Heat

Question 1

Heat energy can be transferred by conduction, convection and radiation.

Distinguish between the three methods of heat transfer.

Answer 1

Conduction is the transfer of heat energy in a solid by passing on kinetic energy from molecule to molecule.

Convection is the transfer of heat energy by the circulation of the heated parts of a liquid or gas.

Radiation is the transfer of heat energy as electromagnetic (EM) waves, without the need for a medium.

Question 2

What is the thermometric property (i) of a thermocouple (ii) of a mercury thermometer.

Answer 2

(i) Length of liquid column

(ii) Electrical resistance

Question 3

Heat

Answer 3

Heat is a form of energy and is measured in joules.

Question 4

Temperature

Answer 4

Temperature is a measure of the hotness or coldness of an object.

Question 5

Describe the operation of the heat pump.

Answer 5

Coolant is evaporated inside the fridge.

It takes specific latent heat of vaporisation from the inside of the fridge to do this. (This lowers the temperature of the fridge inside).

The coolant is pumped outside the fridge and is compressed into a high pressure liquid.

It releases its latent heat of vaporisation at this point to the surrounding air.

The liquid coolant is pumped back inside the fridge and the process repeats again

Question 6

State two desirable physical properties of fluid used in a heat pump.

Answer 6

The fluid used in a heat pump would ideally have:
A low boiling point
A high specific latent heat of vaporisation.

Question 7

Thermometric Property

Answer 7

A physical property that changes measurably and repeatedly with temperature change.

Question 8

Emf

Answer 8

Emf is the voltage applied to an electric circuit.

Question 9

What is the SI unit of temperature?

Answer 9

Degrees Celsius

Question 10

What is the advantage of using the SI unit of temperature in scientific measurements?

Answer 10

The absolute temperature scale has the advantage that the temperature measured in Kelvin is directly proportional to the actual average energy of the particles, whose temperature is being measured.

Question 11

Describe a labatory experiment to demonstrate the principle of operation of a thermocouple.

Answer 11

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Question 12

Give the advantage of using a thermocouple thermometer instead of a mercury-in-glass thermometer.

Answer 12

Thermocouples have a number of advantages over mercury thermometers:

• They have a very wide range of temperatures at which they can operate.
• They respond very quickly to changing temperatures.
• They do not make use of such a hazardous material as mercury.

Question 13

On what thermometric properties are the following based: (i) the thermocouple thermometer and (ii) the mercury-in-glass thermometer.

Answer 13

(i) The emf of a thermocouple

(ii) Length of liquid column

Question 14

Latent heat

Answer 14

The latent heat of a substance is the heat energy required to change its state without a change in temperature.

Question 15

Storage heaters are frequently used to heat buildings. State the principle that underlies the operation of an electrical storage heater.

Answer 15

• Large heat capacity

Question 16

Explain why heat does not travel through solids by means of convection.

Answer 16

The particles cannot move freely.

Question 17

Explain why snow is slow to melt as the air temperature rises above 0 degrees celsius.

Answer 17

The latent heat of snow is large.

Question 18

How is energy transferred from the sun to earth?

Answer 18

Radiation

Question 19

Why is it necessary to have a standard thermometer?

Answer 19

Due to different thermometric properties vary differently with temperature.

Question 20

Give an advantage of geothermal heating system over a solar heating system.

Answer 20

• Geothermal system functions all the time

- Solar heating system works only during daytime

Question 21

Heat capacity

Answer 21

The heat capacity of an object is the heat required to change its temperature by 1 kelvin.

Question 22

Specific heat capacity

Answer 22

The amount of heat energy required to change 1kg of the substance by 1 kelvin.

Question 23

Specific latent heat

Answer 23

The amount of heat energy required to change the state of 1kg of the substance without a change in temperature.

Question 24

Specific latent heat of vaporisation

Answer 24

The amount of heat energy required to change 1kg of the substance from a liquid to a gas without a temperature change.

Question 25

Specific latent heat of fusion

Answer 25

The amount of heat energy required to change 1kg of the substance from a solid to a liquid without a temperature change.

Question 26

Solar constant

Answer 26

The amount of solar energy striking 1m^2 of the Earth’s atmosphere every second.

Question 27

U-Value

Answer 27

The amount of heat energy that can be transmitted across 1m^2 of its surface every second, so long as there is a temperature difference of 1K each side of the material.