Physics Questions and Answers

Answer: B

The value of \(t\) is 6 hours.

If the ratio of X to Y atoms is 1 : 7, then X is \(\frac{1}{8}\) of the original amount, since every decayed atom of X became an atom of Y.

\[ \begin{aligned} \left(\frac{1}{2}\right)^{n} &= \frac{1}{8} \Rightarrow n = 3 \\ t &= 3 \times 2 = 6\ \text{hours} \end{aligned} \]

14 hours treats the 7 in the ratio as the number of half-lives, and 4 hours (two half-lives) would leave \(\frac{1}{4}\), a ratio of 1 : 3.

Answer: B

An inertial observer is one moving at constant velocity, so a driver in a car moving with constant velocity is inertial, option B.

A slowing train, an aircraft taking off and a child revolving in a merry-go-round are all accelerating, so they are non-inertial.

Newton's first law holds in inertial frames, which are frames at rest or in uniform motion in a straight line. A car moving at constant velocity is such a frame. A braking train, a climbing aircraft and a merry-go-round are all accelerating frames, in which a free body appears to accelerate for no reason.

Answer: B

A transformer works on the principle of mutual induction: a changing current in the primary coil induces a voltage in the secondary coil, option B.

Self induction is the effect in a single coil, Lenz's law gives the direction of the induced current, and Fleming's right hand rule is the rule for a generator.

A transformer has a primary and a secondary coil wound on the same iron core. When an alternating current flows in the primary, the changing magnetic flux induces a voltage in the secondary, which is mutual induction. This is why transformers work only with alternating current.

Answer: B

No work is done in moving a charge on an equipotential surface, because the potential difference between its points is zero, so the work done is zero, option B.

It is not finite, positive or negative.

An equipotential surface is one on which every point is at the same electric potential. The work done in moving a charge between two points is the charge times the potential difference, so with zero difference the work is zero whatever the path, which is why the answer is zero and not finite, positive or negative.

Answer: C

One rutherford is a unit of radioactivity equal to 10⁶ disintegrations per second, so option C is correct.

The becquerel (1 disintegration per second) and the curie (3.7 x 10¹⁰ per second) are the other units of activity; the values 10⁸ and 10⁴ do not define the rutherford.

The unit is named after Ernest Rutherford and is now obsolete, replaced by the becquerel, and 1 rutherford equals one million becquerels.

Answer: C

The sky is blue because of the scattering of light.

Sunlight is scattered by the molecules of the air, and this Rayleigh scattering varies inversely as the fourth power of the wavelength. Blue light, with its short wavelength, is scattered far more than red, so blue light reaches the eye from every part of the sky.

Reflection and refraction do not separate colours across the sky in this way; refraction is what produces a rainbow inside raindrops. Polarisation is a property that scattered skylight does show, but it is not the cause of its colour.

Answer: C

It is difficult to walk on ice because friction is low.

Walking depends on friction between the foot and the ground. The foot pushes backwards on the ground, and friction pushes the body forwards. Ice is very smooth, and a thin film of water on its surface acts as a lubricant, so the friction it can provide is small, and the foot slips instead of gripping.

High friction would make walking easier, not harder. Pressure is not the reason: the body exerts much the same pressure on ice as on any other floor.

Answer: C

The filament is made of tungsten.

An incandescent bulb gives light because its filament glows white-hot, at well over 2,000°C. Tungsten has the highest melting point of all metals, about 3,400°C, and evaporates slowly, so it can glow for a long time without melting.

Copper and aluminium are good conductors that would melt long before glowing brightly, which is why they are used in wiring. Lead has a very low melting point.

Answer: A

The fathom is a unit of depth.

A fathom equals 6 feet (about 1.83 m), roughly the span of a man's outstretched arms. It is used mainly to measure the depth of water, in navigation and on nautical charts.

Strictly it is a length, but its use is for depth rather than distance along the surface, which is measured in nautical miles. Frequency is measured in hertz and sound level in decibels.

Answer: B

The material that conducts electricity is graphite.

In graphite each carbon atom bonds to only three others, leaving one electron free to move between the layers. These delocalised electrons carry an electric current, which is why graphite is used for electrodes and brushes in motors.

Glass, rubber and PVC have no free charge carriers and are electrical insulators, used to cover wires and handle electrical equipment safely.