Physics 2009 Jamb Examination

When a liquid wets a surface, it indicates that the adhesive force between the liquid and the surface is stronger than the cohesive force among the liquid molecules.
In this case, the adhesive attraction between water molecules and the glass surface exceeds the cohesive attraction among water molecules themselves. This causes the water to spread along the glass surface, forming a thin film.

Heat from a fire travels primarily through radiation. Radiation is the transfer of energy by electromagnetic waves, which can occur even through a vacuum. Since air is a poor conductor and convection mainly affects fluids in motion, the heat felt from a distance is mainly due to radiant energy emitted by the hot charcoal.

Metal wires are used for signal transmission because the speed of sound (and electrical signal propagation) through metals is very high compared to non-metals. Metals have tightly packed particles that transmit vibrations or electrical signals efficiently with minimal delay, making them ideal for telephone communication.

Different musical instruments can produce the same pitch and loudness but still sound different due to variations in the waveform of the sound they emit. This characteristic is known as the ‘quality’ or ‘timbre’ of sound. It arises from the presence of overtones and harmonics that differ among instruments, allowing listeners to distinguish between, for example, a piano and a violin playing the same note.

For a floating body, the weight of the body equals the upthrust exerted by the displaced fluid:\[ \text{Density of object} = \text{Fraction submerged} \times \text{Density of fluid} \]In water:\[ \rho_s = 0.5 \times 1000 = 500 \; \text{kg/m}^3 \]In glycerine:\[ 0.4 = \dfrac{\rho_s}{\rho_g} \Rightarrow \rho_g = \dfrac{\rho_s}{0.4} = \dfrac{500}{0.4} = 1250 \; \text{kg/m}^3 \]Hence, the density of glycerine is 1250 kg/m³.

An optical pyrometer measures high temperatures without direct contact by observing the colour of light emitted from a hot object. The colour changes with temperature, and the instrument compares this colour to a calibrated reference, providing a precise temperature reading. Hence, its reading depends on colour variation.

Water exhibits anomalous expansion, meaning it behaves differently from most substances when cooled. Its density is maximum at 4°C, which means its volume is minimum at that temperature. Below 4°C, water expands as it freezes, which allows ice to float on water.

The electrical energy supplied is converted into heat energy that raises the temperature of the water.\[ P \times t = m c \Delta T \]\( P = 1000 \; \text{W}, \; t = 300 \; \text{s}, \; c = 4200 \; \text{J/kg·K}, \; \Delta T = 100 - 30 = 70 \; \text{K} \)\[ 1000 \times 300 = m \times 4200 \times 70 \]\[ 3.0 \times 10^5 = 294000m \]\[ m = \dfrac{3.0 \times 10^5}{2.94 \times 10^5} = 1.02 \; \text{kg} \]The mass of water is approximately 1.02 kg.

A liquid boils when its vapour pressure equals the external or atmospheric pressure. At this point, bubbles of vapour form within the liquid and rise freely to the surface. Therefore, when the vapour pressure equals atmospheric pressure, the temperature corresponds to the boiling point of the liquid.

The human eye can see clearly at different distances because it adjusts the focal length of the lens by changing its curvature through the action of the ciliary muscles. This process is known as accommodation. When looking at a distant object, the ciliary muscles relax, making the lens thinner, while for a near object, they contract, making the lens thicker. This adjustment ensures that images are properly focused on the retina, allowing for clear vision.