Physics Questions and Answers

Answer: C

Impulse and moment of force do not share dimensions.

Impulse is \(\mathrm{MLT^{-1}}\); moment of force, or torque, is \(\mathrm{ML^2T^{-2}}\). The others match: pressure and Young's modulus are both \(\mathrm{ML^{-1}T^{-2}}\), Planck's constant and angular momentum both \(\mathrm{ML^2T^{-1}}\), and force is by definition the rate of change of momentum.

Answer: B

Angular momentum is the vector.

It is \(\vec{L} = \vec{r} \times \vec{p}\), a cross product, so it has a direction - along the axis of rotation, by the right-hand rule. Heat, time and work are all scalars.

Answer: B

Rectifiers convert alternating current to direct current.

A diode conducts in one direction only, so it passes half of each AC cycle. Four diodes in a bridge give full-wave rectification, and a smoothing capacitor turns the pulses into steady DC. The reverse conversion is done by an inverter.

Answer: D

Radiocarbon dating is used on fossils and other once-living material.

A living organism keeps its carbon-14 in equilibrium with the atmosphere; once it dies the isotope decays with a half-life of 5,730 years, so the remaining fraction gives the age. The method is useful to about 50,000 years, beyond which too little carbon-14 is left. Rocks are dated by potassium-argon or uranium-lead instead.

Answer: C

Superconductors offer no resistance to the flow of electricity.

Below a critical temperature the resistance falls exactly to zero, so a current once started in a ring persists indefinitely. They also expel magnetic flux entirely - the Meissner effect - which is what makes magnetic levitation possible.

Answer: B

Because it can damage the domestic wiring by overloading it.

Appliances on one socket draw current in parallel, so the currents add while the wiring stays the same. The heat produced, \(I^2Rt\), rises as the square of the total current, and the wire can overheat and start a fire before any appliance is harmed.

Answer: C

Sunlight reaches us in about 8 minutes.

\[t = \frac{d}{c} = \frac{1.5\times10^{11}}{3\times10^{8}} = 500\ \mathrm{s} \approx 8\ \text{minutes 20 seconds}\] So we always see the sun as it was eight minutes ago.

Answer: B

A siphon fails if the liquid levels in the two vessels are the same.

The flow is driven by the difference in height between the two surfaces, giving a pressure difference \(\rho g h\). With \(h = 0\) there is nothing to drive the flow. A siphon also fails in a vacuum, since atmospheric pressure is what pushes the liquid up the short limb.

Answer: C

The magnetism at the centre of a bar magnet is zero.

The magnet behaves as though its poles were concentrated near the ends, and at the midpoint the effects of the two opposing poles cancel. Iron filings therefore cling to the ends and not to the middle. The midpoint is called the neutral region.

Answer: D

Because planets are nearer the earth and so present an extended disc rather than a point.

A star is effectively a point source, so the whole of its light is deflected together by a turbulent air cell and the brightness flickers. A planet's disc is a collection of many such points, and their random fluctuations average out, so the light appears steady.