}} / \rho_{w} = V_{\text{displaced}} \end{aligned} \]

The level would rise only if the ice held something denser than water that floated with it, or fall if it held air pockets that escaped; for pure ice in fresh water it neither rises nor falls.