Heat (Thermal energy)
Thermal energy is what we call energy that comes from heat. For example, a cup of hot tea has thermal energy in the form of kinetic energy from its' particles. When you pour some milk into your tea, some of this energy is transferred to the particles in cold milk. What happens next? The cup of tea is cooler because it lost thermal energy to the milk.
We cannot discuss Thermal Energy without touching on Temperature.
The temperature of an object is to do with how hot or cold it is, measured in degrees Celsius (°C). Temperature can also be measured in in a Fahrenheit scale, named after the German physicist called Daniel Gabriel Fahrenheit (1686 – 1736). It is denoted by the symbol 'F'. In Fahrenheit scale, water freezes at 32 °F, and boils at 212 °F.
A thermometer is used to measure the temperature of an object.
Let's look at two examples to see the difference between
heat and temperature.
A swimming pool at 30°C is at a lower temperature than a cup of tea at 80°C. But the swimming pool contains more water, so it stores more thermal energy than the cup of tea.
To boil water we must increase its' temperature to 100°C. It takes longer to boil a large beaker of water than a small beaker because the large beaker contains more water and needs more thermal energy to reach 100°C.
Now let's see three very important forms of heat energy transfer.
These are Conduction, Convection and Radiation
When a substance is heated, its' particles gain energy and vibrate more vigorously. The particles bump into nearby particles and make them vibrate more. The particles pass the thermal energy through the substance by conduction, from the hot end to the cold end.
Click to see Video Illustration of heat transfer by conduction
Substances that allow thermal energy to move easily through them are called conductors. Metals are good conductors of thermal energy. Substances that do not allow thermal energy to move through them easily are called insulators. Air and plastics are insulators.
When particles in liquids and gases get warm, they become less dense, and they rise. The space is quickly replaced by cooler particles that are less dense (because they are heavier). Thermal energy is transferred from hot places to cold places (air or liquid) by convection.
All objects transfer thermal energy by infra-red radiation. The hotter an object is, the more infra-red radiation it gives off. No particles are involved in radiation, unlike conduction and convection. This means that thermal energy transfer by radiation can even work in space, but conduction and convection cannot.
One good example is the sun. Even though it is millions of kilometers away in space, we can still feel its heat. Radiation is how we can feel the heat of the Sun.