Unit 11:  Heat and temperature                                    1   2   3   4   5   6

5. The transference of heat

The propagation of heat between two bodies can be made in three different ways: by conduction, by convection and by radiation.

5.1. Conduction

It is the way heat propagates in solids.

For transference of heat takes place, solids have to be in direct contact between them or with the source of heat. Heat will flow from the hotter body to the cooler body, until both are the same temperature.

When the solid is heated up, its atoms start to move much more quickly, and this increases their kinetic energy. These atoms start to collide with the atoms next to them, and transmit some of their energy to these atoms.

In conduction thermal energy is transmitted, but no matter. The particles vibrate more and separate, but they don't change their positions.

5.2. Convection

It is the way heat propagates in fluids (liquids and gases).

In this case it is also necessary the direct contact between both bodies or with the source of heat, but there is transmission of matter.

When a liquid is heated up in a container, the liquid at the bottom of the container starts to get warm first. As the temperature increases the liquid at the bottom expands and becomes less dense: the particles (molecules or atoms) move quickly separating from each other and occupying more volume (they increase their kinetic energy).

The liquid on the surface is colder, so it’s also denser and it fails. The liquid that has fallen to the bottom then heats up and the water that has risen, cools down. It forms a cyclical circulation called convection current.

As the particles move, the thermal energy is transferred.

5.3. Radiation

Radiation is the process of heat transfer between bodies in the vacuum.

So that it is not necessary the direct contact between them and matter is not transmitted.

The energy emitted by bodies during this process is called radiant energy (electromagnetic waves). So, for example, the Earth receives radiant energy from the Sun.

All matter absorbs radiation, but it can also reflect (throw back) some of it.

Animation: Heat transfer (Wisc-online)

Animation: Energy transfer and storage (BBC Bitesize)

5.1. Answer the questions:

a. Why are small gaps placed between sections of a road?

b. Why does the density of a liquid or a gas decrease when heated?

c. Why are heaters placed usually under windows near the floor

    while air conditioning units are placed close to the ceiling?

5.2. Indicate which type of transference of heat occur in each case:

a. In the handle of a pan that is put to the fire.

b. In the stove that is heating a room.

c. In the water of a saucepan that is put to the fire.

d. In chops that are grilled on a barbecue.

Heat transfer (linguaframe)