2024 - JAMB Physics Past Questions and Answers - page 6

The efficiency of a cell is given by: Efficiency = (Output power / Input power) * 100%

The output power is the power dissipated in the external resistor (X), and the input power is the total power generated by the cell. Expressing efficiency in terms of resistance:

Efficiency = (X / (X + r)) * 100%

Where: X = external resistance, r = internal resistance

Given: Efficiency = 75%, r = 2 Ω

0.75 = X / (X + 2)

0.75 * (X + 2) = X → 0.75X + 1.5 = X → 1.5 = 0.25X → X = 6 Ω

When thermal energy (heat) is added to a solid, its temperature increases. If enough heat is added, the solid will change its state from solid to liquid. This process is called melting.

The velocity ratio (VR) of a machine is defined as the ratio of the distance moved by the effort to the distance moved by the load. A VR of 4 means that for every unit distance the load moves, the effort moves 4 units.

The capacitance of a capacitor is directly proportional to the dielectric constant of the material between its plates. An insulating material has a dielectric constant greater than 1. Therefore, when an insulating sheet is inserted between the plates of a capacitor, the capacitance increases.

1. Calculate the load (output force): W = m g = 60 kg * 10 m/s² = 600 N

2. Calculate the Mechanical Advantage (MA): MA = Load / Effort = 600 / 40 = 15

1. Principle: A floating object displaces its weight in the fluid, meaning the buoyant force equals the weight of the hydrometer.

2. Submerged volume: 80% of total volume = 0.8 * (2y x 10^-5 m³) = 1.6y x 10^-5 m³

3. Weight of hydrometer: W = mass * g. Using buoyant force = weight: ρ_fluid * V_submerged * g = y * g → ρ_fluid = y / (1.6y x 10^-5) ≈ 6.25 x 10⁴ kg/m³

1. Stress formula: Stress = Force / Area

2. Area of the wire (circular cross-section): Area = π r²

3. Radius: r = diameter / 2 = y / (2 π) m

4. Area: Area = π * (y / 2π)² = y² / 4π m²

5. Stress: Stress = Force / Area = y² / (y² / 4π) = 4π N/m²

The degree of precision of a vernier caliper refers to the smallest measurement it can accurately make. Standard vernier calipers typically have a precision of 0.01 cm.

Relative density of liquid X = (Height of water) / (Height of liquid X) = 0.3 / 0.5 = 0.6

The correct answer is the dynamo because it operates based on the magnetic effect of electric current.

• Dynamo: A generator that converts mechanical energy into electrical energy using electromagnetic induction.
• Electric heater: Operates based on the heating effect of electric current.
• Photocell: Converts light energy into electrical energy using the photoelectric effect.
• Cold cathode: Operates based on thermionic or field emission principles.