Energy and temperature

Temperature is a measure of the average kinetic energy of the particles in a substance. It shows how hot something is, and is measured with a thermometer or a temperature sensor. There are two commonly used temperature scales:

  • degrees Celsius (°C)
  • kelvin (K)

The difference between 1°C and 2°C is the same as the difference between 1 K and 2 K.

Temperature does not show the energy store associated with an object.

How do energy and temperature differ?

The energy stored in a substance or system is measured in joules (J). When energy is transferred by heating, it can:

  • change the energy stored in the system
  • change the physical state of the system
  • make chemical reactions occur

When energy is transferred to water in the liquid state, water molecules gain kinetic energy and speed up. The larger the mass of water, the more energy needed to raise the temperature by a given amount. This is because more molecules need to have their speed changed.

Example

One large beaker and one small beaker filled with water, being heated by bunsen burners. The smaller beaker of water is bubbling and turning into steam.

In the diagram above, the water in the two beakers has been heated by the same Bunsen burner for the same amount of time. This means that the same amount of energy is transferred to both. However, the smaller volume of water increases in temperature more. The same amount of energy has been transferred to a smaller number of particles, so on average each one moves faster than the particles in the other beaker.

Imagine a teaspoon of boiling water at 100°C and a large bowl of water at room temperature (about 20°C):

  • the teaspoon of water could not melt an ice cube - even though each particle has a greater energy store because there are relatively few of them
  • the bowl of water could melt an ice cube - even though on average each particle has a smaller energy store because there are many more of them, so the total energy available for transfer is much greater