2025 - JAMB Physics Past Questions and Answers - page 1

When an electric current flows through a coil of wire, it produces a magnetic field around the coil. This principle is known as electromagnetism.

An electric bell contains a coil wound around a soft iron core. When current passes through the coil, the core becomes magnetized and behaves like a magnet. This magnet attracts an iron armature, which moves and produces the ringing action of the bell.

This shows that the device functions by using the magnetic effect of electric current. Therefore, the correct answer is Electric bell.

The amount of substance deposited during electrolysis depends on the total charge passed and the electrochemical equivalent of the substance. The required relationship is

\[ m = \frac{Q M}{n F} \]

The total charge is found from current and time. Since the time is given in minutes, it must first be converted to seconds.

\[ t = 25 \times 60 = 1500 \, \text{s} \]

The charge passed is

\[ Q = I t = 0.8 \times 1500 = 1200 \, \text{C} \]

Substituting the values of molar mass \( M = 107.87 \, \text{g/mol} \), valency \( n = 1 \), and Faraday constant \( F = 96485 \, \text{C/mol} \),

\[ m = \frac{1200 \times 107.87}{96485} \]

\[ m = \frac{129444}{96485} \approx 1.34 \, \text{g} \]

Therefore, the mass of silver deposited is \(1.34 \, \text{g}\).

The gravitational force between two objects is determined by the expression

\[ F = G \frac{m_1 m_2}{r^2} \]

This shows that the force is directly proportional to the product of the two masses.

If each mass becomes twice its original value, then the new masses are \(2P\) and \(2Q\). Their product becomes

\[ (2P)(2Q) = 4PQ \]

This means the new force becomes four times the original force because the distance and gravitational constant remain unchanged.

\[ F_{new} = 4 \times 10 = 40 \, \text{N} \]

Therefore, the new gravitational force is \(40 \, \text{N}\).

When an object floats, the fraction of its volume immersed equals the ratio of the density of the object to the density of the liquid.

The density of the block is obtained from its relative density. Since relative density compares the block to water,

\[ \text{density of block} = 0.4 \times 1000 = 400 \, \text{kg m}^{-3} \]

The fraction immersed is

\[ \frac{400}{1600} = 0.25 \]

This means one quarter of the total volume is submerged.

Therefore, the fraction of the block immersed is 0.25.

A positively charged electroscope already has positive charges distributed throughout its metal parts, including the leaves. These like charges repel each other, causing the leaves to separate.

When a positively charged rod is brought close to the cap, electrons inside the electroscope are attracted toward the cap. This leaves the leaves with an even greater positive charge.

Since the leaves now carry more positive charge than before, the repulsive force between them becomes stronger. Stronger repulsion makes the leaves move farther apart.

Therefore, the divergence of the leaves increases.

A photovoltaic cell converts light energy directly into electrical energy. This process occurs when light strikes a semiconductor material such as silicon.

The semiconductor absorbs the light energy and releases electrons, allowing current to flow through the circuit. This conversion of light into electrical energy is known as the photovoltaic effect.

Because this process occurs specifically in semiconductor materials, the operation of a photovoltaic cell depends on a semiconductor.

Therefore, the correct answer is Semi-conductor.

The power of a lens describes how strongly the lens converges or diverges light. It is defined as the reciprocal of the focal length when the focal length is measured in metres.

This relationship is written as

\[ P = \frac{1}{f} \]

This means that shorter focal lengths correspond to greater lens power, while longer focal lengths correspond to smaller lens power.

Therefore, the power of a lens in diopters is given by \( \frac{1}{f} \).

The electric field produced by a point charge is given by

\[ E = \frac{kq}{r^2} \]

Rearranging to make \(r^2\) the subject gives

\[ r^2 = \frac{kq}{E} \]

Substituting the given values,

\[ r^2 = \frac{(9.0 \times 10^9)(1.2 \times 10^{-7})}{4.8 \times 10^{-4}} \]

First multiplying the numerator,

\[ 9.0 \times 1.2 = 10.8 \]

\[ r^2 = \frac{10.8 \times 10^2}{4.8 \times 10^{-4}} \]

\[ r^2 = 2.25 \times 10^6 \]

Taking the square root,

\[ r = \sqrt{9 \times 10^6} = 3000 \, \text{m} \]

\[ r = 3.0 \, \text{km} \]

Therefore, the required distance is 3.0 km.

Heat capacity is defined as the amount of heat required to produce a unit rise in temperature. The relationship is

\[ C = \frac{Q}{\Delta T} \]

The heat given is 48 kJ, which must be converted to joules:

\[ Q = 48000 \, \text{J} \]

Substituting into the formula,

\[ C = \frac{48000}{53} \]

\[ C \approx 905.66 \, \text{J/ºC} \]

Rounding to one decimal place gives

\[ C = 905.7 \, \text{J/ºC} \]

Therefore, the heat capacity is \(905.7 \, \text{J K}^{-1}\).

The relationship between the angle of incidence and angle of refraction is given by the refractive index formula

\[ n = \frac{\sin i}{\sin r} \]

To find the sine of the angle of refraction, this expression is rearranged so that sin r becomes the subject

\[ \sin r = \frac{\sin i}{n} \]

The sine of the angle of incidence is given as 0.5 and the refractive index of the glass is 1.5. Substituting these values gives

\[ \sin r = \frac{0.5}{1.5} \]

\[ \sin r = 0.333 \]

This value rounds to 0.3.

Therefore, the sine of the angle of refraction is 0.3.