S11_2024
Official Answer Key
|
Q |
Ans |
Q |
Ans |
Q |
Ans |
Q |
Ans |
|
1 |
C |
11 |
C |
21 |
D |
31 |
B |
|
2 |
B |
12 |
C |
22 |
B |
32 |
B |
|
3 |
A |
13 |
D |
23 |
B |
33 |
D |
|
4 |
A |
14 |
D |
24 |
A |
34 |
D |
|
5 |
C |
15 |
A |
25 |
B |
35 |
C |
|
6 |
D |
16 |
C |
26 |
B |
36 |
B |
|
7 |
D |
17 |
B |
27 |
A |
37 |
D |
|
8 |
A |
18 |
D |
28 |
B |
38 |
C |
|
9 |
C |
19 |
D |
29 |
C |
39 |
B |
|
10 |
B |
20 |
C |
30 |
B |
40 |
B |
Detailed Explanations
Q1 — Correct answer: C
The balance reading changes from 20 g to 54 g.
Mass of object:
54 – 20 = 34 g
The water level changes from 10 cm³ to 20 cm³.
Volume of object:
20 – 10 = 10 cm³
So the object has:
mass = 34 g
volume = 10 cm³
Answer C.
Why others are wrong:
- A: gives wrong mass.
- B: uses the change in mass incorrectly as volume.
- D: volume is not 17 cm³; it is the rise in water level.
Q2 — Correct answer: B
A vector has magnitude and direction.
Force is a vector because it acts in a direction.
Why others are wrong:
- A, distance: scalar; no direction.
- C, mass: scalar.
- D, speed: scalar; velocity would be vector.
Q3 — Correct answer: A
From 0–2 s and 2–4 s, the car travels 10 m in each 2 s interval.
So speed is constant from 0–4 s.
After that, distances in equal time intervals are:
11 m, 13 m, 16 m, 23 m
The increases are not equal, so acceleration is non-uniform.
Answer A.
Why others are wrong:
- B: acceleration after 4 s is not uniform because the distance increases are not regular.
- C and D: the car is not at rest from 0–4 s because it travels 10 m every 2 s.
Q4 — Correct answer: A
Density = mass / volume
The two cubes have the same mass.
Side of X is twice side of Y.
Volume of cube = side³
So:
volume of X = 2³ × volume of Y = 8 × volume of Y
Since density is inversely proportional to volume for the same mass:
density of X / density of Y = 1/8 = 0.125
Answer A.
Why others are wrong:
- B: would be true only if volumes were equal.
- C and D: wrongly suggest X is denser, but X has larger volume for same mass, so it is less dense.
Q5 — Correct answer: C
The surface pushes back on the object with a contact force normal to the surface.
“Normal” means perpendicular to the slope.
So the force from the surface acts at right angles to the slope, away from the surface.
Answer C.
Why others are wrong:
- A: along the slope, not perpendicular.
- B: vertical upward, not normal to the slope.
- D: into the slope, impossible for the surface pushing back.
Q6 — Correct answer: D
Stopping distance is made of:
stopping distance = thinking distance + braking distance
So:
thinking distance = stopping distance – braking distance
Thinking distance is affected by the speed of the car, because during the driver’s reaction time the car continues moving.
Answer D.
Why others are wrong:
- A and B: use braking distance + stopping distance, which is wrong.
- C: expression is correct, but load carried mainly affects braking distance, not thinking distance.
Q7 — Correct answer: D
Spring obeys Hooke’s law, so extension is proportional to load.
6.0 N gives extension 4.0 cm.
3.0 N is half the load, so extension is half:
2.0 cm
Original length = 15.0 cm
Total length:
15.0 + 2.0 = 17.0 cm
Answer D.
Why others are wrong:
- A: gives extension only, not total length.
- B: halves the original length incorrectly.
- C: subtracts/uses the wrong length idea.
Q8 — Correct answer: A
Moment of a force is the turning effect of a force.
Formula:
moment = force × perpendicular distance from pivot
So statements 1 and 2 are correct.
Statement 3 is wrong because pressure on the pivot is not used in calculating moment.
Answer A.
Why others are wrong:
- B: includes false statement 3.
- C: misses statement 1.
- D: statement 3 is false.
Q9 — Correct answer: C
An object is most stable when its centre of gravity is lowest and remains well inside the base.
Object C has the lowest centre of gravity among the options.
So it is the most stable.
Why others are wrong:
- A and B: centre of gravity is higher.
- D: centre of gravity is low-ish, but C is lower and better positioned.
Q10 — Correct answer: B
Change in gravitational potential energy:
GPE = mgh
Only vertical height matters, not distance along the slope and not speed.
m = 5.0 kg
g = 9.8 N/kg
h = 3.0 m
So the calculation is:
5.0 × 9.8 × 3.0
Answer B.
Why others are wrong:
- A and C: these are kinetic energy-style calculations.
- D: uses 8.0 m, but GPE depends on vertical height, not horizontal distance.
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: C
Work done against gravity:
work = mgh
Only vertical distance matters.
m = 60 kg
g = 9.8 N/kg
h = 1.5 m
Work done:
60 × 9.8 × 1.5 = 882 J
Closest answer:
880 J
Answer C.
Why others are wrong:
- A and B: too small; likely ignore mass or g.
- D: uses the horizontal distance or wrong total distance.
Q12 — Correct answer: C
Solar cells produce electricity directly from light using the photovoltaic effect.
They do not involve a turbine.
Why others are wrong:
- A: hydroelectric stations use flowing water to turn turbines.
- B: nuclear stations use steam turbines.
- D: wind generators use rotating blades/turbines.
Q13 — Correct answer: D
Input power = 6.0 W
Time = 3.0 s
Input energy:
E = Pt
E = 6.0 × 3.0
E = 18 J
Wasted energy = 2.7 J
Useful energy:
18 – 2.7 = 15.3 J
Efficiency:
efficiency = useful output / total input
Efficiency = 15.3 / 18
Efficiency = 0.85
Answer D.
Why others are wrong:
- A: uses wasted/input incorrectly.
- B: equals wasted energy divided by input energy.
- C: not the correct useful/input ratio.
Q14 — Correct answer: D
Total pressure underwater:
total pressure = atmospheric pressure + water pressure
Water pressure:
p = ρgh
ρ = 1000 kg/m³
g = 9.8 N/kg
h = 8.0 m
p = 1000 × 9.8 × 8.0
p = 78 400 Pa
Total pressure:
100 000 + 78 400 = 178 400 Pa
Closest answer:
178 000 Pa
Answer D.
Why others are wrong:
- A: far too low.
- B: gives water pressure only and ignores atmospheric pressure.
- C: too low; does not add correct water pressure.
Q15 — Correct answer: A
Particles are in fixed positions in a solid.
Ice is solid water, so its particles vibrate about fixed positions.
Answer A.
Why others are wrong:
- B: liquid water particles move around each other.
- C: air is gas; particles move freely.
- D: water vapour is gas; particles move freely.
Q16 — Correct answer: C
The gas expands, so particles move further apart.
Temperature remains constant, so average kinetic energy remains constant.
Average speed of particles depends on temperature, so it remains unchanged.
Answer C.
Why others are wrong:
- A: average speed does not decrease if temperature is constant.
- B: average speed does not increase if temperature is constant.
- D: particles do not stay the same distance apart; the gas expands.
Q17 — Correct answer: B
For constant temperature, Boyle’s law applies:
P1V1 = P2V2
Pressure change is smallest when the ratio V1/V2 is smallest.
Check the ratios:
A: 50/10 = 5
B: 100/50 = 2
C: 400/25 = 16
D: 400/100 = 4
Smallest change is option B.
Why others are wrong:
- A: pressure increases by factor 5.
- C: pressure increases by factor 16, largest change.
- D: pressure increases by factor 4.
Q18 — Correct answer: D
In a liquid-in-glass thermometer, the liquid expands when heated.
The physical property that changes is the volume of the liquid.
Answer D.
Why others are wrong:
- A: the thermometer length itself does not measure temperature.
- B: mass of liquid stays constant.
- C: specific heat capacity is not what is directly observed.
Q19 — Correct answer: D
When a liquid is boiling, thermal energy is used as latent heat to separate particles.
So during boiling, the temperature remains constant.
Answer D.
Why others are wrong:
- A: boiling occurs throughout the liquid, not only at the surface.
- B: evaporation can occur at any temperature.
- C: evaporation can cool a liquid, so it can change its temperature.
Q20 — Correct answer: C
When water at the base is heated:
- it expands
- its density decreases
- it rises
Cooler, denser water sinks to replace it, producing a convection current.
Answer C.
Why others are wrong:
- A: convection is not caused by evaporation here.
- B: molecules do not individually rise because they “expand”; the water becomes less dense.
- D: water at the side falls because it is denser, not less dense.
Q21 — Correct answer: D
Shiny metal foil is a poor emitter of infrared radiation.
So it reduces heat loss by radiation and helps keep the food hot.
Answer D.
Why others are wrong:
- A: trapping air reduces convection; it does not increase it.
- B: metal is generally a good thermal conductor.
- C: shiny metal is a poor absorber, not a good absorber.
Q22 — Correct answer: B
Frequency is the number of complete waves or wavelengths passing a point per second.
Answer B.
Why others are wrong:
- A: amplitude is maximum displacement from rest position.
- C: speed is distance travelled per second.
- D: wavelength is distance between two corresponding points on a wave.
Q23 — Correct answer: B
A plane mirror forms an image that is:
- same size as the object
- virtual
- upright
- same distance behind the mirror as the object is in front
Answer B.
Why others are wrong:
- A: image is not real.
- C: image is not smaller and not real.
- D: virtual is correct, but image is not smaller.
Q24 — Correct answer: A
Use Snell’s law:
n = sin i / sin r
Given:
n = 1.8
i = 45°
So:
sin r = sin 45° / 1.8
sin r = 0.707 / 1.8
sin r = 0.393
r ≈ 23°
Answer A.
Why others are wrong:
- B: close but not correct.
- C: would mean no refraction.
- D: impossible here because ray bends towards normal entering glass.
Q25 — Correct answer: B
A converging lens brings parallel or spreading rays closer together.
Diagram B shows rays being refracted so that they converge.
Why others are wrong:
- A, C and D: do not show the correct converging behaviour of rays through a converging lens.
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: B
White light disperses in a prism.
Violet light is refracted more than red light.
In the diagram, the light at X is less deviated than the light at Y, so X is red and Y is violet.
Red light has lower frequency than violet light.
Answer B.
Why others are wrong:
- A: colours travel at different speeds in glass, causing dispersion.
- C: light at X refracts less, not more, than Y.
- D: when entering glass from air, light bends towards the normal, not away.
Q27 — Correct answer: A
Remote controls commonly use infrared radiation.
Communication from Earth to satellites uses microwaves.
Answer A.
Why others are wrong:
- B: ultraviolet is not used for satellite communication.
- C: remote controls use infrared, not visible light in this context.
- D: both parts are wrong.
Q28 — Correct answer: B
There are 11 claps, so there are 10 echo intervals between the first and eleventh clap.
Time for one echo interval:
9.4 / 10 = 0.94 s
This is the time for sound to travel to the wall and back.
Total distance travelled by sound:
distance = speed × time
distance = 320 × 0.94
distance = 300.8 m
Distance to wall:
x = 300.8 / 2
x = 150.4 m
Closest answer:
150 m
Answer B.
Why others are wrong:
- A: uses incorrect number of intervals or rounding.
- C: forgets correct interval calculation.
- D: forgets to divide by 2 for the echo journey.
Q29 — Correct answer: C
A compass needle is a permanent magnet.
The end of a compass that points towards the south pole of a nearby magnet is its north pole, because opposite poles attract.
From the diagram, X is attracted towards the magnet’s opposite pole, so X is a north magnetic pole.
Answer C.
Why others are wrong:
- A and B: compass needle is not an induced magnet; it is permanent.
- D: X is not south in the shown position.
Q30 — Correct answer: B
When plastic and wool are rubbed, electrons are transferred.
In this case:
- plastic gains electrons
- wool loses electrons
So:
plastic becomes negative
wool becomes positive
Answer B.
Why others are wrong:
- A: plastic gaining electrons cannot remain neutral.
- C: losing electrons would make plastic positive, not negative.
- D: wool gaining electrons would not be neutral.
Q31 — Correct answer: B
The correct circuit must contain:
- a heater symbol
- a fuse symbol
Circuit B contains both.
Why others are wrong:
- A, C and D: each misses either the fuse or the heater, or includes the wrong component combination.
Q32 — Correct answer: B
An LDR has high resistance in darkness and low resistance in light.
The lamp must switch on when there is no light.
The correct circuit places the LDR and fixed resistor as a potential divider so that in darkness the output switches the lamp on.
Answer B.
Why others are wrong:
- A, C and D: give the wrong LDR/lamp switching arrangement, so the lamp would not turn on correctly in darkness.
Q33 — Correct answer: D
Energy transferred electrically:
E = VIt
Given:
V = 4.0 V
I = 3.0 A
t = 10 s
E = 4.0 × 3.0 × 10
E = 120 J
Answer D.
Why others are wrong:
- A: gives power only, not energy over 10 s.
- B: uses current × time only.
- C: uses voltage × time only.
Q34 — Correct answer: D
The diagram shows a generator.
In a simple a.c. generator:
- the rotating coil is connected to slip rings
- the opposite pole to N is S
So:
X = S pole
Y = slip rings
Answer D.
Why others are wrong:
- A and C: split-ring commutator is used in a d.c. motor/generator arrangement, not this simple a.c. generator.
- B: slip rings are correct, but X should be S, not N.
Q35 — Correct answer: C
A loudspeaker works using an alternating current.
The current in the coil changes direction, producing a changing magnetic field.
This magnetic field interacts with the permanent magnet’s field, producing a force on the coil attached to the cone.
The cone vibrates, producing sound.
Statements correct:
2 and 3
Answer C.
Why others are wrong:
- 1: current is not direct; it is alternating.
- 4: the cone does not spin; it vibrates backwards and forwards.
Q36 — Correct answer: B
Transformer equation:
Vs / Vp = Ns / Np
Given:
Vp = 240 V
Vs = 12 V
Np = 600
Ns / 600 = 12 / 240
Ns = 600 × 12 / 240
Ns = 30
Answer B.
Why others are wrong:
- A: too few turns.
- C: would give no voltage change.
- D: would be a step-up transformer, not step-down.
Q37 — Correct answer: D
The alpha scattering experiment showed that:
- most of the atom is empty space
- the nucleus is very small
- most of the mass is concentrated in the nucleus
- nucleus is positively charged
It did not conclude that the nucleus contains protons and neutrons. That is later nuclear structure knowledge.
Answer D.
Why others are wrong:
- A, B and C: these are conclusions supported by the alpha scattering experiment.
Q38 — Correct answer: C
Gamma radiation is not affected by magnetic fields and is reduced by distance/shielding.
Moving the source closer to the detector increases the count rate because more gamma photons reach the detector.
Answer C.
Why others are wrong:
- A: lead absorbs gamma, so count rate decreases.
- B: gamma is uncharged, so magnetic field does not increase count.
- D: moving detector further away decreases count rate.
Q39 — Correct answer: B
Nuclear fusion is the joining together of two smaller nuclei to form a larger nucleus.
Answer B.
Why others are wrong:
- A: radioactive decay is not fusion.
- C: nuclei do not “melt”.
- D: splitting a nucleus is nuclear fission, not fusion.
Q40 — Correct answer: B
The Earth orbits the Sun in an ellipse.
Answer B.
Why others are wrong:
- A: orbital speed should use circumference based on orbital radius, not average diameter in that expression.
- C: the Moon orbits Earth in about one month, not the Sun.
- D: light from the Sun takes about 8 minutes 20 seconds, around 500 s, not 5.0 minutes.
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.