Back to: Jss2 Basic Science (BST)

Introduction

Thermal refers to anything that has to do with heat. You will learn about heat flow and heat transfer in this lesson.

Heat flow

Because the temperatures are different, heat is the transfer of energy from one part of a body to another or from one body to another. Heat is a form of energy that is in motion. It always moves from a hot object to a cold object, as long as the sizes of the objects don’t change. Heat does not move from something cold to something hot. For example, if you put a metal spoon in a fire (like a stove) to heat it up, the heat from the fire flows to the spoon and warms up the handle. There will come a time when the spoon is too hot to hold. You can do this activity with the help of your teacher to really get a feel for how heat moves.

Heat transfer

Heat transfer is the process of moving heat energy from one body to another that is at a different temperature. Most of the time, heat moves through a system by conduction, convection, or radiation.

Conduction

This is how solids move heat from one place to another. For example, when a metal rod is heated in a fire, heat moves from the end that is closest to the fire to the end that is far away. Conductors are things like aluminium, silver, steel, iron, and copper that allow heat to move through them. Some things, like wool, glass, ceramic, and paper, don’t let heat move through them very well. These things are called insulators. Conductors like aluminium and steel are often used to make cooking utensils because they can transfer heat. Insulators like plastic and wood are used to make the handles of spoons, forks, knives, and pots because they can’t transfer heat. Wool clothes, which don’t let heat escape well, are often worn when it’s cold because they keep body heat from escaping.

Convection

This is how liquids and gases move heat from one place to another. When a liquid or gas is heated, the hotter particles at the bottom rise and the colder particles at the top sink. For example, when a pot of water is heated on a stove, the molecules of water at the bottom that are hotter move up, while the molecules of water at the top that are cooler sink. When a liquid or gas is heated, the molecules move around because of the convection current. 

Uses of convection

• Using convection to cool the engine of a car
• bodies of water like lakes and rivers, where the top is usually warmer than the bottom.
• The bottom floor of a building is usually colder than the top floor.

Radiation

This is the transfer of heat between two objects that are not touching. For example, heat energy from the sun gets to the earth through radiation, even though the sun and the earth don’t touch each other. Black surfaces are good at letting heat out, while white surfaces that shine are not. Radiation is used to: move heat energy from the sun to the earth; make black clothes warmer than brightly coloured ones; and make the sun shine brighter.