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### Heat and Temperature

#### temperatures

• Heat energy is the total kinetic energy of the particles in a substance (in Joules).
• Temperature (in Kelvin) is proportional to the average kinetic energy of the particles.
• Temperature differences tell us how easily heat is transferred. The bigger the temperature difference between an object and its surroundings the more easily heat will be transferred.
• Heat always flows from hot to cold

## MCQ Questions

1. If the two wires in the thermocouple are made from the same metal and are placed in different temperatures, the voltage produced will be
a. the minimum
b. the maximum
c. zero
d. constantly changing

2. Thermocouples are used to measure high temperatures provided the temperatures measured
a. do not exceed the melting points of the wires in the thermocouple
b. are equal to the melting points of the wires in the thermocouple
c. are not more than the boiling point of water
d. are above 0K

3. Which one of the following is suitable for measuring rapidly changing temperatures?
a. alcohol-in-glass thermometer
b. mercury-in-glass thermometer
c. clinical thermometer
d. thermocouple

4. Which of the following determines the range of a mercury thermometer?
a. length of the stem
b. thickness of the bulb
c. volume of the bulb
d. volume of the stem
e. volumes of the bulb and stem

5. The lengths of mercury thread in the uniform tube above the bulb of a mercury thermometer are:
20mm when the bulb is in melting ice
170mm when the bulb is in the steam above boiling water
50mm when the bulb is in liquid X.
What is the temperature of liquid X?

6. If heat energy is removed from an object, its temperature will normally
a. fall
b. fall then rise
c. stay the same
d. rise
e rise then fall

7. How can the sensitivity of a liquid-in-glass thermometer be increased?
a. use a liquid which is a better conductor of heat
b. use a thinner-walled bulb
c. use a longer tube
d. use a liquid of higher boiling point
e. use a tube with a narrower bore

8. Which of the following determines the range of a mercury thermometer?
a. length of the stem
b. thickness of the bulb
c. volume of the bulb only
d. volume of the stem only
e. volumes of the bulb and stem

9. Thermal expansion and contraction of metals are used in each of the following except
a. riveting steel plates
b. fixing axles for wheels
c. pressure cooker
d. fire alarm

10. A biimetallic strip is used in temperature control devices because the two metals used in a bimetallic strip
a. expand unequally when heated
b. are good conductors of heat
c. conduct heat at different rates
d. can bend easily

11. The density of a solid decreases when it is heated because
a. its mass decreases
b. its mass increases
c. its volume decreases
d. its volume increases

12. The reason for the bursting of water pipes during very cold weather is that
a. water pipes contract when cooled
b. water expands on freezing
c. ice expands on melting
d. the structure of the material for the pipe is weakened at low temperature

13. The volume of a gas when heated increases much more than for a solid or liquid because
a. the particles of a gas expand more
b. the attractive forces between particles of a gas are negligible
c. the particles of solids and liquids cannot move
d. the molecules of gas are lighter

14. When a narrow-necked glass vessel containing water at room temperature is immersed in hot water, the level of the water is seen to go down a little before it begins to rise. This is because
a. the initial heating causes water to contract
b. the glass vessel expands
c. the water evaporates
d. glass is a poor conductor of heat

15. Which of the following types of energy remains constant during a change of state?
a. internal energy
b. potential energy
c. kinetic energy
d. heat energy

16. The energy required for a change of state is called
a. chemical energy
b. state energy
c. latent energy
d. heat energy

17. If a substance expands on melting, increased pressure acting on it will
a. not change its melting point
b. increase its melting point
c. decrease its melting point
d. condense the substance

18. The reversed process of melting is
a. condensing
b. freezing
c. sublimation
d. burning

19. The boiling point of water in a pressure cooker is raised by
a. increasing the volume of water in the cooker
b. increasing the internal volume of the cooker
c. increasing the pressure in the cooker
d. increasing the pressure outside the cooker

20. If a sample of water boils at a temperature above 100ºC, the water
a. is pure
b. contains impurities
c. boils at a pressure lower than normal atmospheric pressure
d. boils in vacuum

21. Which one of the following properties of a liquid is determined by comparing its boiling point with its standard value?
a. purity of the liquid
b. density of the liquid
c. mass of the liquid
d. pressure in the liquid

22. A liquid evaporates at
a. temperatures above its boiling point
b. its boiling point
c. temperatures below its boiling point
d. temperatures above and below its boiling point

23. A liquid that evaporates easily is called
a. an evaporating liquid
b. a volatile liquid
c. an unstable liquid
d. a saturated liquid

24. The rate of evaporation is higher on a windy day because
a. the temperature of a windy day is higher
b. the temperature of a windy day is lower
c. the particles which escape are carried away by the wind
d. the liquid particles move faster

25. Which one of the following factors does not affect the rate of evaporation of water?
a. density of water
b. humidity of the surrounding air
c. motion of the surrounding air
d. temperature of the water

26. Which one of the following liquids is used as the cooling agent in a household refrigerator?
a. alcohol
b. ether
c. freon
d. liquid hydrogen

27. The specific latent heat of fusion is absorbed by a substance when it changes from
a. a liquid to a gas
b. a gas to a liquid
c. a solid to a liquid
d. a liquid to a solid

28. The latent heat of fusion supplied to a substance is used
a. to keep its particles in their fixed positions
b. by particles to break away from their fixed positions
c. to increase the average speed of its particles in their random motion
d. to decrease the average speed of its particles in their random motion

29. Steam at ordinary atmospheric pressure has more energy than boiling water because it has
a. a high specific heat capacity
b. a low specific heat capacity
c. the latent heat of fusion
d. the latent heat of vaporization

30. Ab evaporating liquid feels cool because it
a. gives out latent heat of fusion
b. absorbs latent heat of fusion
c. gives out latent heat of vaporization
d. absorbs latent heat of vaporization

31. In a vacuum flask, which methods of heat transfer are prevented by the vacuum?
a. conduction only
b. convection only
c. conduction and convection only

32. The temperature shown by a mercury-in-glass thermometer increases. Which of the following is constant?
a. density of the mercury
b. internal energy of the mercury
c. mass of the mercury
d. volume of the mercury

33. Which of the following increases when a liquid becomes a gas at its boiling point?
a. the average kinetic energy of the molecules
b. the molecular size
c. the molecular spacing
d. the total number of molecules

1. c
2. a
3. a
4. e
5. a
6. a
7. e
8. e
9. c
10. a
11. d
12. b
13. b
14. b
15. c
16. c
17. b
18. b
19. c
20. b
21. a
22. c
23. b
24. c
25. a
26. c
27. c
28. b
29. d
30. d
31. d
32. c
33. c

## Structured Questions and Solutions

1. A solid substance is placed in an oven which is set to temperature T. The graph illustrates how the temperature of the substance changes with time.
a. What is the state of the substance when the time is 1.0 min?
b. What is the state of the substance when the time is 4.0 min?
c. What is the state of the substance when the time is 8.0 min?
d. What is the state of the substance when the time is 9.0 min?
e. What is the state of the substance when the time is 13.0 min?
f. What is the state of the substance when the time is 14.0 min?
g. What is the state of the substance when the time is 15.0 min?
h. What is the melting point of the substance?
i. What is the boiling point of the substance?
j. What is the termperature T?
k. Explain why the rate of transfer of thermal energy into the substance is higher at the beginning than towards the end of the process?
l. Why is the temperature unchanged between the 15th and and the 16th minute?

Solution

1a. solid
1b. solid-liquid
1c. liquid
1d. liquid
1e. liquid-gaseous
1f. gaseous
1g. gaseous
1h. -100ºC
1i. 300ºC
1j. 350ºC
1k. The oven has a fixed temperature. As thermal energy flow from the oven to the substance, the temperature of the substance increases and so the temperature difference between the oven and the substance decreases. The temperature difference between two objects is a factor which determines the rate of transfer of thermal energy. A bigger temperature difference will cause a higher rate of transfer of thermal energy.
1l. The oven has a fixed temperature and the substance will eventually gain enough thermal energy from the oven to reach this temperature. When thermal equilibrium is reached between the substance and the oven, the substance stops increasing in temperature.