S1 Heat

Question 1

1. Describe the difference between heat and temperature.

Answer 1

Heat is a type of energy and its units are Joules (J). It is a measure of the total energy in a material so depends on how much of the material there is.

Temperature is a measure of how fast the atoms in a material are moving or their average kinetic energy. It doesn’t depend on the amount of material. It is measured using a thermometer and its units are Degrees Celsius/Centigrade (ºC).

Question 2

2. Describe what happens to how fast the particles are moving when a substance gains heat energy (heats up) or loses heat energy (cools down)

Answer 2

When a material gains heat energy (heats up), the particles in it speed up.

When a material loses heat energy (cools down), the particles in it slow down.

Question 3

3. Explain changes in temperature in terms of heat being absorbed or emitted.

Answer 3

Heat is absorbed by an object to increase its temperature.
Heat is emitted by an object to decrease its temperature.

Question 4

4. Describe what is mean by melting and evaporation, including what happens to the heat energy.

Answer 4

Melting means changing from a solid to a liquid.

Evaporating means changing from a liquid to a gas.

Heat energy is absorbed (gained) when a substance melts or evaporates.

Question 5

5. Describe what is mean by solidifying and condensing, including what happens to the heat energy.

Answer 5

Solidifing means changing from a liquid to a solid.

Condensing means changing from a gas to a liquid.

Heat energy is emitted (lost) when a substance solidifies or condenses.

Question 6

6. What happens to temperature during changes of state?

Answer 6

Temperature remains constant during changes of state.

Question 7

7. E_h = ml

(Define symbols and units)

Answer 7

E_h - Heat Energy (J)

m - mass (g)

l - Change of State Constant (J/g)

Question 8

7. Example

Change of State Constant for melting ice is 334 J/g.
How much energy will it take to melt 3g of ice?

Answer 8

E_h = ?
m = 3g
l = 334 J/g
(Sometimes this is given in the question, sometimes you have to look it up in a table)

E<sub>h</sub> = ml
E<sub>h</sub> = 3 x 334
E<sub>h</sub> = 1002 J

Question 9

7. Example

Change of State Constant for melting ice is 334 J/g.
How many g of ice is there when 668 J of heat energy is needed to melt it?

Answer 9

E_h = 668 J
m = ?
l = 334 J/g
(Sometimes this is given in the question, sometimes you have to look it up in a table)

E_h = ml
668 = m x 334
(swap sides)
m x 334 = 668
(divide by 334 to get m on its own)
m = 668/334
m = 2 g

Question 10

8. Explain the conduction of heat.

Answer 10

Conduction is the main method of heat transfer in solids.

When you heat one part of a solid the atoms vibrate faster. They will then bump into the neighbouring atoms making them vibrate faster. The heat is transferred through the material while the atoms stay where they are.

Question 11

9. Explain what is meant by a thermal conductor and a thermal insulator, giving an example of each.

Answer 11

Materials which are good at transferring heat are called thermal conductors (e.g. copper, brass and other metals).

Materials which are poor at transferring heat by conduction are called insulators (e.g. glass, wood, air, most gases and liquids)

Question 12

9. Example: Explain why stone flooring usually feels colder than carpet to your bare feet.

Answer 12

The stone flooring and carpet will both be at the same temperature which is colder than your foot. The carpet is a good insulator so little heat escapes from your foot and it stays warm. The stone is a better conductor so more heat escapes from your foot and it feels cold.

Question 13

9. Example: Explain why the base of a frying pan is usually made of metal and the handle made of wood or plastic.

Answer 13

The base of a frying pan is made of a conductor to transfer the heat from the cooker to the food. The handle is made of an insulator to prevent the heat transferring to the user and burning them.

Question 14

9. Example: Give an example of thermal insulators used in your house and explain its purpose.

Answer 14

Thermal insulators are used in homes and buildings to keep them warm in winter. They do this by preventing the heat from escaping. Examples include bricks, loft insulation, double glazing and cavity wall insulation.

Question 15

10. Explain what is meant by convection.

Answer 15

Convection is the main method of heat transfer in fluids (liquids and gases). When a fluid is heated, it rises, taking the heat with it. When the fluid cools, it falls.

These loops of rising and falling are called convection currents.

Question 16

11. Explain what is meant by thermal radiation.

Answer 16

When an object is hotter than its surroundings it gives out waves known as thermal (or infra-red) radiation. This is another method of heat transfer. It can work through a vacuum like space since the waves don’t need a material to travel. This is how the Earth gets heat from the sun.

Question 17

12. Describe the effectiveness of dull black and white/shiny surfaces in emitting and absorbing heat radiation

Answer 17

Dull and dark objects are better at both emitting and absorbing heat radiation than shiny, light coloured objects.

Question 18

13. Describe what thermal images show.

Answer 18

Thermal images use the heat radiation emitted by objects to build a picture of their temperature. Hotter objects give out more heat radiation and appear brighter than colder or well-insulated objects.

Question 19

13. Explain the use of thermal images to investigate the effectiveness of insulation systems in homes or other buildings.

Answer 19

If insulation is effective then an image taken from outside will appear dark as there is little heat escaping. Bright spots on the image indicate areas where the insulation is not as effective.

If insulation is effective then an image taken from inside will appear bright as there is little heat escaping. Dark spots on the image indicate areas where the insulation is not as effective.