Hot and cold
Touch can tell us whether something is hot or cold, but it is not reliable. Put one hand in hot water and the other in cold water for a minute, then put both into lukewarm water. One hand feels it as cold and the other as warm.
To measure how hot something is, we need a thermometer. The degree of hotness is called temperature, measured in degrees Celsius (°C).
Heat and temperature are not the same thing. Heat is a form of energy that flows from a hotter object to a colder one. Temperature tells us how hot an object is, and therefore which way heat will flow.
The clinical thermometer
A clinical thermometer measures human body temperature.
Range: 35 °C to 42 °C, because body temperature never goes far outside this range.
Normal human body temperature is about 37 °C.
It has a kink (a narrow bend) near the bulb. The kink stops the mercury from falling back on its own, so the reading can be read after the thermometer is taken out of the mouth.
How to use it:
Wash it, preferably with an antiseptic solution.
Hold it firmly and give it a few jerks so the mercury falls below 35 °C.
Place the bulb under the tongue for about one minute.
Read it with your eye level with the mercury thread.
Because mercury is toxic, many hospitals now use digital thermometers.
The laboratory thermometer
A laboratory thermometer measures the temperature of other objects and liquids.
Range: usually −10 °C to 110 °C.
It has no kink, so the reading must be taken while the bulb is still in the substance.
Keep it upright and make sure the bulb is surrounded by the substance without touching the bottom or sides of the container.
Never use a clinical thermometer to measure high temperatures, such as a flame or boiling water. It can break.
How heat travels
Conduction
In a solid, heat passes from the hotter part to the colder part, from particle to particle. This is conduction. Hold one end of a metal spoon in hot tea and the other end soon becomes hot.
Good conductors let heat pass easily: metals such as aluminium, copper and iron.
Poor conductors (insulators) do not: wood, plastic, glass, air.
This is why cooking pans are made of metal but have plastic or wooden handles.
Convection
In liquids and gases, the heated part becomes lighter and rises, and colder, heavier parts sink to take its place. These currents carry heat through the whole liquid or gas. This is convection. It is how water heats up in a pot.
Sea breeze and land breeze are convection on a large scale:
During the day, land heats up faster than the sea. Air over the land rises, and cooler air from the sea moves in. This is a sea breeze.
At night, land cools faster than the sea. The air over the sea is warmer and rises, and air from the land moves towards the sea. This is a land breeze.
Radiation
Heat from the Sun reaches us across empty space, where there are no particles to conduct or carry it. This transfer, which needs no medium, is radiation. You also feel radiation when you sit near a fire.
Clothes and colours
Dark surfaces absorb more heat than light ones. That is why we prefer light-coloured clothes in summer and dark clothes in winter.
Woollen clothes keep us warm because wool traps air between its fibres. Air is a poor conductor, so it stops body heat from escaping.
Summary
Mode | Needs a medium? | Main in |
|---|---|---|
Conduction | yes | solids |
Convection | yes | liquids and gases |
Radiation | no | can travel through a vacuum |