HKCEE 1990 - 1998
PHYSICS - Long Question
Heat
(1990-CE-PHY I - 4)
4.
Figure 5 shows the cooling curve of a substance changing from liquid to solid state.
(a)
Given a boiling tube half filled with this substance in its solid state, describe, with the
help of a diagram, an experiment to obtain the cooling curve of the substance.
(5 marks)
(b)
Read from the above cooling curve the melting point of the substance.
(1 mark)
(c)
Explain why the temperature remains constant as the substance solidifies at its melting
point, even though heat is lost to the surroundings.
(2 marks)
(d)
If the mass of substance used is 0.05 kg and the rate of heat loss to the surroundings at its
melting point is 25 W, find the specific latent heat of fusion of the substance.
(3 marks)
(e)
Describe the arrangement and motion of the molecules of the substance in the
(i)
solid state, and
(ii)
liquid state.
(4 marks)
(1991-CE-PHY I - 5)
5.
Figure 7
Figure 7 shows the apparatus of an experiment to study the absorption of solar energy by gases.
Identical flasks A and B are filled with carbon dioxide and air respectively. They are placed
under sunlight and their temperatures are taken at 1-minute intervals. The results are as follows :
Time/minutes
0
3
6
9
12
15
18
Temperature in A /C
28.0
38.4
44.0
46.2
47.8
48.8
48.8
Temperature in B / C
28.0
37.8
41.8
43.7
45.2
46.0
46.0
(a)
Using a scale that 2 cm represents 5C and 2 cm represents 3 minutes, plot the
temperature-time graphs for carbon dioxide and air on the same graph paper.
(5 marks)
(b)
Why does each of the gases reach a steady temperature ?
(2 marks)
(c)
The mass of carbon dioxide in flask A is 0.00196 kg and the mass of air in flask B is
0.00125 kg. The specific heat capacities of carbon dioxide and air are 640 J kg-1 K-1 and
740 J kg-1 K-1 respectively. Which flask of gas gains more energy to reach its steady
temperature ? Show your calculations.
(5 marks)
(d)
Each year the amount of carbon dioxide in the atmosphere is increased by billions of
tons.
(i)
Suggest a possible effect on the mean temperature of the earth.
(ii)
Suggest two methods to reduce the amount of carbon dioxide in the atmosphere.
(3 marks)
(1992-CE-PHY I - 4)
4.
Figure 6
A student uses the experimental set-up shown in Figure 6 to find the specific heat capacity of a
metal. The cylindrical metal block is heated by an immersion heater of unknown power. The
following results are obtained :
Mass of metal block = 1 kg
Initial temperature of metal block = 29C
Final temperature of metal block = 41C
Energy supplied by the heater = 12300J
(a)
Describe, with the help of a diagram, a method to measure the energy supplied by the
heater.
(4 marks)
(b)
Calculate the specific heat capacity of the metal.
(2 marks)
(c)
The value obtained in (b) is found to be higher than the actual specific heat capacity of
the metal. Suggest a reason for this and explain your answer briefly.
(3 marks)
(d)
Suggest TWO improvements on the set-up to increase the accuracy of the experiment.
(3 marks)
(e)
Is the above method suitable for finding the specific heat capacity of wood ? Explain
briefly.
(3 marks)
(1993-CE-PHY I - 4)
4.
Figure 5
A student performs an experiment to find the specific latent heat of vaporization of water. A
beaker containing water is placed on an electronic balance. The water is heated by a 100 W
immersion heater, which is immersed in the water such that it does not touch the beaker, as
shown in Figure 5.
(a)
It is found that there is a slight decrease in the mass of water in the beaker before the
water boils. Explain briefly in terms of molecular motion.
(3 marks)
(b)
When the water boils, the reading of the balance is taken. After 240 s, the reading of the
balance is taken again. The following results are obtained :
Initial reading of the balance = 525.4 g
Final reading of the balance = 515.2 g
Calculate
(i)
the energy supplied by the heater in 240 s,
(ii)
the specific latent heat of vaporization of water.
(4 marks)
(c)
The value obtained in (b) (ii) is found to be higher than the actual specific latent heat of
vaporization of water. Suggest a reason for this and explain briefly.
(3 marks)
(d)
If the student covers the beaker with a lid, how would the result of the experiment be
affected ? Explain briefly.
(3 marks)
(e)
Suggest TWO improvements on the set-up to increase the accuracy of the
experiment.
(2 marks)
(1994-CE-PHY I - 4)
4.
Figure 7
A student uses the set-up shown in Figure 7 to study the relationship between the pressure and
volume of a fixed mass of gas at constant temperature. The piston is pushed in or pulled out to
vary the volume of gas and the corresponding pressure is measured by the Bourdon gauge.
(a)
The following results are obtained in the experiment :
Pressure P/kPa
80
Volume V/m3
2.60
10
-4
Plot a graph of P against
100
120
140
160
x 2.10
x 1.75
x 1.50
x 1.31
10
-4
10
-4
10
-4
10
x
-4
1
on graph paper, with P ranging from 0 to 160 kPa and
V
ranging from 0 to 8000 m-3.
What physical law is the student attempting to verify ?
(5 marks)
(b)
Should the rubber tubing be long or short ? Explain briefly.
State TWO other precautions that should be taken to improve the accuracy of the
experiment.
(4 marks)
(c)
Explain, in terms of the kinetic theory of gases, the increase in the pressure of the gas
when the volume decreases at a constant temperature.
(3 marks)
(d)
Figure 8
The student uses another set of apparatus as shown in Figure 8 to measure the volume of
a container. Initially the readings of the syringe and the gauge are 1.8 x 10 -4 m3 and 100
kPa respectively. The piston is then completely pushed in and the reading of the gauge
becomes 210 kPa. Assuming that temperature remains unchanged, calculate the volume
of the container.
(3 marks)
(1997-CE-PHY I - 4)
4.
Figure 5
A student uses the apparatus shown in Figure 5 to perform an experiment to measure the
specific latent heat of fusion of ice. He uses a joulemeter to measure the energy required to melt
a certain amount of ice.
(a)
Draw a diagram to show how the apparatus can be set up for the experiment.
(3 marks)
(b)
The following data are obtained in the experiment :
Initial joulemeter reading
= 28 000 J
Final joulemeter reading
= 40 400 J
Mass of water collected in the beaker = 0.045 kg.
Calculate the specific latent heat of fusion of ice.
(3 marks)
(c)
Why should the ice used in the experiment be crushed ?
(2 marks)
(d)
A teacher comments that the result of this experiment is not accurate. She points out that
a control experiment is required in order to improve the accuracy of the experiment.
*(i)
Describe how the control experiment can be set up and explain its function.
(5 marks)
(ii)
After setting up the control experiment, the student repeats the above experiment.
Would you expect the specific latent heat of fusion obtained to be higher or lower
than that obtained in (b) ? Explain your answer.
(2 marks)