Thermal Conduction, Convection and Radiation

Question 1

Metals are especially good conductors of thermal energy because they contain:

Accepted Answer

free electrons — In metals, free (delocalised) electrons move quickly and carry energy through the metal, in addition to particle vibration, making metals the best conductors.

Question 2

Convection can take place in:

Accepted Answer

fluids (liquids and gases) — Convection needs particles that are free to move and carry energy as they flow, so it happens in fluids (liquids and gases) but not in solids or a vacuum.

Question 3

Energy reaches the Earth from the Sun mainly by:

Accepted Answer

radiation — Space between the Sun and Earth is a vacuum, so energy is transferred by infrared radiation, the only method that needs no medium.

Question 4

Which surface is the BEST absorber and emitter of thermal radiation?

Accepted Answer

Dull black — Dull, black surfaces are the best absorbers and best emitters of infrared radiation, while shiny, light-coloured surfaces are good reflectors and poor emitters.

Question 5

A shiny silvered surface is used on the inside of a vacuum flask mainly to:

Accepted Answer

reduce heat loss by radiation — A shiny silvered surface is a poor emitter and reflects radiation back, reducing energy loss (or gain) by radiation.

Question 6

In a convection current, hot fluid rises because it is:

Accepted Answer

less dense than the cooler fluid — When a fluid is heated it expands and becomes less dense, so it rises while denser, cooler fluid sinks, setting up a convection current.

Question 7

Which of these is the best thermal insulator?

Accepted Answer

Trapped air — Trapped air is a poor conductor and cannot easily convect when trapped, making it a good insulator, which is why it is used in double glazing and wool.

Question 8

A vacuum flask keeps a drink hot. The vacuum between its walls mainly prevents heat loss by:

Accepted Answer

conduction and convection — A vacuum has no particles, so it stops conduction and convection. Radiation is reduced separately by the silvered walls.

Question 9

During the day a sea breeze forms because the land heats up faster than the sea. This causes:

Accepted Answer

warm air over the land to rise, drawing in cooler air from the sea — The land warms faster, so air above it becomes less dense and rises (convection). Cooler denser air from over the sea flows in to replace it, forming a sea breeze.

Question 10

Compared with solids, most liquids and gases are:

Accepted Answer

poor conductors of thermal energy — In liquids and gases the particles are further apart, so conduction is slow; they are poor conductors and transfer most of their energy by convection instead.

Three ways heat travels

Conduction and convection

Radiation