Rate of Heat Transfer

Question 1

What are the 3 things that effect the rate of heat transfer?

Answer 1

1. Surface area
2. Volume
3. The type of material

Question 2

Heat energy is ____ from the ____ of an object

Answer 2

Heat energy is radiated from the surface of an object

Question 3

How does the surface area affect the amount of infrared waves emmitted (or absorbed by) the surface?

How does this affect heat transfer?

Answer 3

A bigger surface area = more infrared waves emitted / absorbed

So the quicker the heat transfer

Question 4

Why do car and motorbike engines often have fins?

Answer 4

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

Question 5

What are heat sinks designed to do?

Example?

Answer 5

Transfer heat away from objects they’re in contact with

e.g. computer components

Question 6

How are heat sinks designed for their purpose?

Answer 6

They have fins (giving it a larger overall surface area) and a large surface area so they can emit heat as quickly as possible

Question 7

If two objects have the same surface area but a different volume, which if them will cool quicker?

Why?

Answer 7

The object with the smaller volume.

A higher proportion of the object will be in contact with its surroundings

Question 8

How does an objects material affect its rate of heat transfer?

Answer 8

Objects made from good conductors transfer heat more quickly than insulating materials e.g. plastic

Question 9

As well as the material the object is made of, what else affects the object’s rate of heat trandfer?

Answer 9

The materials the object is in contact with e.g. whether they are insulators or conductors

Question 10

What happens to the rate of heat transfer when an object is in contact with a conductor?

And an insulator?

Answer 10

The heat will be conducted away much faster than if it were in contact with a good insulator.

Question 11

How can a human reduce the rate of heat transfer frmo their body?

Answer 11

By curling up into a ball to reduce their surface area, reducing the infrared waves that can be emitted from their skin.

Question 12

What are the 4 ways a vacuum flask is designed to reduce the transfer of heat energy?

Answer 12

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

Question 13

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

Answer 13

To stop ALL conduction and convection through the sides

Question 14

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

Answer 14

To keep heat loss by radiation to a minimum

Question 15

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

Answer 15

To minimise heat conduction to or from the outer glass bottle

Question 16

Why is the stopper of a vacuum flask made of plstic and filled with cork or foam?

Answer 16

Because they are insulating materials and will reduce any heat conduction through it

Question 17

Why do the hairs on your skin ‘stand up’ in the cold? (2 parts)

Answer 17

1. To trap a thick layer of insulating air around the body.
2. This limits the amount of heat lost by convection

Question 18

Why do people go pink when they are hot? (2 parts)

Answer 18

1. When you are too warm, your body diverts more blood to flow near the surface of your skin
2. So that heat can be lost by radiation

Question 19

Why have arctic foxes evolved to have small ears with a small surface area?

Answer 19

To minimise heat loss by radiation and conserve body heat

Question 20

Why have desert foxes evolved to have large ears with a large surface area?

Answer 20

To allow them to lose heat by radiation easily and keep cool