Chapter 11 Thermal Processes

An experiment is set up to find out which metal is the best conductor of heat.

Balls are stuck with wax to rods made from different metals, as shown in diagram 1.

The rods are heated at one end. Some of the balls fall off, leaving some as shown in diagram 2.

Which labelled metal is the best conductor of heat?

• A

• B

• C

• D

Food is kept in a cool-box which uses two ice packs to keep it cool.

Where should the ice packs be placed to keep all the food as cool as possible?

• both at the bottom of the box

• both at the top of the box

• one at the front and one at the back of the box

• one on the left and one on the right of the box

Why does convection take place in a liquid when it is heated?

• Liquids expand when they are heated.

• Liquids start to bubble when they get close to boiling point.

• Molecules in the liquid expand when they are heated.

• Molecules near to the surface of the liquid escape into the air.

Two plastic cups are placed one inside the other. Hot water is poured into the inner cup and a lid is put on top, as shown.

Which statement is correct?

• Heat loss by radiation is prevented by the small air gap.

• No heat passes through the sides of either cup.

• The bench is heated by convection from the bottom of the outer cup.

• The lid is used to reduce heat loss by convection.

The diagram shows the cross-section of a vacuum flask containing a hot liquid in a cold room.

X and Y are points on the inside surfaces of the walls of the flask.

How is thermal energy transferred between X and Y?

• by conduction and convection

• by conduction only

• by radiation and convection

• by radiation only

Which statement about the transfer of thermal energy is correct?

• Convection can occur in air, but only when the air is trapped.

• Convection can only occur in a gas.

• Radiation cannot occur in air.

• Radiation can occur in a vacuum, but convection cannot.

Two otherwise identical cars, one black and one white, are at the same initial temperature. The cars are left in bright sunshine and their temperatures increase. During the night, their temperatures decrease.

Which car shows the greater rate of temperature increase and which car shows the greater rate of temperature decrease?

• A

• B

• C

• D

A liquid is heated in a beaker.

The density of the liquid changes as its temperature increases. This causes energy to be transferred throughout the liquid.

How does the density change and what is this energy transfer process?

• A

• B

• C

• D

Which processes occur in a metal to cause thermal conduction?

• A

• B

• C

• D

Four objects, made of the same material and having the same mass, are at the same temperature. The objects have different surfaces and different surface areas.

Which object emits infra-red radiation at the greatest rate?

• A

• B

• C

• D