Heat is the transfer of energy from one thing to another.  When we say that an object is getting hotter, we mean that energy is being transferred to it.  If an object is getting colder, that means that energy is being transferred from it.

As an object gains heat, its thermal energy – its total kinetic and potential energy – increases.  Temperature is a measure of the average thermal energy of all the particles in a substance. People often use the terms “heat” and “thermal energy” are interchangeably.

Heat naturally flows from hotter to colder substances, unless energy is used to prevent this transfer, for example, when you use an air conditioner to prevent hot summer air from heating up the air in your home.

Convection is the transfer of thermal energy in liquids and gases is known.  When thermal energy is transferred in solids, we call this process heat conduction.

Weather and climate depend on convection in our air and oceans

Heat conduction within the Earth plays an important role in the dynamics of our planet.

Here are some experiments that will teach you more about heat.

Simple Thermometer

Liquids expand when they are heated and contract when they are cooled.  This is because an increase in kinetic energy when a liquid is heated causes its particles to move further apart from each other. The opposite happens when the liquid is cooled.

Old-fashioned liquid thermometers, such as those made with mercury or ethanol, depend on this principle.

Equipment Needed

1 glass tube
1 cork
1 bottle
1 bowl

The Experiment

Pour a cupful of water in the bottle. Stand the bottle upright in the bowl.

Insert the glass tube through the cork.

Place the cork in the bottle, making sure that it fits tightly and that the end of the tube reaches well below the surface of the water in the bottle.

Heat some water in the kettle and pour it in the bowl, so that it heats up the water inside the bottle, causing it to expand.

Place a mark on the side of the bottle to show the water level.

As the water in the bottle expands, it will rise up the glass tube.

Using Convection to Move Water

Like all gases, warm air takes up more space than cool air because the molecules in warm air have more kinetic energy. This means that warm air is less dense than cool air and warm air will rise above hot air.  You may notice that the air in a room is warmer near the ceiling than near the floor.  The movement of warm air above cold air has an important effect on climate and weather.

Equipment needed

1 candle stub
1 bottle with no lid
1 flat soup bowl - flat enough for the bottle to remain upside down in it without falling over

The Experiment

Fill the dish with water.

Light the candle stub and carefully float it in the dish.

Once the flame is burning steadily, turn the bottle upside down and use it to cover the candle.

Eventually the flame will go out.

As the air inside the bottle becomes warmer and expands, the air pressure in the bottle will decrease.

To equalize the pressure, the air outside of the bottle will press down upon the surface of the water of the dish, attempting to enter the bottle. This will force more water to enter the bottle, so the water level in the bottle will rise.