Physics Questions and Answers

Answer: C

The order is air, water, iron.

Speed is \(v = \sqrt{E/\rho}\), and the elastic modulus rises far more sharply from gas to liquid to solid than the density does. Sound travels at about \(343\ \mathrm{m\,s^{-1}}\) in air, \(1{,}500\) in water and \(5{,}100\) in iron.

Answer: B

Because soft iron has large susceptibility and small retentivity.

High susceptibility means it magnetises strongly and concentrates the flux; low retentivity means it loses that magnetism almost entirely the moment the current is switched off - exactly what an electromagnet must do. Steel, with high retentivity, is used for permanent magnets.

Answer: B

The sphere reaches the bottom first.

For a body rolling without slipping, \[a = \frac{g\sin\theta}{1 + k^2/R^2}\] The ratio \(k^2/R^2\) is \(2/5\) for a solid sphere and \(1/2\) for a disc, so the sphere has the larger acceleration. Mass and radius cancel out entirely - only the shape matters.

Answer: A

All operate at nearly the same propagation speed.

Microwaves and radio waves are both electromagnetic and travel at \(3\times10^{8}\ \mathrm{m\,s^{-1}}\) in air. Light in an optical fibre goes at \(c/n\), about two-thirds of that, so fibre is if anything slightly slower - its advantage is enormous bandwidth, not speed.

Answer: B

Viscosity is measured in newton second per square metre.

From Newton's law of viscous flow, \[F = \eta A \frac{dv}{dx} \Rightarrow \eta = \frac{F}{A\,(dv/dx)}\] which gives \(\mathrm{N\,s\,m^{-2}}\), also written as the pascal second. The CGS unit is the poise, and \(1\ \mathrm{Pa\,s} = 10\ \mathrm{poise}\).

Answer: A

Shadows are explained by the rectilinear propagation of light.

Light travels in straight lines in a uniform medium, so an opaque body blocks the rays behind it and leaves a dark region of the same outline. The sharpness depends on the source: a point source gives a crisp umbra, an extended source a fuzzy penumbra as well.

Answer: A

The brighter bulb has the smaller resistance.

Household bulbs are in parallel, so both have the same voltage across them: \[P = \frac{V^2}{R}\] With \(V\) fixed, power varies inversely with resistance, so the lower-resistance bulb dissipates more and glows brighter. In a series circuit the opposite would hold.

Answer: C

The two scales read alike at −40°.

Setting the readings equal in the conversion formula: \[F = \frac{9}{5}C + 32,\qquad x = \frac{9}{5}x + 32\]

\[-\frac{4}{5}x = 32 \Rightarrow x = -40\]

So −40 °C is the same temperature as −40 °F.

Answer: A

The kilowatt-hour is a unit of energy.

It is power multiplied by time, so \[1\ \mathrm{kWh} = 1000\ \mathrm{W} \times 3600\ \mathrm{s} = 3.6\times10^{6}\ \mathrm{J}\] The kilowatt is the unit of power; the electricity bill charges for energy consumed, which is why the meter counts kilowatt-hours.

Answer: D

The maximum and minimum separations are 3.3 km and 0.30 km.

The two listeners are at \[d_1 = 300\times5 = 1500\ \mathrm{m},\qquad d_2 = 300\times6 = 1800\ \mathrm{m}\] from the source. They are furthest apart when they lie on opposite sides of it, \(1500+1800 = 3300\ \mathrm{m}\), and closest when they lie on the same side, \(1800-1500 = 300\ \mathrm{m}\).