W11_2023
Official Answer Key
|
Q |
Ans |
Q |
Ans |
Q |
Ans |
Q |
Ans |
|
1 |
D |
11 |
D |
21 |
C |
31 |
C |
|
2 |
C |
12 |
C |
22 |
C |
32 |
B |
|
3 |
B |
13 |
B |
23 |
B |
33 |
D |
|
4 |
A |
14 |
B |
24 |
A |
34 |
D |
|
5 |
B |
15 |
D |
25 |
C |
35 |
C |
|
6 |
D |
16 |
D |
26 |
C |
36 |
C |
|
7 |
C |
17 |
A |
27 |
B |
37 |
D |
|
8 |
A |
18 |
D |
28 |
B |
38 |
B |
|
9 |
D |
19 |
D |
29 |
A |
39 |
B |
|
10 |
A |
20 |
C |
30 |
A |
40 |
B |
Detailed Explanations
Q1 — Correct answer: D
A scalar quantity has:
- magnitude
- unit
- no direction
So the correct description is magnitude and unit only.
Why others are wrong:
- A: direction only is not enough.
- B: direction and unit only is not a scalar.
- C: direction + magnitude + unit describes a vector.
Q2 — Correct answer: C
On a distance-time graph:
- gradient = speed
- decreasing gradient = deceleration
- horizontal line = stationary
Region C shows the graph becoming less steep, so the car is slowing down.
Why others are wrong:
- A: speed is increasing or not decelerating.
- B: constant gradient means constant speed.
- D: horizontal section means stationary, not decelerating.
Q3 — Correct answer: B
The wooden block moves at constant speed.
Constant speed means:
resultant force = 0
So the pushing force forward must equal the frictional force backward.
Answer B.
Why others are wrong:
- A: friction does not keep increasing if speed is constant.
- C: if friction were greater, the block would decelerate.
- D: if friction were less, the block would accelerate.
Q4 — Correct answer: A
The car begins moving, speeds up, then reaches constant speed.
During the journey:
- acceleration changes: it is positive while speeding up, then becomes zero at constant speed
- velocity changes: it increases until the car reaches constant speed
So both acceleration and velocity change.
Answer A.
Why others are wrong:
- B: velocity also changes.
- C: acceleration also changes.
- D: both do change.
Q5 — Correct answer: B
Initial spring length = 5.0 cm
Length with 10 N load = 8.0 cm
Extension with 10 N:
8.0 – 5.0 = 3.0 cm
For 20 N, load doubles, so extension doubles:
3.0 × 2 = 6.0 cm
New length:
5.0 + 6.0 = 11 cm
Answer B.
Why others are wrong:
- A: too small; does not add extension correctly.
- C: likely doubles total length incorrectly.
- D: doubles 8.0 cm, but spring length does not work like that.
Q6 — Correct answer: D
Moment of a force:
moment = force × perpendicular distance from pivot to the line of action of the force
The correct perpendicular distance from O to the line of action of F is d4.
So moment = F × d4.
Answer D.
Why others are wrong:
- A, B, C: use distances that are not the perpendicular distance from pivot to the force’s line of action.
Q7 — Correct answer: C
Take moments about the pivot.
The metre rule is uniform, so its weight W acts at the 50 cm mark.
Pivot is at 25 cm.
Distance of W from pivot:
50 – 25 = 25 cm
Force F acts at 100 cm.
Distance of F from pivot:
100 – 25 = 75 cm
For balance:
F × 75 = W × 25
F = W/3
Now use vertical forces.
Upward forces = downward forces
P + F = W
P + W/3 = W
P = 2W/3
Answer C.
Why others are wrong:
- A and B: too small.
- D: too large; it would not satisfy vertical force balance.
Q8 — Correct answer: A
Gravitational potential energy depends on height:
GPE = mgh
The chairs have equal mass, so the highest chair has the most gravitational potential energy.
Chair A is highest.
Why others are wrong:
- B, C and D: lower height means lower gravitational potential energy.
Q9 — Correct answer: D
Hydroelectric power uses the water cycle, so it is renewable.
Tidal power uses the regular motion of tides, so it is also renewable.
Answer D.
Why others are wrong:
- A: both are wrongly called non-renewable.
- B: hydroelectric is renewable.
- C: tidal is renewable too.
Q10 — Correct answer: A
Biofuels are renewable because they come from recently living material, such as plants or organic waste.
They can also be stored and used when needed, so they are available at all times.
Answer A.
Why others are wrong:
- B: nuclear fuel is not renewable.
- C: solar radiation is renewable, but not available at all times.
- D: wind is renewable, but intermittent.
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
Energy transferred:
E = Pt
Power = 10 W
Time = 5.0 minutes = 300 s
E = 10 × 300
E = 3000 J
Answer D.
Why others are wrong:
- A: divides instead of multiplying.
- B: uses 5 minutes as 5 seconds.
- C: misses a factor of 10.
Q12 — Correct answer: C
Liquid pressure:
pressure = density × g × depth
The pressure is greatest when:
- density is greater
- depth is greater
Liquid X has greater density than Y.
Beaker C has liquid X and the greatest relevant depth.
Answer C.
Why others are wrong:
- A: same liquid as C, but smaller depth.
- B and D: liquid Y has lower density.
Q13 — Correct answer: B
Pressure due to a liquid:
p = ρgh
Given:
density = 1000 kg/m³
g = 9.8 N/kg
depth = 5.0 m
So:
p = 5.0 × 1000 × 9.8 Pa
Answer B.
Why others are wrong:
- A: divides by g instead of multiplying.
- C: misses g.
- D: divides by depth instead of multiplying by depth.
Q14 — Correct answer: B
Initial gas pressure is 200 kPa above atmospheric pressure.
Atmospheric pressure = 100 kPa
So absolute pressure initially:
200 + 100 = 300 kPa
Volume doubles at constant temperature, so Boyle’s law applies:
P1V1 = P2V2
If volume doubles, pressure halves:
300 / 2 = 150 kPa
Answer B.
Why others are wrong:
- A: subtracts atmospheric pressure wrongly.
- C: ignores doubling of volume.
- D: pressure should decrease, not increase.
Q15 — Correct answer: D
On a heating curve:
- sloping section = temperature rises
- flat section = change of state
At X, the solid is melting.
At Y, the liquid becomes hotter.
At Z, the liquid is boiling.
Answer D.
Why others are wrong:
- A: puts boiling at X and melting at Z, reversed.
- B: misses melting.
- C: says X is solid heating, but X is the flat melting section.
Q16 — Correct answer: D
The fastest cooling happens when evaporation is fastest.
Wet paper provides water for evaporation.
A breeze removes water vapour, increasing evaporation.
Evaporation removes thermal energy from the bottle, cooling it.
Answer D.
Why others are wrong:
- A: aluminium foil reduces radiation loss.
- B: sunny place heats the bottle.
- C: foam is an insulator and reduces heat transfer.
Q17 — Correct answer: A
Thermal radiation emission depends on:
- surface temperature
- surface area
- surface colour/texture
Hotter objects emit more radiation.
Larger surface area emits more radiation.
Black/dull surfaces emit better than white/shiny surfaces.
So all three suggestions are correct.
Answer A.
Why others are wrong:
- B, C, D: each leaves out one correct factor.
Q18 — Correct answer: D
A plane mirror forms an image that is:
- virtual
- upright
- same size as the object
- same distance behind the mirror as the object is in front
Answer D.
Why others are wrong:
- A: not real and not inverted.
- B: not real.
- C: virtual is correct, but it is not inverted.
Q19 — Correct answer: D
Light travels from air into glass.
At the surface:
- part of the light reflects, with angle of incidence = angle of reflection
- part refracts into glass
- when entering glass, it bends towards the normal
Diagram D shows correct reflection and refraction.
Why others are wrong:
- A, B and C: either reflection angle is wrong, or the refracted ray bends the wrong way.
Q20 — Correct answer: C
During total internal reflection:
- all the light is reflected inside the denser medium
- angle of incidence = angle of reflection
Answer C.
Why others are wrong:
- A: TIR happens when light goes from denser to less dense medium, not air into glass.
- B: larger refractive index gives smaller critical angle.
- D: TIR happens when incidence angle is greater than the critical angle.
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.
Q21 — Correct answer: C
Linear magnification:
magnification = image height / object height
Image height = 6.0 cm
From the scale diagram, object height = 3.0 cm.
Magnification:
6.0 / 3.0 = 2.0
Answer C.
Why others are wrong:
- A: would mean the image is half the object size.
- B: too small.
- D: uses image height alone instead of the ratio.
Q22 — Correct answer: C
A healthy human ear hears approximately:
20 Hz to 20 000 Hz
Option C is:
50–5000 Hz
This range lies completely inside the audible range.
Answer C.
Why others are wrong:
- A: includes frequencies below 20 Hz.
- B: includes 5 Hz, below the audible range.
- D: includes 50 000 Hz, above the audible range.
Q23 — Correct answer: B
The aircraft becomes charged because of friction between the aircraft and the air.
Friction can transfer electrons, leaving the aircraft with a net charge.
Answer B.
Why others are wrong:
- A: background radiation is not the usual cause.
- C: heating alone does not explain charging.
- D: radio waves do not cause the aircraft to become charged in this context.
Q24 — Correct answer: A
Repulsion occurs only between like charges.
If both spheres are positive, they repel.
Answer A.
Why others are wrong:
- B: opposite charges attract.
- C and D: having the same number of electrons or protons does not necessarily explain repulsion; the important point is same net charge.
Q25 — Correct answer: C
The circuit contains a diode in series with the lamp.
A diode only allows current in one direction.
The lamp does not light because the diode is reverse-biased.
Reversing the battery connections makes the diode forward-biased, so current flows and the lamp lights.
Answer C.
Why others are wrong:
- A: changing to 12 V does not solve reverse bias and may damage a 6 V lamp.
- B: a fuse does not make current flow.
- D: reversing lamp connections makes no difference because a filament lamp works either way.
Q26 — Correct answer: C
Circuit P has two resistors in series:
Rtotal = R + R = 2R
Circuit Q has two resistors in parallel:
1/Rtotal = 1/R + 1/R
1/Rtotal = 2/R
Rtotal = R/2
Answer C.
Why others are wrong:
- A and B: circuit P is not R.
- D: circuit Q is not 2R; parallel resistance is smaller.
Q27 — Correct answer: B
Energy transferred electrically:
E = VIt
Rearrange:
V = E / It
E = 20 J
I = 2.0 A
t = 5.0 s
V = 20 / (2.0 × 5.0)
V = 20 / 10
V = 2.0 V
Answer B.
Why others are wrong:
- A: too small.
- C: uses wrong arrangement of the formula.
- D: far too large.
Q28 — Correct answer: B
Double insulation means the appliance has two layers of insulation or reinforced insulation.
Student P is correct:
- double-insulated appliances do not need an earth wire
Student Q is correct:
- double insulation protects the user from electric shock
Student R is wrong:
- double-insulated appliances can still contain fuses; double insulation does not remove the need for overcurrent protection.
Answer B.
Why others are wrong:
- A: includes R, which is false.
- C: includes R and misses Q.
- D: includes R and misses P.
Q29 — Correct answer: A
The original trace has a negative pulse followed by a positive pulse.
The new trace has the opposite order: positive then negative.
That means the opposite pole of the magnet enters the coil first.
The new pulses are further apart in time, meaning the magnet takes longer to pass through the coil. Since speed is constant, the coil must be longer.
Answer A.
Why others are wrong:
- B: same pole would not reverse the pulse order.
- C: shorter coil would make pulses closer together, not further apart.
- D: same pole and shorter coil are both wrong.
Q30 — Correct answer: A
When the magnet leaves the solenoid, the solenoid opposes the change.
The south pole of the magnet is moving away from end Y.
To oppose this, end Y becomes a north pole to attract the south pole back.
So:
- end Y is north
- force is attraction
Answer A.
Why others are wrong:
- B: north is correct, but the force is not repulsion.
- C: south would repel the leaving south pole, not oppose the change correctly.
- D: both polarity and force are wrong.
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.
Q31 — Correct answer: C
A simple a.c. generator produces an alternating voltage.
The voltage changes smoothly with time, going positive and negative.
This gives a sinusoidal waveform.
Graph C shows this.
Why others are wrong:
- A: only positive output; not alternating.
- B: square wave, not the output of a simple rotating-coil a.c. generator.
- D: triangular wave, not the normal generator output.
Q32 — Correct answer: B
Transformer equation:
Vs / Vp = Ns / Np
Given:
Vp = 230 V
Vs = 5.0 V
Np = 2500
Ns / 2500 = 5.0 / 230
Ns = 2500 × 5.0 / 230
Ns ≈ 54
Answer B.
Why others are wrong:
- A: too small.
- C: too large.
- D: would be a step-up transformer, not step-down.
Q33 — Correct answer: D
A d.c. motor uses a split-ring commutator.
The current reverses once every half-turn when the plane of the coil is perpendicular to the magnetic field.
This keeps the turning effect in the same direction.
Answer D.
Why others are wrong:
- A and B: slip rings are used in a.c. generators, not d.c. motor commutation.
- C: split ring is correct, but the orientation is wrong.
Q34 — Correct answer: D
A transformer needs:
- an iron core/rod to carry and strengthen the changing magnetic field
- an a.c. supply, because a changing current is needed to induce a voltage in the secondary coil
So:
rod = iron
supply = a.c.
Answer D.
Why others are wrong:
- A and B: plastic is not suitable for the transformer core.
- C: iron is correct, but d.c. will not produce continuous transformer action.
Q35 — Correct answer: C
The nucleus contains:
- protons
- neutrons
Electrons are outside the nucleus.
Answer C.
Why others are wrong:
- A: electrons are not in the nucleus.
- B: electrons are not in the nucleus and neutrons are missing.
- D: includes electrons incorrectly.
Q36 — Correct answer: C
Alpha particles:
- strongly ionising
- weakly penetrating
Beta particles:
- less ionising than alpha
- more penetrating than alpha
So:
- weaker ionising effect = beta
- weaker penetrating power = alpha
Answer C.
Why others are wrong:
- A: alpha does not have weaker ionising effect.
- B: both columns wrong.
- D: beta does not have weaker penetrating power than alpha.
Q37 — Correct answer: D
Beta particles are negatively charged.
In the magnetic field diagram, the force on the beta beam makes it follow path 3.
In the electric field, the positive plate is above and the negative plate is below. A beta particle is negative, so it is attracted towards the positive plate. It follows path 6.
Answer D.
Why others are wrong:
- A: gives wrong magnetic and electric deflection.
- B: magnetic path is wrong.
- C: no deflection in the electric field is wrong because beta particles are charged.
Q38 — Correct answer: B
Half-life is the time taken for half the radioactive nuclei in a sample to decay.
Radon-222 has a half-life of 3.8 days.
Starting with 1.0 mg:
after 3.8 days, half remains:
1.0 / 2 = 0.50 mg
Answer B.
Why others are wrong:
- A: 1.9 days is half of the half-life, so not enough time for half to decay.
- C: half-life does not depend on sample mass.
- D: after 7.6 days, two half-lives have passed, leaving 0.25 mg, not zero.
Q39 — Correct answer: B
The first stage in the life cycle of stars is a cloud of gas and dust.
Gravity pulls this material together to form a protostar.
Answer B.
Why others are wrong:
- A: black hole is a possible final stage for massive stars.
- C: red supergiant is a later stage.
- D: supernova is near the end of a massive star’s life.
Q40 — Correct answer: B
Redshift is:
the increase in observed wavelength of light from galaxies moving away from Earth.
As galaxies recede, their light is stretched to longer wavelengths.
Answer B.
Why others are wrong:
- A: frequency decreases, not increases.
- C: blue light/blueshift is linked to approaching objects, not receding galaxies.
- D: redshift refers to observed wavelength being increased, not simply light being emitted red.
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.