Rate of heat transfer

Question 1

Where is heat energy radiated from?

Answer 1

The surface of an object.

Question 2

What does the rate of heat energy transfer depend on?

Answer 2

• The surface area of the object.
• The volume of the object.
• The type of material.
• What materials the object is on contact with.

Question 3

Why does the rate of heat energy transfer depend on the surface area of an object?

Answer 3

The bigger the surface area, the more infrared waves that can be emitted from/absorbed by the surface, so the transfer of heat is quicker.

Question 4

Why do objects have large surface areas?

Answer 4

To maximise the amount of heat they transfer.

Question 6

Why do car and motorbike engines have fins?

Answer 6

They increase the surface area so heat is radiated away quicker, so the engine cools quicker.

Question 7

Why do heat sinks have fins and a large surface area?

Answer 7

Because they are devices designed to transfer heat away from objects they’re in contact with, so they need to emit heat as quickly as possible.

Question 8

Why does the rate of heat energy transfer depend on the volume of the object?

Answer 8

Because if the two objects at the same temperature have the same surface area but a different volume, the one with the smaller volume will cool more quickly as a higher proportion of the object will be in contact with its surroundings.

Question 9

Why does the rate of heat energy transfer of the object depend on the type of material?

Answer 9

Because objects made from good conductors transfer heat away more quickly than insulating materials (e.g. Plastic).

Question 10

What type of materials does the object being in contact with affect the rate of heat energy transfer?

Answer 10

• Insulators.

- Conductors.

Question 11

What objects have large surface areas to maximise the amount of heat they transfer?

Answer 11

• Radiators.
• Car and motorbike engines have fins.
• Heat sinks have fins (e.g. In computer components).

Question 12

What product is designed to reduce the rate of heat energy transfer?

Answer 12

A vacuum flask.

Question 13

How is the vacuum flask designed to reduce the rate of heat energy transfer?

Answer 13

• The glass bottle is double-walled with a vacuum between the two walls.
• The walls either side of the vacuum are silvered.
• The bottle is supported using insulating foam.
• The stopper is made of plastic and filled with cork or foam.

Question 14

Why is the glass bottle of the vacuum flask double-walled with a vacuum between the two walls?

Answer 14

It stops all conduction and convection through the sides.

Question 15

Why are the walls either side of the vacuum in the vacuum flask silvered?

Answer 15

To keep heat loss by radiation to a minimum.

Question 16

Why is the bottle of the vacuum flask supported using insulating foam?

Answer 16

It minimises heat conduction to or from the outer glass bottle.

Question 17

Why is the stopper in the vacuum flask made of plastic and filled with cork or foam?

Answer 17

To reduce any heat conduction through it.

Question 18

Why does the rate of heat energy transfer depend on the materials the object is in contact with?

Answer 18

If the object is in contact with a conductor, the heat will be conducted away much faster than if it is in contact with a good insulator.

Question 19

How do humans control heat transfer?

Answer 19

• When it’s cold the hairs on your skin stand up.

- When you’re too warm, your body diverts more blood flow near the surface of your skin.

Question 20

Why do the hairs on your skin stand up when it’s cold to control heat transfer?

Answer 20

This traps a thicker layer of insulting air around the body. This limits the amount of heat loss by convection.

Question 21

Why does your body divert more blood flow near the surface of your skin when you’re too warm to control heat transfer?

Answer 21

So that more heat can be lost by radiation (that’s why some people go pink when they get hot).

Question 22

How do animals control heat transfer?

Answer 22

• Some animals fur also stands up in the cold.

- In warm climates animals have larger ears than those in cold climates.

Question 23

What example of an animal has smaller ears in cold climates to control heat transfer?

Answer 23

Arctic foxes.

Question 24

Why do arctic foxes have smaller ears to control heat transfer?

Answer 24

This means they have a smaller surface area to minimise heat loss by radiation and conserve body heat.

Question 25

What example of an animal has huge ears in warm climates to control heat transfer?

Answer 25

Desert foxes.

Question 26

Why do desert foxes have huge ears to control heat transfer?

Answer 26

This means they have a large surface area to allow them to lose heat by radiation easily and keep cool.