0625_s21_qp_42-Solved/ IGCSE Physics Past Exam Paper Solution

This document has 16 pages.
Cambridge IGCSE™
PHYSICS 0625/42
Paper 4 Theory (Extended) May/June 2021
1 hour 15 minutes
You must answer on the question paper.
No additional materials are needed.
INSTRUCTIONS
● Answer all questions.
● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs.
● Write your name, centre number and candidate number in the boxes at the top of the page.
● Write your answer to each question in the space provided.
● Do not use an erasable pen or correction fluid.
● Do not write on any bar codes.
● You may use a calculator.
● You should show all your working and use appropriate units.
● Take the weight of 1.0kg to be 10N (acceleration of free fall = 10m/s2).
INFORMATION
● The total mark for this paper is 80.
● The number of marks for each question or part question is shown in brackets [ ].
DC (ST/JG) 198540/2
© UCLES 2021 [Turn over
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SOLVED BY SMART EXAM RESOURCES/SMART EDU HUB
By Smart Edu Hub at 2:43 pm, Nov 28, 2024

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© UCLES 2021
1 (a) Fig. 1.1 shows a sealed weather balloon which is stationary in still air.
weather
balloon
instruments
Fig. 1.1
State whether the overall density of the balloon and its instruments is greater than, less than,
or the same as the density of the surrounding air.
............................................................................................................................................. [1]
(b) At night, the gas inside the balloon cools. The pressure of the air outside the balloon remains
the same.
(i) State whether the balloon rises, falls or remains stationary.
..................................................................................................................................... [1]
(ii) Explain your answer.
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [2]
It is same as density of surrounding air.
It falls
The volume of the balloon decreases and hence the density increases.
As a result the balloon falls.

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(c) An object is released from the balloon. It starts at rest and eventually reaches a constant
speed.
(i) On the axes of Fig. 1.2, sketch a speed–time graph to show this motion.
Fig. 1.2
[3]
(ii) State the values of the initial acceleration and the final acceleration of the object.
initial acceleration ..............................................................................................................
final acceleration ...............................................................................................................
[2]
[Total: 9]
10 m / s2 down
0

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© UCLES 2021
2 (a) Define the moment of a force.
............................................................................................................................................. [1]
(b) Fig. 2.1 shows an object of negligible weight. The object is in equilibrium.
12cm
20cm
pulley
50kg
mass
object
pivot P
rope
force F
Fig. 2.1
The object is free to rotate about its pivot P.
Calculate the value of force F.
F = ......................................................... [2]
(c) Describe an experiment involving vertical forces to show that there is no net moment on an
object in equilibrium. You may draw a diagram in the space provided.
[3]
[Total: 6]
Moment of a force is equal to force × perpendicular distance from pivot
(F1d1 = F2d2 =) 500 × 20 = F × 12
F = 10 000 / 12
= 830 N
830 N
Materials Needed
A uniform meter ruler
A pivot (such as a knife edge or a fulcrum)
Weights (masses with known values)
A clamp stand (optional, for stability)
Procedure
Set Up the Ruler on the Pivot:
Place the meter ruler horizontally on the pivot at its center (50 cm mark). Ensure that the ruler is balanced and level.
Add Weights to Create Forces:
Place weights at known distances from the pivot on both sides of the ruler. For example, place a 200g weight at the 30 cm
mark on one side and another 200g weight at the 70 cm mark on the other side.
You can use multiple weights to create different force scenarios.

Ensure Equilibrium:
Adjust the position of the weights if necessary to ensure the ruler remains horizontal and balanced.
Verify the Net Moment:
Measure the distances of the weights from the pivot.
Calculate the moments (force × distance) for each weight about the pivot.
Sum the moments on both sides of the pivot.
Calculations and Analysis
Assume the weight of the ruler is negligible or uniform.
Calculate the moments on either side of the pivot:
Moment on left side = 200g × (50 cm - 30 cm) = 200g × 20 cm
Moment on right side = 200g × (70 cm - 50 cm) = 200g × 20 cm
The moments are equal and opposite, ensuring there is no net moment, and the ruler remains in equilibrium.
Conclusion
When the ruler is balanced, it shows that the sum of the clockwise moments equals the sum of the counterclockwise
moments.
This demonstrates that there is no net moment on the object, confirming the condition for rotational equilibrium.

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3 Fig. 3.1 shows water flowing at very slow speed over a cliff edge.
water
rocks
15m
cliff edge
[4]
(b) 30kg of water flows over the cliff edge every second.
Calculate the force exerted by the rocks on the falling water. Ignore any splashing.
force = ......................................................... [3]
[Total: 7]
Fig. 3.1
The water falls 15m onto the rocks below.
(a) Show that the velocity of the water when it strikes the rocks is 17m/s.
P.E=K.E
mgh= (1/2) m v2 [ m gets cancelled from both sides]
2gh = v2
v2= 2 x 10 x 15
v2= 300
v= √300
v= 17.32m/s
F = change of p / change of time
F = 30 × 17.32 /1 [ note : momentum= mass x velocity]
F = 520 N
520 N

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© UCLES 2021
4 (a) Pollen particles are mixed into a liquid. They are seen to move when observed through a
microscope.
(i) Describe this movement.
(ii) Explain this movement in terms of the molecules of the liquid and the pollen particles.
...........................................................................................................................................
[3]
(b) (i) Medical professionals sometimes rub ethanol over the skin of a patient. Ethanol
evaporates readily at room temperature and has a high specific latent heat of vaporisation.
State whether the patient experiences heating, cooling or neither at the site where the
ethanol is applied. Explain your answer.
explanation ........................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [3]
(ii) State any effect on the rate of evaporation of ethanol when a fan blows air over the
patient’s skin.
..................................................................................................................................... [1]
[Total: 8]
This movement can be describes as random [ or / haphazard / zig-
[1]
The liquid water molecules move fast and collide with more massive
pollen particles. This causes the random movement of the liquid
statement
The thermal energy needed to evaporate ethanol is taken
from skin. As a result, most energetic liquid molecules
escape, leaving least energetic molecules behind. Hence
evaporation causes cooling
Rate of evaporation increases

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5 (a) A machine delivers a hot drink in a plastic cup, which is uncomfortably hot to hold.
Fig. 5.1 shows the cup with the hot drink.
hot drink
plastic cup
Fig. 5.1
Fig. 5.2a shows the cup with the hot drink and a holder for the sides of the cup.
Fig. 5.2b shows a cross-section through the holder. The holder is made from two strong paper
cylinders separated by a wavy piece of strong paper to make air gaps.
hot drink
plastic cup holder
holder
Fig. 5.2a Fig. 5.2b
Explain how using the holder makes it more comfortable to hold the cup.
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [3]
(b) A student carries out experiments on the cooling of the hot drink described in (a), with and
without the holder in place. He finds that the holder only reduces the rate of cooling slightly.
Suggest and explain another action that reduces the rate of cooling more effectively.
suggestion ................................................................................................................................
explanation ...............................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [3]
(c) State the method of thermal energy transfer from a star through the vacuum of space.
............................................................................................................................................. [1]
[Total: 7]
Air is a poor conductor. It reduces conduction.As a result, the temperature
of outside of holder is lower than cup and hence less thermal energy is
conducted to the hand.
put a lid on the cup
Because the convection from the surface is less when lid is
present
radiation

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© UCLES 2021
6 (a) Fig. 6.1 shows a ray of green light passing through a prism.
Fig. 6.1
A ray of blue light is directed towards the prism on the same path as the ray of green light.
On Fig. 6.1, draw the path of the blue light through and out of the prism. [3]
(b) The wavelength of the blue light in air is 4.8 × 10–7 m.
Calculate the frequency of the blue light.
frequency = ......................................................... [3]
[Total: 6]
v = fλ
Hence:
f= v / λ
= 3 × 108 ÷ 4.8 × 10–7
= 6.3 × 1014 Hz 6.3 × 1014

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7 (a) Fig. 7.1 shows two magnets and the gap between the N pole of one magnet and the S pole of
the other magnet.
Fig. 7.1
On Fig. 7.1, draw three lines to show the pattern and direction of the magnetic field in the
gap. [2]
(b) (i) Fig. 7.2 is a repeat of Fig. 7.1 showing the two magnets.
On Fig. 7.2, draw the position of a plotting compass needle when it comes to rest in the
gap between the N pole and the S pole.
Fig. 7.2 [1]
(ii) Explain why the needle comes to rest in this position.
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [2]
(c) Describe a method of demagnetising a bar magnet.
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [2]
[Total: 7]
Hammer the magnet , with the magnet lying in the magnetically E–W
direction
This is because the compass aligns with the field.N pole of the needle is
attracted to the S pole of the magnet

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© UCLES 2021
8 (a) Two identical radioactive sources emit α-particles and γ-rays into two vacuum tubes.
(i) Fig. 8.1 shows two electrically charged plates on either side of one of the vacuum tubes.
source
initial path of
beam of α-particles
and γ-rays
plate at +2500V
plate at –2500V
vacuum
Fig. 8.1
Write the symbol α once in Table 8.1 to indicate any deflection of the α-particles.
Write the symbol γ once in Table 8.1 to indicate any deflection of the γ-rays.
Table 8.1
into page out of page no deflection
towards bottom of
page
towards top of
page
[2]
α
γ

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(ii) Fig. 8.2 shows the poles of a very strong magnet on either side of the other vacuum
tube.
source
initial path of
beam of α-particles
and γ-rays
S pole of
strong magnet
N pole of
strong magnet
vacuum
S
N
Fig. 8.2
Write the symbol α once in Table 8.2 to indicate any deflection of the α-particles.
Write the symbol γ once in Table 8.2 to indicate any deflection of the γ-rays.
Table 8.2
into page out of page no deflection
towards bottom of
page
towards top of
page
[2]
α γ

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© UCLES 2021
(b) Fig. 8.3 shows a simple direct current (d.c.) electric motor with a split-ring commutator.
X
coil
brush
split-ring
N S
Fig. 8.3
(i) State and explain the direction of rotation of the coil as seen from point X.
statement ..........................................................................................................................
explanation ........................................................................................................................
..................................................................................................................................... [3]
(ii) The coil rotates through 90° from the position shown.
State what happens to the moment in this position.
..................................................................................................................................... [1]
(iii) The coil is rotated through 180° from the position shown. By considering the forces on
the coil, explain how the split-ring commutator enables the motor to turn continuously.
...........................................................................................................................................
..................................................................................................................................... [2]
[Total: 10]
clockwise
Because there is a force on L wire up and force on RH wire
down
There is zero moment
The urrent in coil changes direction , but the forces on the wires
still acts up on L and down on R

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9 (a) Fig. 9.1 shows a circuit.
Fig. 9.1
On Fig. 9.1, draw two clearly labelled arrows to show the direction of the electron flow and
the direction of the conventional current in the circuit. [2]
(b) The current in the motor is 13A. The charge on an electron is 1.6 × 10–19 C.
Calculate the number of electrons that pass through the motor every second.
number of electrons = ......................................................... [3]
[Total: 5]
Q=It
Hence: Q= It=13 x 1= 13C
Also;
Q= n x e
Hence
n= Q/e
n=13/1.6 x 10-19C
n= 8.1 x 1019 8.1 x 1019

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© UCLES 2021
10 (a) Fig. 10.1 shows the potential difference–current graph for a circuit component K.
0
0 1.0 2.0 3.0 4.0 5.0
current/mA
6.0
2.0
4.0
6.0
potential difference/V
8.0
10.0
Fig. 10.1
Calculate the resistance of component K when the current in it is 4.0mA.
resistance = ......................................................... [2]
From The Graph:
At I= 4.0mA=0.004A , V= 9.2V
V=IR
Hence;
R = V/I
= 9.2 / 0.004 = 2300Ω
2300Ω

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(b) Fig. 10.2 shows a circuit containing component K.
point X
resistor R
component K
Fig. 10.2
At low temperature, component K has a much greater resistance than resistor R.
At high temperature, component K has a much smaller resistance than resistor R.
State and explain the effect on the lamp when the temperature changes from very low to very
high.
Refer to the voltage at point X in your explanation.
statement ..................................................................................................................................
explanation ...............................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [4]
(c) State the name of component K.
............................................................................................................................................. [1]
[Total: 7]
thermistor
There is a much greater current in lamp
The resistance of thermistor is reduced .Hence the pd across R
increases and there is a larger current in the lamp

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© UCLES 2021
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Cambridge Assessment International Education is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of the University of
Cambridge Local Examinations Syndicate (UCLES), which itself is a department of the University of Cambridge.
11 (a) A student investigates a radioactive substance in a laboratory.
Fig. 11.1 is a graph showing the count rate detected as the substance decays for 7.5 minutes.
0
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
50
100
150
200
250
time/min
count rate
counts/min
Fig. 11.1
The background radiation is 20 counts/min.
(i) Determine the half-life of the substance.
half-life = ......................................................... [3]
(ii) Calculate the count rate detected at time = 9.6 minutes.
count rate = .......................................counts/min [2]
(b) The substance emits α-particles and γ-rays. The student suggests that it is safe to store the
substance in a plastic container of thickness 2mm.
State and explain whether the student’s suggestion is correct.
statement ..................................................................................................................................
explanation ...............................................................................................................................
............................................................................................................................................. [3]
[Total: 8]
incorrect
The container is 2mm plastic and does nor abosorb γ and hence
is penetrated by γ
initial count rate adjusted for background radiation= 220– 20 =) 200
After 1 half-life count rate adjusted for background radiation =) 100
Hence Half life= 2.4 min
(13 + 20 =) 32 33
Note 9.6 mins=4 half lives

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