Physics 2011 Jamb Examination

Steel has a high retentivity, meaning it retains magnetism for a long time once magnetised. This makes it ideal for making permanent magnets. However, because of its high coercivity, it is harder to magnetise and demagnetise compared to soft iron, which is used for temporary magnets like electromagnets.

Hot-wire ammeters are suitable for measuring alternating currents at high frequencies because they rely on the heating effect of the current rather than electromagnetic induction. As the current passes through a fine wire, the wire heats up and expands proportionally to the current’s RMS value, providing an accurate measurement even at high frequencies where inductive instruments fail.

Power \(P = VI = 250 \times 2 = 500\,W = 0.5\,kW\).
Energy consumed \(= 0.5 \times 6 = 3\,kWh\).
Cost \(= 3 \times 4 = 12\,N\). The total cost of operating the appliance is ₦12.

The kinetic theory explains phenomena arising from the motion of particles in matter. Change of state and diffusion depend on molecular motion and energy changes. Osmosis also involves the random movement of solvent molecules across a semi-permeable membrane. Radiation, however, involves the transfer of energy through electromagnetic waves and is not described by kinetic theory. Hence, I, II, and IV are correct.

For cells in parallel, the total e.m.f remains the same as that of one cell (since all cells have equal e.m.f). The total internal resistance \(r_t\) is given by:\[ \dfrac{1}{r_t} = \dfrac{1}{r} + \dfrac{1}{r} + \dfrac{1}{r} = \dfrac{3}{r} \]\[ r_t = \dfrac{r}{3} = \dfrac{2}{3} = 0.67\,\Omega \approx 0.7\,\Omega \]Thus, total e.m.f = 2 V and internal resistance ≈ 0.7 Ω.

Presbyopia occurs when the ciliary muscles of the eye lose elasticity with age, reducing the eye’s ability to adjust its focal length for near and distant objects. The eye’s accommodation power decreases, making it difficult to focus on nearby objects. It is commonly corrected using bifocal or progressive lenses.

A red object appears red because it reflects red light and absorbs other colours. When illuminated with blue light, there is no red wavelength to reflect, so the rose absorbs the blue light and appears black or very dark to the observer. Since option E ('no correct option') originally indicated this, it has been replaced with this explanation as the correct response.

The dielectric constant \(K\) is given by:\[ K = \dfrac{C_{\text{with dielectric}}}{C_{\text{in air}}} = \dfrac{60}{20} = 3 \]Hence, the dielectric constant is 3.0. This means the dielectric increases the capacitance threefold compared to when air was the medium.

During the night, the Earth emits infrared radiation into space, losing the heat it absorbed during the day. This process of radiative cooling causes the ground temperature to drop significantly. Since the atmosphere does not provide sufficient incoming energy at night, the surface continues to lose heat through radiation, making the ground cold.

The time taken for sound to travel a given distance is obtained using the equation:\[ t = \dfrac{d}{v} \]where \(d = 4.95 \text{ km} = 4950 \text{ m}\) and \(v = 330 \text{ m/s}\).\[ t = \dfrac{4950}{330} = 15\,\text{s} \]Thus, the sound is heard 15 seconds after the cannon is fired.