Physics Questions and Answers

Answer: B

Pulsars are rotating neutron stars.

A neutron star is the extremely dense core left when a massive star explodes as a supernova. Pulsars are neutron stars that spin rapidly, up to hundreds of times a second, and emit beams of radiation from their magnetic poles. As a beam sweeps past the Earth, it is detected as regular pulses, like a lighthouse.

A pulsar is a single object, not a group of stars. The explosion that creates it is a supernova, and the radio waves are what we detect from a pulsar, not the pulsar itself.

Answer: B

The object should be at the centre of curvature.

For a concave mirror, an object placed at the centre of curvature C forms a real, inverted image also at C, of the same size as the object. This follows from the mirror formula: with \(u = -2f\), the image distance \(v = -2f\) and the magnification is \(-1\).

An object at the focus gives an image at infinity. An object between F and C gives a real, inverted, enlarged image beyond C, and an object beyond C gives a real, inverted, diminished image between F and C.

Answer: A

A photoelectric cell converts light energy into electric energy.

A photoelectric cell (photocell) works on the photoelectric effect: light falling on a photosensitive surface releases electrons, producing an electric current that depends on the intensity of light. Photocells are used in automatic doors, burglar alarms, light meters and street lights that switch on at dusk.

Converting electric energy into light is what a bulb or LED does, and a photocell does not store energy; a battery stores electrical energy.

Answer: D

The answer is None of the above.

A liquid conducts electricity when it contains ions that can move. Salted water contains sodium and chloride ions, and orange juice and lemon juice contain citric acid and dissolved salts that ionise. All three are good enough conductors to light a small bulb in a simple circuit test, so none of them is a bad conductor.

Bad conductors among liquids are those without ions, such as distilled water, kerosene or vegetable oil.

Answer: C

They would have same velocity at any instant.

Near the Moon's surface, as near the Earth's, every freely falling object has the same acceleration due to gravity, whatever its mass (about 1.6 m/s² on the Moon), and the Moon has no air to slow lighter objects. Released together from rest, both objects therefore have the same velocity at every instant.

They do not have different accelerations. Inertia depends on mass, which does not change as they fall. The gravitational forces on them are proportional to their masses, so they are not of the same magnitude.

Answer: B

AC can be produced by a dynamo.

A dynamo, or electric generator, produces current by electromagnetic induction: a coil rotating in a magnetic field cuts magnetic lines of force, and the induced current reverses direction every half turn. With slip rings, it delivers alternating current (an AC generator).

A choke coil only limits AC current with little power loss, and a transformer only changes the voltage of an existing alternating current; neither produces AC from mechanical energy.

Answer: B

Current density is a vector quantity.

Current density \(\vec{J}\) is the current per unit cross-sectional area at a point, with a direction: the direction in which positive charges flow there. It is measured in amperes per square metre and is related to current by \(I = \int \vec{J} \cdot d\vec{A}\).

Current itself is treated as a scalar, but current density is a vector. It is not dimensionless, since it has units of A/m².

Answer: A

The key gives a-2, b-3, c-4, d-1.

• Magenta: a mixture of red and blue light (2).
• Teal: a dark blue-green, matched with blue, green and white (3).
• Mauve: a pale purple, matched with blue, red and white (4).
• Cyan: green and blue light combined (1).

Magenta and cyan are the two clear anchors: both are mixtures of two primary colours of light, red with blue and green with blue. Of the codes, only the first places both correctly and pairs the lighter tints teal and mauve with the combinations that include white.

Answer: B

Sound travels fastest in brass.

Sound travels faster in solids than in liquids, and faster in liquids than in gases, because the particles are more tightly bound. At 25°C sound moves at about 4,700 m/s in brass, about 1,200 m/s in ethanol, about 346 m/s in air and about 316 m/s in oxygen.

Brass is the only solid among the options, so the speed is greatest in it.

The general rule is that sound travels faster where the medium is more rigid, which is why solids carry sound fastest.

Answer: D

The mass is 13 kg.

Gravitational potential energy is \(E_{p} = mgh\). Rearranging for the mass:

\[ \begin{aligned} m &= \frac{E_{p}}{gh} \\ &= \frac{520}{10 \times 4} = \frac{520}{40} \\ &= 13\ \text{kg} \end{aligned} \]

Check: \(13 \times 10 \times 4 = 520\) J. 10, 11 and 12 kg give 400, 440 and 480 J, which are all short of 520 J.