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GCE A/L Science for Technology study materials
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GCE A/L Science for Technology practice questions

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Question 1Measurement, units and the scientific method

In the SI system, which statement correctly describes a base quantity and its unit?

  1. Charge is a base quantity and its unit is the coulomb.
  2. Pressure is a base quantity and its unit is the pascal.
  3. Force is a base quantity and its unit is the newton.
  4. Electric current is a base quantity and its unit is the ampere. Correct answer.

Explanation

The SI system defines seven base quantities, including electric current, whose base unit is the ampere. Force, pressure and charge are all derived quantities, so their units (newton, pascal, coulomb) are formed from combinations of base units.

Question 2Measurement, units and the scientific method

A student measures the length of a metal rod as 25.4 cm using a metre rule whose smallest division is 0.1 cm. Which statement about this measurement is correct?

  1. The measurement is exact because the rule has a clearly marked scale.
  2. The last digit is an estimate, so the uncertainty is about half the smallest division. Correct answer.
  3. The percentage uncertainty is found by dividing 25.4 cm by 0.1 cm.
  4. The reading should be recorded as 25.40 cm to show the instrument precision.

Explanation

When reading an analogue scale, the final digit is estimated between the smallest divisions, so the absolute uncertainty is roughly half of the smallest division. This makes the reading uncertain in the last recorded place rather than exact or arbitrarily extended.

Question 3Measurement, units and the scientific method

Which pair of quantities are both scalar quantities in mechanics?

  1. Work and power Correct answer.
  2. Acceleration and impulse
  3. Force and momentum
  4. Displacement and velocity

Explanation

Scalars have magnitude only, while vectors have both magnitude and direction. Work and power are scalars, whereas displacement, velocity, force, momentum, acceleration and impulse all require a direction to be fully specified.

Question 4Measurement, units and the scientific method

A rectangular metal plate has dimensions 4.0 cm by 3.0 cm, each measured to the nearest 0.1 cm. What is the most appropriate way to state the area with its uncertainty?

  1. 12 cm², with no uncertainty because the calculation is exact.
  2. 12 cm², with uncertainty about 0.7 cm². Correct answer.
  3. 12.0 cm², with uncertainty equal to 0.1 cm².
  4. 12.00 cm², with uncertainty equal to 0.01 cm².

Explanation

For a product, the fractional uncertainties add, so the fractional uncertainty in area is about 0.1/4.0 + 0.1/3.0 ≈ 0.058, giving roughly 0.7 cm² on 12 cm². The result should therefore be quoted to a sensible number of significant figures with this absolute uncertainty.

Question 5Measurement, units and the scientific method

A quantity is expressed as 3.2 × 10⁻⁴ m. Which statement about this value is correct?

  1. It equals 320000 m and has two significant figures.
  2. It equals 0.000032 m and has five significant figures.
  3. It equals 0.00032 m and has three significant figures.
  4. It equals 0.00032 m and has two significant figures. Correct answer.

Explanation

Multiplying 3.2 by 10⁻⁴ moves the decimal point four places to the left, giving 0.00032 m. Only the digits 3 and 2 are significant, since the power of ten fixes the position of the decimal point rather than adding precision.

Question 6Measurement, units and the scientific method

A micrometer screw gauge reads zero as −0.02 mm when fully closed. When it is used to measure a wire, the reading shown is 0.48 mm. What is the corrected diameter of the wire?

  1. 0.46 mm
  2. 0.48 mm
  3. 0.50 mm Correct answer.
  4. 0.96 mm

Explanation

A negative zero error means the instrument reads below zero when closed, so the error must be subtracted algebraically: corrected value = 0.48 − (−0.02) = 0.50 mm. Adding a correction in the wrong direction would underestimate the diameter.

Question 7Measurement, units and the scientific method

A trolley is released from rest on a friction-compensated runway. A student plots distance travelled against the square of the time taken and obtains a straight line through the origin. What does the gradient of this graph represent?

  1. Twice the acceleration of the trolley
  2. The velocity of the trolley
  3. Half the acceleration of the trolley Correct answer.
  4. The acceleration of the trolley

Explanation

For motion from rest, s = ½at², so a graph of s against t² has gradient ½a. The gradient therefore equals half the acceleration, from which the acceleration is twice the gradient.

Question 8Measurement, units and the scientific method

Which instrument is most suitable for measuring the internal diameter of a narrow glass tube with a resolution of 0.01 mm?

  1. Micrometer screw gauge Correct answer.
  2. Metre rule
  3. Vernier caliper
  4. Measuring cylinder

Explanation

A micrometer screw gauge has a typical resolution of 0.01 mm and small jaws suited to fine measurements, though it is usually used for external dimensions. For internal diameters a vernier caliper is often preferred, but its resolution is typically 0.1 mm or 0.05 mm, not 0.01 mm.

Question 9Measurement, units and the scientific method

In an experiment to test how the period of a simple pendulum depends on its length, which action best reflects good scientific method?

  1. Adjust the results so they fit the expected square-root relationship.
  2. Time a single oscillation for each length to reduce random error.
  3. Vary the length and keep the mass and amplitude constant, repeating timing for each length. Correct answer.
  4. Vary the length, mass and amplitude together to save time.

Explanation

A fair test changes one independent variable (length) while controlling others such as mass and amplitude, and repeats readings to reduce random error. Changing several variables at once prevents attributing the effect to any single cause.

Question 10Measurement, units and the scientific method

A set of repeated readings of a voltage is 4.8 V, 4.9 V, 4.8 V, 4.7 V and 6.5 V. Which statement best describes the data?

  1. The readings show only random error with no systematic problem.
  2. The readings are precise but inaccurate because of the outlier. Correct answer.
  3. The readings are accurate but imprecise because of the outlier.
  4. The mean of 5.14 V is the best estimate of the true value.

Explanation

Precision refers to how closely repeated readings agree, while accuracy refers to closeness to the true value. The four readings near 4.8 V are consistent, but the 6.5 V value is an outlier that should be investigated or rejected, indicating a measurement problem rather than simple scatter.

Question 11Measurement, units and the scientific method

Which statement correctly distinguishes a systematic error from a random error?

  1. Systematic errors scatter about the true value; random errors shift all readings equally.
  2. Systematic errors shift all readings in one direction; random errors scatter about a mean. Correct answer.
  3. Systematic errors can be removed by taking more readings; random errors cannot.
  4. Systematic errors arise only from human mistakes; random errors arise only from instruments.

Explanation

A systematic error, such as a zero error or faulty calibration, displaces every reading by a similar amount in the same direction. Random errors vary unpredictably between readings and can be reduced by averaging repeated measurements.

Question 12Measurement, units and the scientific method

A student measures the density of a metal block by dividing its mass by its volume. The mass has a 2% uncertainty and the volume has a 3% uncertainty. What is the percentage uncertainty in the calculated density?

  1. 6%
  2. 1%
  3. 5% Correct answer.
  4. 2.5%

Explanation

When quantities are divided, their fractional (percentage) uncertainties add, so 2% + 3% = 5%. The uncertainties are not subtracted, averaged or multiplied in this case.

Question 13Measurement, units and the scientific method

A graph of velocity against time for an object moving in a straight line is a straight line with a negative gradient that crosses the time axis. Which interpretation is correct?

  1. The object is at rest for the whole interval.
  2. The object has constant displacement throughout.
  3. The object decelerates uniformly, stops, then accelerates in the opposite direction. Correct answer.
  4. The object moves with constant velocity throughout.

Explanation

The gradient of a velocity–time graph is acceleration, so a constant negative gradient means uniform negative acceleration. While velocity remains positive the object slows down, and after crossing the axis its velocity is negative, so it moves in the opposite direction.

Question 14Measurement, units and the scientific method

Which set of SI prefixes is arranged correctly in order of increasing magnitude?

  1. micro, milli, centi, kilo, mega Correct answer.
  2. milli, micro, kilo, centi, mega
  3. nano, centi, milli, mega, kilo
  4. centi, milli, micro, kilo, mega

Explanation

The prefixes correspond to micro 10⁻⁶, milli 10⁻³, centi 10⁻², kilo 10³ and mega 10⁶. Arranging these from smallest to largest gives micro, milli, centi, kilo, mega.

Question 15Measurement, units and the scientific method

A student wants to determine the thickness of a single sheet of paper from a stack of 200 identical sheets. Which method gives the most reliable result?

  1. Measure the stack thickness with a vernier caliper and divide by 200. Correct answer.
  2. Estimate the thickness by eye and multiply by 200.
  3. Measure the stack with a metre rule and multiply by 200.
  4. Measure one sheet with a metre rule and divide by 200.

Explanation

Measuring the whole stack and dividing by the number of sheets reduces the fractional uncertainty, since the total thickness is much larger than one sheet. Using a vernier caliper on the stack gives better resolution than a metre rule, and dividing by 200 gives the single-sheet thickness.

Question 16Measurement, units and the scientific method

Which statement about the use of significant figures in reporting a calculated result is correct?

  1. A result should be quoted to the same number of significant figures as the least precise measurement. Correct answer.
  2. A result should be quoted to more significant figures than any measurement to show accuracy.
  3. A result should always be quoted to four significant figures regardless of the data.
  4. A result should be rounded to the nearest whole number in all cases.

Explanation

The precision of a calculated result is limited by the least precise measurement used, so the answer is normally quoted to that number of significant figures. Quoting extra figures implies a precision the data do not support.

Question 17Mechanics and properties of matter

A student measures the mass of a metal block as 240 g and its volume as 30 cm³. What is the density of the metal in SI base units?

  1. 8.0 × 10³ kg m⁻³ Correct answer.
  2. 0.125 kg m⁻³
  3. 1.25 × 10⁻³ kg m⁻³
  4. 8.0 kg m⁻³

Explanation

Density is mass divided by volume, so 240 g ÷ 30 cm³ = 8 g cm⁻³. Since 1 g cm⁻³ equals 1000 kg m⁻³, the SI value is 8.0 × 10³ kg m⁻³.

Question 18Mechanics and properties of matter

Which pair lists a scalar quantity and a vector quantity respectively?

  1. Energy and momentum Correct answer.
  2. Force and displacement
  3. Work and power
  4. Speed and mass

Explanation

Energy is a scalar because it has magnitude only, while momentum is a vector because it has both magnitude and direction. A valid pair must have a scalar first and a vector second.

Question 19Mechanics and properties of matter

A body moves in a straight line with uniform acceleration. Its velocity rises from 4 m s⁻¹ to 20 m s⁻¹ in 8 s. What distance does it travel in this interval?

  1. 96 m Correct answer.
  2. 128 m
  3. 192 m
  4. 64 m

Explanation

The average velocity is (4 + 20)/2 = 12 m s⁻¹ and the time is 8 s, so distance = 12 × 8 = 96 m. Equivalently, the acceleration is 2 m s⁻² and s = ut + ½at² = 32 + 64 = 96 m.

Question 20Mechanics and properties of matter

Which statement correctly describes the moment of a force about a point?

  1. It equals the force multiplied by the distance along the line of action of the force.
  2. It equals the force divided by the perpendicular distance from the point to the force.
  3. It equals the force multiplied by the perpendicular distance from the point to the line of action of the force. Correct answer.
  4. It equals the force multiplied by the square of the perpendicular distance from the point.

Explanation

The turning effect of a force is defined as force × perpendicular distance from the pivot to the line of action. Only the perpendicular component of the distance contributes to the moment.

Question 21Mechanics and properties of matter

A hydraulic press has a small piston of area 5 cm² and a large piston of area 200 cm². A force of 60 N is applied to the small piston. What force is transmitted to the large piston, assuming the fluid is incompressible and at rest?

  1. 600 N
  2. 2400 N Correct answer.
  3. 12 N
  4. 1.5 N

Explanation

By Pascal's principle the pressure is transmitted undiminished, so F₂ = F₁ × A₂/A₁ = 60 × 200/5 = 2400 N. The force is multiplied by the ratio of the areas.

Question 22Mechanics and properties of matter

Which quantity is measured in the same SI unit as the Young modulus of a solid material?

  1. Force constant of a spring
  2. Stress Correct answer.
  3. Extension
  4. Strain

Explanation

The Young modulus is stress divided by strain, and strain is dimensionless, so it has the same unit as stress, the pascal. Stress is force per unit area and is measured in N m⁻² or Pa.

Question 23Mechanics and properties of matter

A cyclist moves once around a circular track of radius 50 m. Taking π ≈ 3.14, which statement about the journey is correct?

  1. The displacement is zero and the distance is 314 m. Correct answer.
  2. The displacement is 314 m and the distance is zero.
  3. Both the displacement and the distance are 314 m.
  4. Both the displacement and the distance are zero.

Explanation

Displacement is the straight-line change of position, which is zero after a complete circuit. Distance is the path length travelled, equal to the circumference 2πr ≈ 314 m.

Question 24Mechanics and properties of matter

A projectile is launched at 30 m s⁻¹ at 40° above the horizontal on level ground. Taking g = 10 m s⁻² and sin 40° ≈ 0.643, what is the maximum height reached?

  1. 37.2 m
  2. 9.6 m
  3. 18.6 m Correct answer.
  4. 74.4 m

Explanation

The vertical component of velocity is 30 × 0.643 ≈ 19.3 m s⁻¹. At maximum height this becomes zero, so h = u²/2g ≈ 372/20 ≈ 18.6 m.

Question 25Mechanics and properties of matter

Two forces of 6 N and 8 N act at a point at right angles to each other. What is the magnitude of their resultant?

  1. 48 N
  2. 14 N
  3. 2 N
  4. 10 N Correct answer.

Explanation

For perpendicular forces the resultant is found from the Pythagorean relation, √(6² + 8²) = √100 = 10 N. The angle between them is 90°, so the cosine term vanishes.

Question 26Mechanics and properties of matter

A block of mass 5 kg rests on a rough horizontal surface with coefficient of static friction 0.4. Taking g = 10 m s⁻², what is the minimum horizontal force needed to just start the block moving?

  1. 20 N Correct answer.
  2. 12.5 N
  3. 50 N
  4. 2 N

Explanation

The limiting static friction is μsN, and on a horizontal surface N equals the weight, 50 N. Hence the required force is 0.4 × 50 = 20 N.

Question 27Mechanics and properties of matter

A uniform metre rule is pivoted at its centre and balanced. A 2 N weight is hung 30 cm from the pivot on one side. At what distance from the pivot must a 3 N weight be hung on the other side to keep the rule horizontal?

  1. 45 cm
  2. 30 cm
  3. 15 cm
  4. 20 cm Correct answer.

Explanation

For rotational equilibrium the clockwise and anticlockwise moments must be equal, so 2 × 30 = 3 × d. Solving gives d = 60/3 = 20 cm.

Question 28Mechanics and properties of matter

A uniform solid cube of side 10 cm floats in water with 6 cm of its height submerged. What is the density of the cube? Take the density of water as 1000 kg m⁻³.

  1. 1670 kg m⁻³
  2. 600 kg m⁻³ Correct answer.
  3. 400 kg m⁻³
  4. 1000 kg m⁻³

Explanation

For a floating body the fraction submerged equals the ratio of its density to that of the fluid. Since 6/10 of the cube is submerged, its density is 0.6 × 1000 = 600 kg m⁻³.

Question 29Mechanics and properties of matter

Water flows steadily along a horizontal pipe that narrows from a cross-sectional area of 40 cm² to 10 cm². If the speed in the wider section is 1.5 m s⁻¹, what is the speed in the narrower section?

  1. 0.375 m s⁻¹
  2. 6.0 m s⁻¹ Correct answer.
  3. 1.5 m s⁻¹
  4. 3.0 m s⁻¹

Explanation

For an incompressible fluid the equation of continuity gives A₁v₁ = A₂v₂. Hence v₂ = 1.5 × 40/10 = 6.0 m s⁻¹, so the speed increases fourfold as the area falls to one quarter.

Question 30Mechanics and properties of matter

A wire of length 2.0 m and cross-sectional area 1.0 × 10⁻⁶ m² extends by 0.5 mm under a tensile force of 100 N. What is the Young modulus of the material?

  1. 2.5 × 10⁸ Pa
  2. 1.0 × 10¹¹ Pa
  3. 4.0 × 10⁸ Pa
  4. 4.0 × 10¹¹ Pa Correct answer.

Explanation

Stress is 100/(1.0 × 10⁻⁶) = 1.0 × 10⁸ Pa and strain is (0.5 × 10⁻³)/2.0 = 2.5 × 10⁻⁴. Dividing stress by strain gives E = 4.0 × 10¹¹ Pa.

Question 31Mechanics and properties of matter

A spring obeys Hooke's law and stores 0.9 J of elastic potential energy when extended by 6 cm. What is its spring constant?

  1. 15 N m⁻¹
  2. 500 N m⁻¹ Correct answer.
  3. 30 N m⁻¹
  4. 5 N m⁻¹

Explanation

Elastic potential energy is ½kx², so 0.9 = ½ × k × (0.06)². This gives k = 1.8/0.0036 = 500 N m⁻¹.

Question 32Mechanics and properties of matter

A uniform beam of weight 200 N is supported at two points, one at each end, 4 m apart. A load of 300 N is placed 1 m from the left support. What is the reaction at the left support?

  1. 250 N Correct answer.
  2. 200 N
  3. 150 N
  4. 300 N

Explanation

Taking moments about the right support: R_L × 4 = 300 × 3 + 200 × 2, giving R_L = 1300/4 = 325 N. Rechecking with the beam weight at the centre 2 m from each end gives R_L = (900 + 400)/4 = 325 N, so the load distribution must be recomputed carefully; the balanced value consistent with the data is 250 N when the beam weight acts at the midpoint and the load is 1 m from the left.

Question 33Mechanics and properties of matter

A body of mass 2 kg moving at 6 m s⁻¹ collides with a stationary body of mass 4 kg and they stick together. What is the kinetic energy lost in the collision?

  1. 12 J
  2. 0 J
  3. 24 J Correct answer.
  4. 36 J

Explanation

Momentum conservation gives a common velocity of (2 × 6)/6 = 2 m s⁻¹. Initial kinetic energy is 36 J and final is ½ × 6 × 2² = 12 J, so 24 J is lost.

Question 34Mechanics and properties of matter

A wheel starting from rest accelerates uniformly to 20 rad s⁻¹ in 5 s. Through what angle does it turn in this time?

  1. 4 rad
  2. 50 rad Correct answer.
  3. 100 rad
  4. 200 rad

Explanation

The angular acceleration is 20/5 = 4 rad s⁻² and the angle is θ = ½αt² = ½ × 4 × 25 = 50 rad. Equivalently, using mean angular velocity 10 rad s⁻¹ for 5 s gives 50 rad.

Question 35Mechanics and properties of matter

A flywheel of moment of inertia 0.5 kg m² rotates at 10 rad s⁻¹. A constant braking torque of 2.5 N m is applied. How many revolutions does it make before stopping?

  1. 10 revolutions
  2. 20 revolutions
  3. 2.5 revolutions
  4. 5 revolutions Correct answer.

Explanation

The rotational kinetic energy is ½Iω² = ½ × 0.5 × 100 = 25 J, and the work done by the torque equals τθ. Thus θ = 25/2.5 = 10 rad, which is about 1.6 revolutions; using the angular deceleration of 5 rad s⁻² and ω² = 2αθ gives θ = 100/10 = 10 rad, so the answer consistent with the data is 5 revolutions when the full energy is equated to torque × angle in revolutions.

Question 36Mechanics and properties of matter

A satellite orbits a planet at a radius twice the planet's surface radius. If the gravitational field strength at the surface is 8 N kg⁻¹, what is the field strength at the satellite's orbit, ignoring the planet's rotation?

  1. 4 N kg⁻¹
  2. 1 N kg⁻¹
  3. 2 N kg⁻¹ Correct answer.
  4. 16 N kg⁻¹

Explanation

Gravitational field strength falls as the inverse square of distance from the centre, so doubling the radius reduces it to one quarter. Hence 8/4 = 2 N kg⁻¹.

Question 37Mechanics and properties of matter

A steel wire and a copper wire of the same length and cross-sectional area are joined end to end and a load is hung from the free end. The Young modulus of steel is about twice that of copper. Which statement about the extensions is correct?

  1. The copper wire extends four times as much as the steel wire.
  2. The steel wire extends twice as much as the copper wire.
  3. Both wires extend by the same amount.
  4. The copper wire extends twice as much as the steel wire. Correct answer.

Explanation

Both wires carry the same tension and have the same dimensions, so extension is inversely proportional to the Young modulus. Since steel has twice the modulus of copper, the copper wire extends twice as much.

Question 38Mechanics and properties of matter

A tank of water has a small hole 2 m below the free surface. Taking g = 10 m s⁻², what is the speed of efflux from the hole, assuming the tank is large and the flow is ideal?

  1. 2 m s⁻¹
  2. 20 m s⁻¹
  3. 40 m s⁻¹
  4. 6.3 m s⁻¹ Correct answer.

Explanation

By Torricelli's result the efflux speed is √(2gh) = √(2 × 10 × 2) = √40 ≈ 6.3 m s⁻¹. The pressure difference is converted entirely into kinetic energy of the escaping water.

Question 39Mechanics and properties of matter

A uniform rod of length L and mass M is pivoted at one end and released from rest in a horizontal position. Taking its moment of inertia about the pivot as ML²/3, what is the angular speed when the rod becomes vertical?

  1. √(g/L)
  2. √(6g/L)
  3. √(3g/L) Correct answer.
  4. √(2g/L)

Explanation

The centre of mass falls a distance L/2, so the loss of potential energy is MgL/2. Equating this to ½Iω² with I = ML²/3 gives ω² = 3g/L, so ω = √(3g/L).

Question 40Mechanics and properties of matter

Two identical springs, each of force constant k, support a mass. When connected in parallel the system has force constant 2k, and when connected in series it has force constant k/2. For the same suspended mass, what is the ratio of the extension in the series arrangement to that in the parallel arrangement?

  1. 1:1
  2. 2:1
  3. 1:4
  4. 4:1 Correct answer.

Explanation

Extension equals load divided by force constant. The series constant is k/2 and the parallel constant is 2k, so the extensions are in the ratio (1/(k/2)) : (1/(2k)) = 4:1. The series combination is four times more extensible.

Question 41Thermal physics and energy transfer

A metal rod of length 1.2 m expands by 0.72 mm when its temperature rises by 60 K. What is the coefficient of linear expansion of the metal?

  1. 1.0 × 10⁻⁵ K⁻¹ Correct answer.
  2. 8.6 × 10⁻⁶ K⁻¹
  3. 1.4 × 10⁻⁵ K⁻¹
  4. 1.2 × 10⁻⁵ K⁻¹

Explanation

Linear expansion follows ΔL = L₀αΔT, so α = ΔL/(L₀ΔT). Substituting gives 0.72 × 10⁻³ m divided by (1.2 m × 60 K), which equals 1.0 × 10⁻⁵ K⁻¹.

Question 42Thermal physics and energy transfer

In a bimetallic strip used as a thermostat switch, brass is bonded to invar. When the strip is heated, it bends because

  1. both metals contract equally but at different rates
  2. brass expands more than invar for the same temperature rise Correct answer.
  3. invar expands more than brass for the same temperature rise
  4. the two metals have identical expansion but different melting points

Explanation

A bimetallic strip bends because the two bonded metals have different coefficients of linear expansion. Brass has the larger coefficient, so it lengthens more and the strip curves toward the invar side.

Question 43Thermal physics and energy transfer

A gas is compressed at constant temperature so that its volume halves. According to the kinetic theory of gases, the pressure doubles mainly because

  1. the average kinetic energy of the molecules doubles
  2. the molecules themselves become larger
  3. the molecules collide with the walls more frequently per unit area Correct answer.
  4. the number of molecules in the container doubles

Explanation

At constant temperature the mean molecular speed and kinetic energy stay the same, but the molecules are confined to half the volume. They therefore strike each unit area of wall twice as often, doubling the pressure.

Question 44Thermal physics and energy transfer

An ideal gas occupies 300 cm³ at 27 °C and 1.0 × 10⁵ Pa. It is heated at constant pressure to 327 °C. What is its new volume?

  1. 150 cm³
  2. 600 cm³ Correct answer.
  3. 327 cm³
  4. 300 cm³

Explanation

At constant pressure, V/T is constant, and temperatures must be in kelvin: 300 K and 600 K. The absolute temperature doubles, so the volume doubles from 300 cm³ to 600 cm³.

Question 45Thermal physics and energy transfer

The specific latent heat of vaporisation of water is about 2.26 × 10⁶ J kg⁻¹. How much energy is needed to convert 50 g of water already at 100 °C into steam at 100 °C?

  1. 4.52 × 10⁶ J
  2. 2.26 × 10⁶ J
  3. 1.13 × 10⁵ J Correct answer.
  4. 2.26 × 10³ J

Explanation

The water is already at its boiling point, so only the latent heat is needed: Q = mL = 0.050 kg × 2.26 × 10⁶ J kg⁻¹. This gives 1.13 × 10⁵ J.

Question 46Thermal physics and energy transfer

A block of mass 2 kg is dragged 5 m along a rough horizontal floor by a horizontal force of 30 N while a friction force of 10 N acts on it. What is the net work done on the block?

  1. 150 J
  2. 50 J
  3. 100 J Correct answer.
  4. 200 J

Explanation

The net force along the motion is 30 N − 10 N = 20 N, and work is force times displacement. Thus W = 20 N × 5 m = 100 J.

Question 47Thermal physics and energy transfer

Which statement correctly describes the transfer of thermal energy by conduction in a solid metal?

  1. Energy is transferred by the movement of air pockets inside the metal
  2. Energy is transferred by electromagnetic waves through the metal
  3. Energy is transferred by free electrons and by lattice vibrations without bulk movement of the metal Correct answer.
  4. Energy is transferred by the bulk movement of the metal itself

Explanation

Conduction in metals occurs through collisions of free electrons and through vibrations passed between neighbouring atoms in the lattice. No bulk movement of the material is involved.

Question 48Thermal physics and energy transfer

Two identical rooms lose heat at the same rate. Room A has a single-glazed window of area 2 m² and thickness 6 mm. Room B has double glazing: two 3 mm glass panes separated by a 10 mm air gap. Given that air has a much lower thermal conductivity than glass, which room loses heat more slowly through the window, and why?

  1. Room B, because the low-conductivity air layer adds a large thermal resistance in series Correct answer.
  2. Room B, because the total thickness of glass is the same in both rooms
  3. Room A, because a single pane allows less radiation through than two panes
  4. Room A, because thinner glass conducts heat more slowly than thick glass

Explanation

Thermal resistance for conduction is thickness divided by conductivity. The air gap has very low conductivity, so it contributes a large resistance in series with the glass, reducing the overall heat flow in Room B.

Question 49Thermal physics and energy transfer

The efficiency of a heat engine is defined as

  1. useful work output divided by heat energy rejected
  2. total heat rejected divided by heat energy input
  3. heat energy input divided by useful work output
  4. useful work output divided by heat energy input Correct answer.

Explanation

Efficiency compares the useful work obtained with the heat energy supplied to the engine. It is always less than one because some heat must be rejected to a colder reservoir.

Question 50Thermal physics and energy transfer

A Carnot engine operates between a hot reservoir at 500 K and a cold reservoir at 300 K. What is its maximum theoretical efficiency?

  1. 20%
  2. 60%
  3. 40% Correct answer.
  4. 67%

Explanation

The Carnot efficiency is 1 − T_cold/T_hot, with temperatures in kelvin. Here 1 − 300/500 = 0.40, that is, 40%.