3.1 Thermal Physics
3.1 Thermal Physics
3.1 Thermal Physics
A. conduction.
B. radiation.
2. This question is about modelling the thermal processes involved when a person is running.
When running, a person generates thermal energy but maintains approximately constant
temperature.
(a) Explain what thermal energy and temperature mean. Distinguish between the two
concepts.
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(4)
The following simple model may be used to estimate the rise in temperature of a runner
assuming no thermal energy is lost.
A closed container holds 70 kg of water, representing the mass of the runner. The water is
heated at a rate of 1200 W for 30 minutes. This represents the energy generation in the runner.
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(b) (i) Show that the thermal energy generated by the heater is 2.2 10 J.
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(2)
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(ii) Calculate the temperature rise of the water, assuming no energy losses from the
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water. The specific heat capacity of water is 4200 J kg K .
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(3)
(c) The temperature rise calculated in (b) would be dangerous for the runner. Outline three
mechanisms, other than evaporation, by which the container in the model would transfer
energy to its surroundings.
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(6)
A further process by which energy is lost from the runner is the evaporation of sweat.
(d) (i) Describe, in terms of molecular behaviour, why evaporation causes cooling.
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(3)
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(ii) Percentage of generated energy lost by sweating: 50%
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Specific latent heat of vaporization of sweat: 2.26 10 J kg
Using the information above, and your answer to (b) (i), estimate the mass of sweat
evaporated from the runner.
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(3)
(iii) State and explain two factors that affect the rate of evaporation of sweat from the
skin of the runner.
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(4)
(Total 25 marks)
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4. The distance between the 0C and 100C marks on a mercury-in-glass thermometer is 20 cm.
When the thermometer bulb is placed in a mixture of ice and salt, the mercury level is 4 cm
below the 0C mark. The temperature of the mixture is
A. +20C.
B. +5C.
C. 5C.
D. 20C.
(1)
Which one of the following gives the rise in temperature, in deg C, and the final temperature,
in C, of the solid?
A. 13 571
B. 13 25
C. 286 571
D. 286 25
(1)
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7. This question is about electrical components.
(a) In the space below, draw a circuit diagram that could be used to determine the current-
voltage (I-V) characteristics of an electrical component X.
com ponent X
(2)
The graph below shows the I-V characteristics for the component X.
I/A 6
0
8 6 4 2 0 2 4 6 8
V /V
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The component X is now connected across the terminals of a battery of emf 6.0 V and negligible
internal resistance.
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(1)
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(2)
A resistor R of constant resistance 2.0 is now connected in series with component X as shown
below.
X R
2 .0
(c) (i) On the graph in (a), draw the I-V characteristics for the resistor R.
(2)
(ii) Determine the total potential difference E that must be applied across component X
and across resistor R such that the current through X and R is 3.0 A.
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(2)
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(d) (i) A resistor is to be used as a temperature-measuring device. List two desirable
properties of such a device.
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(2)
(ii) Explain how a temperature scale could be constructed for this resistance
thermometer.
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(3)
(Total 14 marks)
C. the direction of thermal energy transfer between two bodies in thermal contact.
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9. The nuclear notation for lithium-7 is 3
Li . Which one of the following is the mass of 1 mol of
lithium-7?
A. 10 g
B. 7g
C. 4g
D. 3g
(1)
Two solid copper spheres, having different radii, undergo the same temperature change. A
student states that the change in internal energy of the two objects would be the same. Briefly
discuss this statement.
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(Total 3 marks)
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(2)
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(b) Discuss why even an accurate thermometer may affect the reliability of a temperature
reading.
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(2)
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(2)
(ii) The table below gives data for water and ice.
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specific heat capacity of water 4.2 kJ kg K
specific latent heat of fusion of ice 1
330 kJ kg
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(4)
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(d) (i) Distinguish between evaporation and boiling.
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(3)
(Total 15 marks)
12. A solid is at an initial temperature of 500 K. The solid is heated so that its temperature rises by
50 K.
What are the initial temperature and the temperature rise of the solid, as measured on the
Celsius scale of temperature?
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