Physics Questions and Answers

Answer: C

The heating is due to both the heating effect of the current and hysteresis loss.

Copper loss is the \(I^2R\) dissipation in the windings; iron loss is hysteresis plus eddy currents in the core. Laminating the core cuts eddy currents, and a soft-iron core with a narrow hysteresis loop cuts the other - but neither can be eliminated.

Answer: C

A washing machine's spin cycle works by centrifugation.

The drum spins rapidly, and the water in the fabric, having no centripetal force to hold it in a circle, moves outward and escapes through the perforations while the clothes are held by the drum wall.

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: C

It depends on both the nature of the signal and the nature of the medium.

The signal's frequency and power determine how far it can carry and how much information it holds, while the medium fixes the attenuation, dispersion and noise it will meet. Neither alone decides the quality of transmission.

Answer: D

The gun recoils with the same momentum as the bullet.

Momentum is conserved, and the system was at rest, so \[m_b v_b = m_g v_g\] The gun's momentum is equal in magnitude and opposite in direction. Being much heavier, it moves back far more slowly.

Answer: C

Natural gas is a non-renewable resource.

It was formed over millions of years from buried organic matter and is consumed far faster than it is replaced, so the stock is finite. Wind, solar and hydrogen fuel are replenished continuously.

Answer: C

This is streamline, or laminar, flow.

Every particle passing a given point follows the same path with the same velocity, so the streamlines never cross. Above a critical speed the flow becomes turbulent and the motion at each point varies irregularly with time.

Answer: A

The minimum distance is 4f.

With object distance \(u\) and image distance \(v\), the total separation is \(D = u+v\), and the lens formula gives \[\frac{1}{v} - \frac{1}{-u} = \frac{1}{f} \Rightarrow \frac{1}{u} + \frac{1}{v} = \frac{1}{f}\]

Minimising \(u+v\) subject to that condition gives \(u = v = 2f\), so \[D_{min} = 2f + 2f = 4f\]

Below this separation a convex lens cannot form a real image at all.

Answer: C

In a static liquid, the pressure is the same at every point at the same horizontal level.

Pressure depends only on depth, \(P = P_0 + \rho gh\), so all points at one depth are at the same pressure whatever the shape of the vessel. If they were not, liquid would flow until they were - which is why connected vessels settle at a common level.

Answer: B

Force times velocity is power.

\[P = \frac{dW}{dt} = \frac{\vec{F}\cdot d\vec{s}}{dt} = \vec{F}\cdot\vec{v}\] Force times displacement gives work; force times time gives impulse.