S12_2024
Official Answer Key
|
Q |
Ans |
Q |
Ans |
Q |
Ans |
Q |
Ans |
|
1 |
C |
11 |
D |
21 |
C |
31 |
B |
|
2 |
A |
12 |
C |
22 |
A |
32 |
A |
|
3 |
A |
13 |
A |
23 |
D |
33 |
C |
|
4 |
B |
14 |
B |
24 |
A |
34 |
C |
|
5 |
A |
15 |
A |
25 |
A |
35 |
D |
|
6 |
D |
16 |
D |
26 |
C |
36 |
C |
|
7 |
C |
17 |
C |
27 |
D |
37 |
B |
|
8 |
B |
18 |
C |
28 |
C |
38 |
C |
|
9 |
C |
19 |
B |
29 |
A |
39 |
A |
|
10 |
D |
20 |
A |
30 |
C |
40 |
C |
Detailed Explanations
Q1 — Correct answer: C
A micrometer is used to measure very small lengths accurately, such as the diameter of a wire or thickness of a small object.
So it measures length.
Why others are wrong:
- A, area: area is calculated from length measurements, not directly measured by a micrometer.
- B, current: measured by an ammeter.
- D, mass: measured by a balance.
Q2 — Correct answer: A
The boat moves at constant velocity, so the resultant force on it is zero.
The two rope tensions act forwards at angles. Their resultant is forwards. The drag must act exactly opposite to this resultant to balance it.
So the drag vector is A.
Why others are wrong:
- B, C and D: these do not exactly oppose the resultant of the two rope tensions, so they would not balance the forces.
Q3 — Correct answer: A
Acceleration is the gradient of a speed-time graph.
The largest acceleration is where the graph is steepest.
Section A has the greatest positive gradient.
Why others are wrong:
- B and C: smaller gradients, so smaller acceleration.
- D: not the steepest positive gradient.
Q4 — Correct answer: B
The man measures:
- distance walked
- time taken
Average speed is:
average speed = total distance / total time
So average speed can be calculated.
Why others are wrong:
- A: acceleration needs change in velocity and time, not just distance and time.
- C: average velocity needs displacement and direction, not just distance walked.
- D: power needs work done or energy transferred per time.
Q5 — Correct answer: A
Resistance to change in motion is called inertia.
Mass is the measure of inertia.
So the larger the mass, the harder it is to change the object’s state of rest or motion.
Why others are wrong:
- B: shape may affect air resistance, not inertia.
- C: surface area affects drag/friction effects, not fundamental resistance to motion change.
- D: volume alone does not determine inertia.
Q6 — Correct answer: D
Weight is:
weight = mass × gravitational field strength
On Earth:
weight = 210 × 9.8 = 2058 N
On the Moon, gravitational field strength is 1/6 of Earth’s:
Moon weight = 2058 / 6
= 343 N
Closest answer = 340 N
Why others are wrong:
- A: weight is not zero on the Moon.
- B: 35 N is too small.
- C: 210 N confuses mass value with weight.
Q7 — Correct answer: C
Engine force = 800 N forwards.
If resistive force R is:
- less than 800 N: car accelerates
- equal to 800 N: constant velocity
- greater than 800 N: car decelerates
Option C says R = 900 N and motion is deceleration.
That is correct because the resistive force is greater than the engine force.
Why others are wrong:
- A: R = 500 N gives acceleration, not deceleration.
- B: R = 800 N gives constant velocity, not acceleration.
- D: R = 1000 N gives deceleration, not acceleration.
Q8 — Correct answer: B
Newton’s third law pair must be:
- same type of force
- equal in size
- opposite in direction
- acting on different objects
P = force on Moon due to Earth
R = force on Earth due to Moon
These are the gravitational action-reaction pair between Earth and Moon.
Answer B.
Why others are wrong:
- A: P and Q involve different interacting pairs.
- C: Q and S both involve the Sun, but they are not forces between the same two objects.
- D: R and S involve different force pairs.
Q9 — Correct answer: C
The box moves up the rough slope.
Friction acts opposite to the motion along the surface.
So friction acts down the slope.
That is force C.
Why others are wrong:
- A: likely the normal/contact force.
- B: likely applied force or component direction.
- D: not the friction direction along the slope.
Q10 — Correct answer: D
The most difficult thread to stretch is the one that has the smallest extension for the same load.
On an extension-load graph, this is the line with the smallest gradient.
Thread D extends the least for a given load.
Answer D.
Why others are wrong:
- A, B and C: they stretch more for the same load, so they are easier to stretch.
Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Q11 — Correct answer: D
A satellite moving in a circle needs a centripetal force.
Centripetal force always acts towards the centre of the circle.
Here, the force due to the planet acts towards the planet.
Answer D.
Why others are wrong:
- A: away from the planet, wrong.
- B: along the motion, would change speed rather than keep circular motion.
- C: opposite the motion, would slow the satellite down.
Q12 — Correct answer: C
Take moments about P.
Counterweight moment:
5800 × 2.3 = 13 340 N m
Load moment:
2300 × x
For no moment about P:
2300x = 13 340
x = 13 340 / 2300
x = 5.8 m
Answer C.
Why others are wrong:
- A: too small; would not balance the counterweight.
- B: too small.
- D: too large; would give too much moment from the load.
Q13 — Correct answer: A
Change in momentum:
change in momentum = mass × change in velocity
m = 750 kg
initial speed = 25 m/s
final speed = 35 m/s
Change in velocity:
35 – 25 = 10 m/s
Change in momentum:
750 × 10 = 7500 kg m/s
Answer A.
Why others are wrong:
- B: incorrect multiplication.
- C: uses one of the speeds incorrectly.
- D: likely uses total or doubled speed change incorrectly.
Q14 — Correct answer: B
The ball is dropped from rest, so:
u = 0
v = 14 m/s
g = 9.8 m/s²
Use:
v² = u² + 2as
14² = 0 + 2 × 9.8 × h
196 = 19.6h
h = 196 / 19.6
h = 10 m
Answer B.
Why others are wrong:
- A: too small.
- C: forgets the factor 2.
- D: much too large.
Q15 — Correct answer: A
Natural gas can be burned whenever needed, so it is available constantly over a 24-hour period.
Why others are wrong:
- B: solar cells do not work at night.
- C: tidal power depends on tide times, not constant all day.
- D: wind is variable.
Q16 — Correct answer: D
Input power = 15 W
Time = 30 min = 1800 s
Input energy:
E = Pt
E = 15 × 1800
E = 27 000 J
Wasted energy = 7000 J
Useful energy:
27 000 – 7000 = 20 000 J
Efficiency:
efficiency = useful output / total input
efficiency = 20 000 / 27 000
efficiency = 0.74
Answer D.
Why others are wrong:
- A: likely uses wasted/input incorrectly.
- B: too low.
- C: does not use the correct useful energy.
Q17 — Correct answer: C
Energy, work and latent heat are measured in joules.
Power is measured in watts.
So the quantity not measured in joules is power.
Why others are wrong:
- A: gravitational potential energy is measured in J.
- B: latent heat/energy is measured in J.
- D: work is measured in J.
Q18 — Correct answer: C
A liquid has:
- variable shape because it takes the shape of its container
- fixed volume because it is difficult to compress
Answer C.
Why others are wrong:
- A: fixed shape describes a solid.
- B: fixed shape and variable volume is not a liquid.
- D: variable volume describes a gas.
Q19 — Correct answer: B
When the dividing wall is removed, the gases diffuse.
Diffusion causes particles to spread out and mix evenly.
Since one gas is brown and one is colourless, the final appearance is pale brown gas throughout the box.
Answer B.
Why others are wrong:
- A and D: gases do not stay separated after a long time.
- C: after a long time, the gases are evenly mixed, not in distinct clouds.
Q20 — Correct answer: A
Drops form because water vapour in the warm air touches the cold bottle, cools, and changes into liquid water.
Gas to liquid is condensation.
Answer A.
Why others are wrong:
- B: conduction is heat transfer through contact.
- C: convection is heat transfer by movement of fluid.
- D: evaporation is liquid to gas, the opposite process.
Q21 — Correct answer: C
Specific heat capacity is:
the energy required to raise the temperature of 1 kg of a substance by 1 °C.
Answer C.
Why others are wrong:
- A: this describes temperature range, not specific heat capacity.
- B: this describes specific latent heat idea, not specific heat capacity.
- D: incomplete; it does not mention energy or mass.
Q22 — Correct answer: A
Infrared radiation is part of the electromagnetic spectrum.
All electromagnetic waves travel at the speed of light in a vacuum.
Answer A.
Why others are wrong:
- B: infrared is transverse, not longitudinal.
- C: infrared has lower frequency than ultraviolet.
- D: black surfaces are better emitters than white surfaces.
Q23 — Correct answer: D
Ultrasound has frequency greater than 20 000 Hz.
Use:
frequency = speed / wavelength
Speed = 330 m/s
A: wavelength = 250 cm = 2.5 m
f = 330 / 2.5 = 132 Hz
B: wavelength = 25 cm = 0.25 m
f = 330 / 0.25 = 1320 Hz
C: wavelength = 2.5 cm = 0.025 m
f = 330 / 0.025 = 13 200 Hz
None are above 20 000 Hz.
Answer D.
Why others are wrong:
- A, B and C: all frequencies are below the ultrasound range.
Q24 — Correct answer: A
In a normal eye, light from a distant object is focused clearly on the retina.
Diagram A shows rays focusing on the retina.
Why others are wrong:
- B: focuses behind the retina or incorrectly.
- C: focuses in front of the retina.
- D: does not show correct focusing on the retina.
Q25 — Correct answer: A
In visible light, wavelength increases in this order:
violet → blue → green → yellow → orange → red
From the options:
blue → green → yellow → red
is in increasing wavelength order.
Answer A.
Why others are wrong:
- B: red should come after yellow, not before.
- C: blue has shorter wavelength than green, yellow and red.
- D: this is decreasing wavelength, not increasing.
Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Q26 — Correct answer: C
A magnifying glass uses a converging lens with the object placed inside the focal length.
The image formed is:
- virtual
- upright
- magnified
Linear magnification is:
image length / object length
Answer C.
Why others are wrong:
- A and B: the image is not real in magnifying glass use.
- D: virtual is correct, but the expression is inverted.
Q27 — Correct answer: D
Ultrasound is used for:
- cleaning jewellery
- scanning an unborn baby
- sonar
It is not normally used for sterilising water.
Answer D.
Why others are wrong:
- A: cleaning jewellery is a standard ultrasound use.
- B: prenatal scanning uses ultrasound.
- C: sonar uses ultrasound/sound reflection.
Q28 — Correct answer: C
Potential difference is:
p.d. = work done / charge
or:
V = W / Q
Answer C.
Why others are wrong:
- A: voltage × current = power.
- B: energy / time = power.
- D: should be current × resistance, not resistance / current.
Q29 — Correct answer: A
All resistors are identical.
Circuits 1, 2, 3 and 5 have equivalent resistance equal to the same value. Circuit 4 has a different arrangement and therefore a different resistance.
Answer A.
Why others are wrong:
- B, C and D: include circuit 4 or exclude one of the equal-resistance circuits.
Q30 — Correct answer: C
A hairdryer uses the heating effect of electricity to heat air.
Answer C.
Why others are wrong:
- A: a phone mainly uses electrical energy for electronic functions.
- B: a fan mainly uses a motor effect to produce motion.
- D: a lawnmower mainly uses a motor to produce motion.
Q31 — Correct answer: B
Power = 1000 W = 1.0 kW
Time = 30 minutes = 0.5 h
Energy used:
E = Pt
E = 1.0 × 0.5
E = 0.50 kWh
Answer B.
Why others are wrong:
- A: wrong time conversion.
- C and D: far too large.
Q32 — Correct answer: A
A kettle with a stainless-steel outer casing has a metal case.
Metal-cased appliances must be earthed so the case cannot remain live if a fault occurs.
The fuse must be in the live wire so it disconnects the appliance from the live supply if current is too large.
Answer A.
Why others are wrong:
- B: fuse should not be in the neutral wire.
- C: metal casing must be earthed.
- D: no earth and fuse in neutral are both unsafe.
Q33 — Correct answer: C
The earth wire provides a low-resistance path for current if the live wire touches the metal case.
This causes a large current to flow and blows the fuse, preventing electric shock.
Answer C.
Why others are wrong:
- A: the live and neutral complete the operating circuit, not the earth wire.
- B: earth wire is not for thermal energy transfer.
- D: the fuse protects against excessive current; earth wire mainly prevents shock.
Q34 — Correct answer: C
A current-carrying wire in a magnetic field experiences a force.
Using Fleming’s left-hand rule:
- first finger: magnetic field direction
- second finger: current direction
- thumb: force direction
For the given current and field directions, the force is into the page.
Answer C.
Why others are wrong:
- A and B: force is not horizontally left/right.
- D: opposite direction to the left-hand rule result.
Q35 — Correct answer: D
In a simple d.c. motor, the split-ring commutator reverses the current every half-turn.
The labelled part Q is the split-ring commutator.
Answer D.
Why others are wrong:
- A: coil is the rotating wire loop, not Q.
- B: magnet is the field source, not Q.
- C: slip rings are used in a.c. generators, not simple d.c. motors.
Q36 — Correct answer: C
A transformer works because:
- An alternating voltage across the primary coil produces a changing magnetic field in the iron core.
- A changing magnetic field in the iron core induces an alternating voltage across the secondary coil.
So statements 2 and 3 are correct.
Answer C.
Why others are wrong:
- 1: the secondary voltage is alternating, not unchanging.
- 4: an unchanging voltage does not produce a changing magnetic field.
Q37 — Correct answer: B
The electron has the smallest mass.
Alpha particles are much heavier because they contain 2 protons and 2 neutrons.
Protons and neutrons are also much heavier than electrons.
Answer B.
Why others are wrong:
- A: alpha particle is the heaviest option.
- C and D: neutron and proton are far heavier than an electron.
Q38 — Correct answer: C
Isotopes have:
- same number of protons
- different number of neutrons
Atoms 2 and 4 both have 14 protons, so they are the same element.
They have different numbers of neutrons:
- atom 2 has 22 neutrons
- atom 4 has 24 neutrons
Answer C.
Why others are wrong:
- A: different proton numbers.
- B: different proton numbers.
- D: different proton numbers.
Q39 — Correct answer: A
Radioactive unstable nuclei can emit:
- alpha particles
- beta particles
- gamma rays
X-rays are not normally emitted from the unstable nucleus during radioactive decay; they are produced by electron transitions or deceleration of fast electrons.
Answer A.
Why others are wrong:
- B: misses gamma rays.
- C: misses alpha and beta.
- D: includes X-rays and misses alpha/beta.
Q40 — Correct answer: C
The Sun is powered by nuclear fusion.
Hydrogen nuclei join together to form helium nuclei, releasing energy.
So the reaction is:
fusion of hydrogen into helium
Answer C.
Why others are wrong:
- A: hydrogen does not undergo fission into helium.
- B: helium does not fission into hydrogen to power the Sun.
- D: helium fusing into hydrogen is the wrong direction.
Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.