Unit 2- Thermal physics (2.3 Revision)

One end of a rod of copper is placed in hot water. Thermal energy travels along the rod to make the other end warmer.

What is the behaviour of the copper at an atomic level that accounts for most of the transfer of thermal energy from one end to the other?

• Atoms at the hot end gain kinetic energy and move towards the other end.

• Atoms at the hot end expand, colliding with other atoms and transferring energy.

• Free electrons at the hot end gain energy and move towards the other end, colliding with atoms along the rod.

• Free electrons at the hot end gain energy from the hot water and move directly to the other end.

A surface is made so that it is a good source of infrared radiation.

Which surface is not suitable?

• a surface that is painted matt black

• a surface that is painted white

• a surface that is heated to a high temperature

• a surface that has a large surface area

In countries where it is usually hot, houses are often painted white.

What is the reason for this?

• White surfaces are good reflectors of radiant energy.

• White surfaces are good transmitters of radiant energy.

• White surfaces are good absorbers of radiant energy.

• White surfaces are good emitters of radiant energy.

On a cold day, a shiny metal rod feels colder to the touch than a black plastic rod.

Which statement explains this observation?

• The metal rod is a better absorber of infra-red radiation than the plastic rod.

• The metal rod is a better thermal conductor than the plastic rod.

• The metal rod is a worse absorber of infra-red radiation than the plastic rod.

• The metal rod is a worse thermal conductor than the plastic rod.

The air temperature in a cold room is 15 °C.

A heater that contains water at 50 °C is used to heat the room.

By which process is most of the thermal energy transferred throughout the air in the room?

• conduction

• convection

• evaporation

• radiation

Which statement about convection currents is correct?

• Convection currents occur because, when cooled, liquids contract and become more dense.

• Convection currents occur because, when warmed, liquids expand and become more dense.

• Convection currents only occur in liquids.

• Convection currents only occur in solids and liquids.

The metal surface of a kettle is hot.

What happens to the cool air outside the kettle when it comes into contact with the hot kettle?

• The density of the air decreases and the air falls.

• The density of the air decreases and the air rises.

• The density of the air increases and the air falls.

• The density of the air increases and the air rises.

In which does thermal conduction not occur?

• a gas

• a liquid

• a solid

• a vacuum

100 g of water at 25 °C is poured into an insulating cup. 50 g of ice at 0 °C is added to the water.

The water is stirred until the temperature of the water has fallen to 0 °C.

18 g of ice remains unmelted.

The specific heat capacity of water is 4.2 J / g °C.

Which value does this experiment give for the specific latent heat of fusion of ice?

• 210 J / g

• 330 J / g

• 580 J / g

• 770 J / g

Vacuum flasks usually have silvered walls that help to keep the contents of the flask hot.

Why are the walls silvered?

• to absorb thermal energy from the air around the flask

• to increase the rate of convection inside the flask

• to reduce energy loss to the surroundings by conduction

• to reflect thermal radiation back into the flask