10 Transfer of Thermal Energy 21J 20N 20J [EUT]

Rods of the same shape and size are inserted through corks into a tank of hot water. Each rod is covered with a layer of solid wax that has a low melting point. After a period of time, some wax melts. On which rod will the wax melt first?

• all at the same time

• good thermal conductor

• bad thermal conductor

• moderate thermal conductor

A boy jumps into an indoor swimming pool. He notices that the water appears to get colder as he goes deeper underwater. This is due to convection. Which statement is correct?

• Cold water is more dense than warm water so it sinks to the bottom of the pool.

• Warm water is more dense than cold water so it rises to the surface of the pool.

• The molecules in cold water have more kinetic energy than the molecules in warm water so they move to the bottom of the pool faster.

• The molecules in warm water are closer together than the molecules in cold water so they rise to the surface of the pool.

Which description of the mechanism of thermal conductivity is correct?

• In insulators, conduction takes place by electron transfer and molecular vibrations.

• In insulators, conduction takes place by electron transfer only.

• In metals, conduction takes place by electron transfer and molecular vibrations.

• In metals, conduction takes place by electron transfer only.

A teacher shows his class a polystyrene cup. The cup is made from thick plastic with lots of tiny air bubbles in it. He asks the class why the cup is so good at keeping a hot drink warm. Three suggestions are made. 1. It contains air which is a poor thermal conductor. 2. The air is trapped in tiny bubbles so very little convection is possible. 3. The plastic is a poor thermal conductor. Which suggestions are correct?

• 1 and 2 only

• 1 and 3 only

• 2 and 3 only

• 1, 2 and 3

One ice cube is placed on a metal block. An identical ice cube is placed on a plastic block. The blocks are left next to each other on a table in a laboratory. Which ice cube melts first and why?

• The ice cube on the plastic block melts first because plastic is a good insulator of thermal energy.

• The ice cube on the plastic block melts first because plastic is a good conductor of thermal energy.

• The ice cube on the metal block melts first because metal is a good conductor of thermal energy.

• The ice cube on the metal block melts first because metal is a good insulator of thermal energy.

Two freezers X and Y are identical except that one has a door opening at the front and the other has a door opening at the top. Both doors are the same size and are opened for the same amount of time.

Which freezer gains the least amount of thermal energy in this time and why?

• X - cold air falls

• X - warm air falls

• Y - cold air falls

• Y - warm air falls

A room is heated by a radiator. The diagrams X and Y show two possible circulations of hot air, which heat the room.

Which diagram and reason explain the heating of the room by convection?

• X - air density decreases when air is heated

• X - air density increases when air is heated

• Y - air density decreases when air is heated

• Y - air density increases when air is heated

The diagram shows a vacuum flask used to keep a liquid warm. Which methods of heat loss are reduced by the vacuum between the silvered walls?

• conduction only

• conduction and convection only

• convection and radiation only

• conduction, convection and radiation

Equal volumes of water at 100°C are put in four containers. Two containers have matt black sides and two containers have shiny white sides. One of each type of container has a lid. The containers are left for two minutes.

Which container has the highest temperature?

• Container A (matt black sides)

• Container B (matt black sides with lid)

• Container C (shiny white sides)

• Container D (shiny white sides with lid)

Why are metals better thermal conductors than other solids?

• Metals contain free electrons which help transfer the energy.

• Molecules in metals are in fixed positions.

• Molecules in metals can move freely.

• Molecules in metals vibrate faster than those in other solids.