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Cameroon GCE O-Level Physics study materials
Cameroon GCE O-Level Physics study resources

Cameroon GCE O-Level Physics practice questions

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Question 1Mechanics

A physical quantity is a vector only when a direction must be stated alongside its magnitude. On that basis, which of the following is a vector quantity?

  1. Mass
  2. Velocity Correct answer.
  3. Temperature
  4. Speed

Explanation

Velocity is a vector quantity because it has both magnitude and a specified direction. In contrast, mass, temperature, and speed are scalar quantities possessing magnitude only.

Question 2Mechanics

A car accelerates uniformly from rest to a speed of $20\text{ m/s}$ in $5\text{ seconds}$. Calculate the acceleration of the car.

  1. $2\text{ m/s}^2$
  2. $4\text{ m/s}^2$ Correct answer.
  3. $5\text{ m/s}^2$
  4. $100\text{ m/s}^2$

Explanation

Using the first equation of uniformly accelerated motion: $v = u + at \implies 20 = 0 + a(5) \implies a = \frac{20}{5} = 4\text{ m/s}^2$.

Question 3Mechanics

A net horizontal force of $50\text{ N}$ acts on a stationary mass of $10\text{ kg}$. What acceleration is produced in the object?

  1. $0.2\text{ m/s}^2$
  2. $5\text{ m/s}^2$ Correct answer.
  3. $40\text{ m/s}^2$
  4. $500\text{ m/s}^2$

Explanation

According to Newton's Second Law of Motion ($F = ma$): $a = \frac{F}{m} = \frac{50\text{ N}}{10\text{ kg}} = 5\text{ m/s}^2$.

Question 4Mechanics

A solid metal block has a mass of $540\text{ g}$ and a volume of $200\text{ cm}^3$. What is its density in $\text{g/cm}^3$ and $\text{kg/m}^3$?

  1. $2.7\text{ g/cm}^3$ ($2,700\text{ kg/m}^3$) Correct answer.
  2. $0.37\text{ g/cm}^3$ ($370\text{ kg/m}^3$)
  3. $7.4\text{ g/cm}^3$ ($7,400\text{ kg/m}^3$)
  4. $2.7\text{ g/cm}^3$ ($27\text{ kg/m}^3$)

Explanation

Density $\rho = \frac{\text{Mass}}{\text{Volume}} = \frac{540\text{ g}}{200\text{ cm}^3} = 2.7\text{ g/cm}^3$. In SI units: $2.7 \times 1,000 = 2,700\text{ kg/m}^3$ (the density of aluminium).

Question 5Mechanics

Calculate the work done when a force of $120\text{ N}$ moves a crate through a horizontal distance of $5\text{ m}$ in the direction of the force.

  1. $24\text{ J}$
  2. $125\text{ J}$
  3. $600\text{ J}$ Correct answer.
  4. $1,200\text{ J}$

Explanation

Work done is the product of force and distance in the direction of the force: $W = F \times d = 120\text{ N} \times 5\text{ m} = 600\text{ J}$.

Question 6Mechanics

An electric crane lifts a load of $600\text{ N}$ vertically through a height of $10\text{ m}$ in $4\text{ seconds}$. What is the useful power output of the crane?

  1. $150\text{ W}$
  2. $600\text{ W}$
  3. $1,500\text{ W}$ Correct answer.
  4. $24,000\text{ W}$

Explanation

Work done $= F \times h = 600\text{ N} \times 10\text{ m} = 6,000\text{ J}$. Power $P = \frac{\text{Work}}{\text{Time}} = \frac{6,000\text{ J}}{4\text{ s}} = 1,500\text{ W}$.

Question 7Mechanics

Calculate the hydrostatic pressure exerted at a depth of $20\text{ m}$ below the surface of freshwater. (Take density of water $\rho = 1,000\text{ kg/m}^3$ and $g = 10\text{ m/s}^2$)

  1. $2,000\text{ Pa}$
  2. $20,000\text{ Pa}$
  3. $200,000\text{ Pa}$ Correct answer.
  4. $2,000,000\text{ Pa}$

Explanation

Hydrostatic pressure in liquids: $P = \rho g h = 1,000\text{ kg/m}^3 \times 10\text{ m/s}^2 \times 20\text{ m} = 200,000\text{ Pa} = 200\text{ kPa}$.

Question 8Mechanics

A uniform meter rule is pivoted at its center of gravity ($50\text{ cm}$ mark). A mass of $40\text{ g}$ is suspended at the $20\text{ cm}$ mark. Where must a mass of $60\text{ g}$ be suspended to balance the rule horizontally?

  1. At the $60\text{ cm}$ mark
  2. At the $70\text{ cm}$ mark Correct answer.
  3. At the $80\text{ cm}$ mark
  4. At the $90\text{ cm}$ mark

Explanation

Distance of $40\text{ g}$ mass from pivot $= 50 - 20 = 30\text{ cm}$. Anticlockwise moment $= 40 \times 30 = 1,200\text{ g}\cdot\text{cm}$. For equilibrium, Clockwise moment $= 60 \times d = 1,200 \implies d = \frac{1,200}{60} = 20\text{ cm}$ to the right of pivot. Position on rule $= 50 + 20 = 70\text{ cm}$ mark.

Question 9Mechanics

A stone of mass $0.5\text{ kg}$ is dropped from the top of a cliff and hits the ground after $3\text{ seconds}$. Calculate the height of the cliff. (Take $g = 10\text{ m/s}^2$ and ignore air resistance)

  1. $15\text{ m}$
  2. $30\text{ m}$
  3. $45\text{ m}$ Correct answer.
  4. $90\text{ m}$

Explanation

Using $s = ut + \frac{1}{2}gt^2$: Initial velocity $u = 0$. $s = 0(3) + \frac{1}{2}(10)(3^2) = 5 \times 9 = 45\text{ m}$.

Question 10Mechanics

What is the kinetic energy of an athlete of mass $60\text{ kg}$ sprinting at a steady speed of $8\text{ m/s}$?

  1. $240\text{ J}$
  2. $480\text{ J}$
  3. $1,920\text{ J}$ Correct answer.
  4. $3,840\text{ J}$

Explanation

Kinetic energy formula: $E_k = \frac{1}{2}mv^2 = \frac{1}{2}(60\text{ kg})(8\text{ m/s})^2 = 30 \times 64 = 1,920\text{ J}$.

Question 11Mechanics

A hydraulic press has an effort piston of cross-sectional area $0.02\text{ m}^2$ and a load piston of area $0.80\text{ m}^2$. If a force of $250\text{ N}$ is applied to the small piston, what load force is lifted by the large piston?

  1. $6.25\text{ N}$
  2. $1,000\text{ N}$
  3. $10,000\text{ N}$ Correct answer.
  4. $20,000\text{ N}$

Explanation

By Pascal's principle, pressure is transmitted equally: $\frac{F_1}{A_1} = \frac{F_2}{A_2} \implies F_2 = F_1 \times \frac{A_2}{A_1} = 250\text{ N} \times \frac{0.80\text{ m}^2}{0.02\text{ m}^2} = 250 \times 40 = 10,000\text{ N}$.

Question 12Mechanics

A bullet of mass $0.02\text{ kg}$ is fired horizontally at $400\text{ m/s}$ from a stationary rifle of mass $4.0\text{ kg}$. What is the recoil velocity of the rifle?

  1. $-2.0\text{ m/s}$ Correct answer.
  2. $-4.0\text{ m/s}$
  3. $-8.0\text{ m/s}$
  4. $-20.0\text{ m/s}$

Explanation

By the Principle of Conservation of Linear Momentum: Total initial momentum $= 0$. Total final momentum: $m_b v_b + m_r v_r = 0 \implies (0.02\text{ kg})(400\text{ m/s}) + (4.0\text{ kg})v_r = 0 \implies 8 + 4.0v_r = 0 \implies v_r = -\frac{8}{4.0} = -2.0\text{ m/s}$.

Question 13Mechanics

Which law of physics states that the total extension of an elastic spring is directly proportional to the applied stretching force, provided the elastic limit is not exceeded?

  1. Newton's First Law
  2. Hooke's Law ($F = ke$) Correct answer.
  3. Pascal's Principle
  4. Archimedes' Principle

Explanation

Hooke's Law states that within the elastic limit, the extension $e$ of a spring is directly proportional to the applied force $F$ ($F = ke$, where $k$ is the spring constant).

Question 14Mechanics

A spring has an unstretched length of $10\text{ cm}$. When a weight of $6\text{ N}$ is suspended from it, its length increases to $14\text{ cm}$. What will be the total length of the spring when a weight of $9\text{ N}$ is hung from it, assuming elastic behavior?

  1. $15\text{ cm}$
  2. $16\text{ cm}$ Correct answer.
  3. $18\text{ cm}$
  4. $21\text{ cm}$

Explanation

Original extension for $6\text{ N}$ load $= 14 - 10 = 4\text{ cm}$. Spring constant $k = \frac{F}{e} = \frac{6\text{ N}}{4\text{ cm}} = 1.5\text{ N/cm}$. Extension for $9\text{ N}$ load $= \frac{F}{k} = \frac{9}{1.5} = 6\text{ cm}$. Total length $= \text{Original length} + \text{Extension} = 10 + 6 = 16\text{ cm}$.

Question 15Mechanics

A wooden block of mass $400\text{ g}$ floats in equilibrium in water. What is the magnitude of the upthrust exerted by the water on the floating block? (Take $g = 10\text{ m/s}^2$)

  1. $0.4\text{ N}$
  2. $4.0\text{ N}$ Correct answer.
  3. $40\text{ N}$
  4. $400\text{ N}$

Explanation

According to the Law of Flotation, a floating body displaces its own weight of fluid, meaning Upthrust = Weight of object. Weight $W = mg = (0.4\text{ kg})(10\text{ m/s}^2) = 4.0\text{ N}$. Thus, Upthrust $= 4.0\text{ N}$.

Question 16Mechanics

A vernier calliper has a main scale marked in millimeters. When the jaws are closed around a metal cylinder, the main scale reads $2.4\text{ cm}$ and the 6th vernier division coincides with a main scale mark. What is the measured diameter? (Vernier least count $= 0.01\text{ cm}$)

  1. $2.40\text{ cm}$
  2. $2.46\text{ cm}$ Correct answer.
  3. $2.64\text{ cm}$
  4. $3.00\text{ cm}$

Explanation

Measured value $= \text{Main Scale Reading} + (\text{Coinciding Division} \times \text{Least Count}) = 2.4\text{ cm} + (6 \times 0.01\text{ cm}) = 2.4 + 0.06 = 2.46\text{ cm}$.

Question 17Mechanics

The gradient (slope) of a displacement-time graph represents which physical quantity?

  1. Acceleration
  2. Velocity Correct answer.
  3. Distance
  4. Force

Explanation

On a displacement-time graph, the gradient $\frac{\Delta s}{\Delta t}$ represents velocity. (On a velocity-time graph, the gradient represents acceleration).

Question 18Mechanics

The area under a velocity-time graph represents which physical quantity?

  1. Acceleration
  2. Total distance / displacement Correct answer.
  3. Speed
  4. Power

Explanation

The area enclosed between a velocity-time graph and the time axis is the product of velocity and time ($v \times t$), representing total displacement (or distance traveled).

Question 19Mechanics

A machine with an efficiency of $75\%$ requires $4,000\text{ J}$ of input energy to perform a task. What is the useful work output produced by the machine?

  1. $1,000\text{ J}$
  2. $2,500\text{ J}$
  3. $3,000\text{ J}$ Correct answer.
  4. $5,333\text{ J}$

Explanation

Efficiency formula: $\text{Efficiency} = \frac{\text{Useful Output}}{\text{Total Input}} \times 100\% \implies 75\% = \frac{\text{Output}}{4,000} \times 100\% \implies \text{Output} = 0.75 \times 4,000 = 3,000\text{ J}$.

Question 20Mechanics

A force of $80\text{ N}$ acts on a surface area of $0.05\text{ m}^2$. Calculate the pressure exerted on the surface in Pascals ($\text{N/m}^2$).

  1. $4\text{ Pa}$
  2. $400\text{ Pa}$
  3. $1,600\text{ Pa}$ Correct answer.
  4. $4,000\text{ Pa}$

Explanation

Pressure formula: $P = \frac{F}{A} = \frac{80\text{ N}}{0.05\text{ m}^2} = 80 \times 20 = 1,600\text{ Pa}$.

Question 21Mechanics

A pendulum bob completes 20 full oscillations in $30\text{ seconds}$. What is the periodic time $T$ of the pendulum?

  1. $0.67\text{ seconds}$
  2. $1.50\text{ seconds}$ Correct answer.
  3. $10\text{ seconds}$
  4. $600\text{ seconds}$

Explanation

Periodic time $T$ is the time taken for one complete oscillation: $T = \frac{\text{Total Time}}{\text{Number of Oscillations}} = \frac{30\text{ s}}{20} = 1.50\text{ s}$.

Question 22Mechanics

Two forces of $3\text{ N}$ and $4\text{ N}$ act concurrently on a point at right angles ($90^\circ$) to each other. What is the magnitude of the resultant force?

  1. $1\text{ N}$
  2. $5\text{ N}$ Correct answer.
  3. $7\text{ N}$
  4. $12\text{ N}$

Explanation

For two perpendicular forces, the resultant magnitude is found using Pythagoras' theorem: $R = \sqrt{F_1^2 + F_2^2} = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5\text{ N}$.

Question 23Mechanics

What is the gravitational potential energy of a $5\text{ kg}$ crate raised to a shelf $4\text{ m}$ above the floor? (Take $g = 10\text{ m/s}^2$)

  1. $20\text{ J}$
  2. $50\text{ J}$
  3. $200\text{ J}$ Correct answer.
  4. $400\text{ J}$

Explanation

Gravitational potential energy: $E_p = mgh = 5\text{ kg} \times 10\text{ m/s}^2 \times 4\text{ m} = 200\text{ J}$.

Question 24Mechanics

A crane engine develops a constant power of $5,000\text{ W}$. How much energy does the engine transfer in $2\text{ minutes}$?

  1. $10,000\text{ J}$
  2. $300,000\text{ J}$
  3. $600,000\text{ J}$ Correct answer.
  4. $1,200,000\text{ J}$

Explanation

Energy $E = P \times t$. Convert time to seconds: $t = 2\text{ minutes} = 120\text{ s}$. $E = 5,000\text{ W} \times 120\text{ s} = 600,000\text{ J} = 600\text{ kJ}$.

Question 25Mechanics

What is the acceleration due to gravity on the Moon, where a $60\text{ kg}$ astronaut weighs $96\text{ N}$?

  1. $0.625\text{ m/s}^2$
  2. $1.6\text{ m/s}^2$ Correct answer.
  3. $9.8\text{ m/s}^2$
  4. $10\text{ m/s}^2$

Explanation

Weight formula: $W = mg \implies g = \frac{W}{m} = \frac{96\text{ N}}{60\text{ kg}} = 1.6\text{ m/s}^2$ (approximately one-sixth of Earth's gravity).

Question 26Mechanics

A car travelling at $30\text{ m/s}$ decelerates uniformly to rest in a distance of $90\text{ m}$. Calculate the magnitude of the deceleration.

  1. $3\text{ m/s}^2$
  2. $5\text{ m/s}^2$ Correct answer.
  3. $10\text{ m/s}^2$
  4. $15\text{ m/s}^2$

Explanation

Using $v^2 = u^2 + 2as$: $0 = 30^2 + 2a(90) \implies 0 = 900 + 180a \implies 180a = -900 \implies a = -5\text{ m/s}^2$. The deceleration is $5\text{ m/s}^2$.

Question 27Mechanics

A force of $20\text{ N}$ stretches a spring by $4\text{ cm}$. What is the spring constant $k$ in $\text{N/m}$?

  1. $5\text{ N/m}$
  2. $50\text{ N/m}$
  3. $500\text{ N/m}$ Correct answer.
  4. $5,000\text{ N/m}$

Explanation

Convert extension to meters: $e = 4\text{ cm} = 0.04\text{ m}$. Spring constant $k = \frac{F}{e} = \frac{20\text{ N}}{0.04\text{ m}} = 500\text{ N/m}$.

Question 28Mechanics

A body of mass $2\text{ kg}$ is moving in a circular path of radius $0.5\text{ m}$ at a constant speed of $4\text{ m/s}$. Calculate the centripetal force acting on the body.

  1. $16\text{ N}$
  2. $32\text{ N}$
  3. $64\text{ N}$ Correct answer.
  4. $128\text{ N}$

Explanation

Centripetal force formula: $F = \frac{mv^2}{r} = \frac{2\text{ kg} \times (4\text{ m/s})^2}{0.5\text{ m}} = \frac{2 \times 16}{0.5} = \frac{32}{0.5} = 64\text{ N}$.

Question 29Mechanics

A standard mercury barometer has a vertical column of mercury of height $760\text{ mm}$. What is the atmospheric pressure in Pascals? (Density of mercury $\rho = 13,600\text{ kg/m}^3$ and $g = 9.8\text{ m/s}^2$)

  1. $1,013\text{ Pa}$
  2. $10,130\text{ Pa}$
  3. $101,293\text{ Pa}$ Correct answer.
  4. $1,012,928\text{ Pa}$

Explanation

Atmospheric pressure $P = \rho g h = 13,600\text{ kg/m}^3 \times 9.8\text{ m/s}^2 \times 0.760\text{ m} = 101,292.8\text{ Pa} \approx 1.01 \times 10^5\text{ Pa}$ ($1\text{ atm}$).

Question 30Mechanics

What is the momentum of a vehicle of mass $1,200\text{ kg}$ travelling at a speed of $15\text{ m/s}$?

  1. $80\text{ kg}\cdot\text{m/s}$
  2. $1,215\text{ kg}\cdot\text{m/s}$
  3. $18,000\text{ kg}\cdot\text{m/s}$ Correct answer.
  4. $135,000\text{ kg}\cdot\text{m/s}$

Explanation

Linear momentum formula: $p = mv = 1,200\text{ kg} \times 15\text{ m/s} = 18,000\text{ kg}\cdot\text{m/s}$.

Question 31Mechanics

Cameroon generates most of its electricity from hydroelectric stations on the Sanaga. Which of the following energy sources, unlike hydroelectricity, is non-renewable?

  1. Solar energy
  2. Wind energy
  3. Hydroelectric power
  4. Petroleum (fossil fuel) Correct answer.

Explanation

Petroleum is a fossil fuel formed over millions of years from buried organic matter; once consumed, it cannot be replenished on a human timescale, making it a non-renewable energy resource.

Question 32Electricity Magnetism

Calculate the electric current flowing through an electric kettle element of resistance $R = 48\,\Omega$ connected to a $240\text{ V}$ mains electricity supply.

  1. $0.2\text{ A}$
  2. $5\text{ A}$ Correct answer.
  3. $192\text{ A}$
  4. $11,520\text{ A}$

Explanation

By Ohm's Law ($V = IR$): $I = \frac{V}{R} = \frac{240\text{ V}}{48\,\Omega} = 5\text{ A}$.

Question 33Electricity Magnetism

Two resistors of $6\,\Omega$ and $12\,\Omega$ are connected in parallel. What is their combined equivalent resistance?

  1. $4\,\Omega$ Correct answer.
  2. $6\,\Omega$
  3. $9\,\Omega$
  4. $18\,\Omega$

Explanation

For parallel resistors: $\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} = \frac{1}{6} + \frac{1}{12} = \frac{2 + 1}{12} = \frac{3}{12} = \frac{1}{4} \implies R_T = 4\,\Omega$. Alternatively, $R_T = \frac{R_1 R_2}{R_1 + R_2} = \frac{72}{18} = 4\,\Omega$.

Question 34Electricity Magnetism

An electric iron rated at $1,200\text{ W}$ is used for $3\text{ hours}$. How many kilowatt-hours ($\text{kWh}$) of electrical energy are consumed?

  1. $0.4\text{ kWh}$
  2. $3.6\text{ kWh}$ Correct answer.
  3. $400\text{ kWh}$
  4. $3,600\text{ kWh}$

Explanation

Power in kilowatts $= \frac{1,200\text{ W}}{1,000} = 1.2\text{ kW}$. Energy in kilowatt-hours $= \text{Power (kW)} \times \text{Time (hours)} = 1.2\text{ kW} \times 3\text{ h} = 3.6\text{ kWh}$.

Question 35Electricity Magnetism

In the harmonised colour code used for the flexible cable of a domestic appliance, which colour identifies the live conductor?

  1. Blue
  2. Brown Correct answer.
  3. Green with a yellow stripe
  4. Grey

Explanation

In the harmonised three-core cable colour code the live conductor is brown, the neutral conductor is blue and the earth conductor is green with a yellow stripe. Identifying the live conductor correctly matters because it is the one that must be broken by the switch and protected by the fuse.

Question 36Electricity Magnetism

A step-down transformer has $2,000\text{ turns}$ on its primary coil and $100\text{ turns}$ on its secondary coil. If an alternating voltage of $240\text{ V}$ is supplied to the primary coil, what is the output voltage on the secondary coil?

  1. $12\text{ V}$ Correct answer.
  2. $24\text{ V}$
  3. $48\text{ V}$
  4. $4,800\text{ V}$

Explanation

Applying the transformer equation: $\frac{V_s}{V_p} = \frac{N_s}{N_p} \implies \frac{V_s}{240} = \frac{100}{2,000} = \frac{1}{20} \implies V_s = \frac{240}{20} = 12\text{ V}$.

Question 37Electricity Magnetism

Three identical $6\,\Omega$ resistors are connected in series with a $36\text{ V}$ battery of negligible internal resistance. What is the potential difference across each individual resistor?

  1. $6\text{ V}$
  2. $12\text{ V}$ Correct answer.
  3. $18\text{ V}$
  4. $36\text{ V}$

Explanation

Total series resistance $R_T = 6 + 6 + 6 = 18\,\Omega$. Circuit current $I = \frac{V}{R_T} = \frac{36\text{ V}}{18\,\Omega} = 2\text{ A}$. Potential difference across each resistor $V_1 = IR = 2\text{ A} \times 6\,\Omega = 12\text{ V}$. (Since the resistors are identical, the $36\text{ V}$ divides equally: $36 / 3 = 12\text{ V}$).

Question 38Electricity Magnetism

According to Fleming's Left-Hand Rule for electric motors, what does the Thumb represent when the First Finger points in the direction of the Magnetic Field and the Second Finger points in the direction of the Current?

  1. Direction of the Electric Charge
  2. Direction of Motion (Thrust/Force) Correct answer.
  3. Direction of Induced Electromotive Force
  4. Direction of Gravitational Acceleration

Explanation

In Fleming's Left-Hand Rule: **F**irst finger = Magnetic **F**ield; se**C**ond finger = **C**urrent; **Th**umb = **Th**rust (Direction of Force/Motion).

Question 39Electricity Magnetism

Why are electrical appliances with exposed metal casings (such as washing machines and electric cookers) fitted with an Earth wire?

  1. To complete the live circuit and allow current to return to the power station
  2. To prevent electric shock by providing a low-resistance path to ground that blows the fuse if a live wire touches the metal casing Correct answer.
  3. To reduce the household electricity bill by bypassing the meter
  4. To convert alternating current into direct current

Explanation

The earth wire connects the metal casing to ground. If a fault causes the live wire to touch the metal casing, a massive current flows safely to ground, causing the fuse to melt (or circuit breaker to trip), immediately disconnecting the live supply.

Question 40Electricity Magnetism

A current of $3.0\text{ A}$ flows through an electric lamp for $2\text{ minutes}$. Calculate the total electrical charge that passes through the lamp.

  1. $6\text{ C}$
  2. $180\text{ C}$
  3. $360\text{ C}$ Correct answer.
  4. $720\text{ C}$

Explanation

Charge formula: $Q = I \times t$. Convert time to seconds: $t = 2\text{ minutes} = 120\text{ s}$. $Q = 3.0\text{ A} \times 120\text{ s} = 360\text{ C}$.

Question 41Electricity Magnetism

A cell of electromotive force $\text{EMF} = 6.0\text{ V}$ and internal resistance $r = 0.5\,\Omega$ is connected to an external resistor of $R = 2.5\,\Omega$. What is the terminal potential difference across the cell?

  1. $1.0\text{ V}$
  2. $5.0\text{ V}$ Correct answer.
  3. $6.0\text{ V}$
  4. $7.0\text{ V}$

Explanation

Total circuit resistance $R_T = R + r = 2.5 + 0.5 = 3.0\,\Omega$. Circuit current $I = \frac{\text{EMF}}{R_T} = \frac{6.0}{3.0} = 2.0\text{ A}$. Lost volts $V_{\text{lost}} = Ir = 2.0 \times 0.5 = 1.0\text{ V}$. Terminal voltage $V = \text{EMF} - V_{\text{lost}} = 6.0 - 1.0 = 5.0\text{ V}$ (or $V = IR = 2.0 \times 2.5 = 5.0\text{ V}$).

Question 42Electricity Magnetism

When an uncharged polythene rod is rubbed with a dry woolen cloth, what electrostatic charge is acquired by the polythene rod and why?

  1. Positive charge because it gains protons from the cloth
  2. Negative charge because electrons are transferred from the cloth to the polythene rod Correct answer.
  3. Positive charge because electrons leave the polythene rod
  4. No charge because polythene is an electrical insulator

Explanation

Frictional rubbing causes electrons to transfer from the wool to the polythene rod. Because polythene gains negative electrons, it becomes negatively charged (while the wool becomes positively charged).

Question 43Electricity Magnetism

Which of the following changes will DOUBLE the electrical resistance of a uniform copper wire?

  1. Doubling its length while keeping its cross-sectional area constant Correct answer.
  2. Doubling its cross-sectional area while keeping its length constant
  3. Doubling its diameter
  4. Halving its length

Explanation

Resistance formula: $R = \frac{\rho L}{A}$. Resistance is directly proportional to length $L$. Therefore, doubling the length $L$ will double the resistance $R$.

Question 44Electricity Magnetism

What is the SI unit of magnetic flux density?

  1. Weber (Wb)
  2. Tesla (T) Correct answer.
  3. Henry (H)
  4. Volt (V)

Explanation

The Tesla ($\text{T} = \text{Wb/m}^2 = \text{N}/(\text{A}\cdot\text{m})$) is the SI unit of magnetic flux density ($B$).

Question 45Electricity Magnetism

What happens to the electrical conductivity of a pure semiconductor such as silicon when its temperature is raised, and why?

  1. It decreases, because the increased vibration of the lattice atoms scatters the charge carriers more strongly
  2. It increases, because thermal energy breaks more covalent bonds and frees additional electron-hole pairs Correct answer.
  3. It stays constant, because the number of charge carriers is fixed by the crystal structure
  4. It falls to zero, because the covalent bonds collapse and the crystal turns into a perfect insulator

Explanation

Raising the temperature of a semiconductor supplies enough thermal energy to break more covalent bonds, releasing extra free electrons and leaving mobile holes behind, so the number of charge carriers rises sharply and conductivity increases. This is the opposite of a metallic conductor, whose conductivity falls as temperature rises. Conduction in a metal is by electrons only, while in a semiconductor it is by both electrons and holes.

Question 46Electricity Magnetism

What is the function of a fuse in an electrical circuit?

  1. To step down high voltages from the main generator
  2. To protect the circuit and appliance by melting and breaking the circuit when excessive current flows Correct answer.
  3. To store electrical charge during power outages
  4. To measure the current consumption in amperes

Explanation

A fuse contains a thin wire with a low melting point. If an electrical fault causes the current to exceed the fuse rating, Joule heating ($I^2R$) melts the wire, breaking the circuit and preventing fire or appliance damage.

Question 47Electricity Magnetism

Faraday's Law of Electromagnetic Induction states that the magnitude of an induced electromotive force (EMF) in a conductor is directly proportional to:

  1. The electrical resistance of the magnetic core
  2. The rate of change of magnetic flux linkage Correct answer.
  3. The electrostatic charge on the surrounding air
  4. The mass of the permanent magnet

Explanation

Faraday's Law states that induced $\text{EMF} = -N \frac{\Delta \Phi}{\Delta t}$, meaning the induced EMF is directly proportional to the rate at which magnetic flux lines are cut by the conductor.

Question 48Electricity Magnetism

What is the power dissipated in a $10\,\Omega$ heating resistor carrying an electric current of $4\text{ A}$?

  1. $40\text{ W}$
  2. $80\text{ W}$
  3. $160\text{ W}$ Correct answer.
  4. $400\text{ W}$

Explanation

Electrical power formula: $P = I^2 R = (4\text{ A})^2 \times 10\,\Omega = 16 \times 10 = 160\text{ W}$.

Question 49Electricity Magnetism

Which component in an AC generator converts the induced alternating current in the rotating armature coil into direct current in the external circuit for a DC generator?

  1. Slip rings
  2. Split-ring commutator Correct answer.
  3. Carbon brushes
  4. Soft iron core

Explanation

A split-ring commutator reverses the electrical connections to the external circuit every half-rotation, ensuring that the output current in the external circuit always flows in the same direction (DC).

Question 50Electricity Magnetism

An ideal transformer has $500\text{ turns}$ on the primary coil and $2,500\text{ turns}$ on the secondary coil. If the primary power input is $1,000\text{ W}$ at $200\text{ V}$, what is the secondary output current?

  1. $1.0\text{ A}$ Correct answer.
  2. $5.0\text{ A}$
  3. $10.0\text{ A}$
  4. $25.0\text{ A}$

Explanation

Secondary voltage: $V_s = V_p \times \frac{N_s}{N_p} = 200 \times \frac{2,500}{500} = 200 \times 5 = 1,000\text{ V}$. For an ideal transformer ($P_s = P_p = 1,000\text{ W}$): $I_s = \frac{P_s}{V_s} = \frac{1,000\text{ W}}{1,000\text{ V}} = 1.0\text{ A}$.