Thermal Energy


  • To define thermal energy
  • To differentiate between thermal energy and temperature
  • To define heat
  • To describe how heat is measured
  • To describe the relationship between heat and thermal energy
  • To describe how heat can be transferred

                                                            Thermal Energy

What is thermal energy?

Thermal energy is the total kinetic energy of all particles in a substance. Thermal energy is measured in joules (J). Remember we defined kinetic energy in our last lesson as the energy associated with the state of motion of an object.

What is the difference between thermal energy and temperature?

  • Temperature is related to the average kinetic energy of particles. It is measured in kelvin
  • Thermal energy is the total kinetic energy of all the particles.

For example, a glass of water can have the same temperature as a lake, but the lake has much more thermal energy because the lake contains many more water molecules.

What is heat?

Heat is the energy transferred from an object at a higher temperature to an object at a lower temperature. For example, energy in the form of heat flows from warm drinks to a bucket of ice.

How is heat measured? Heat is measured in two ways:

  • Heat is measured in calories (cal).

One calorie is equal to the amount of energy needed to raise the temperature of 1 g of a substance by 1 °C.

  • Heat is also measured in joules (J) because heat is a measurement of energy.

One calorie is equal to 4.18 joules.

How is heat measured?

In nutrition, 1 calorie (don’t forget heat is measured in calories) is actually 1 kilocalorie, or 1,000 calories. To find out how many calories are in food, a sample of food is burned inside an instrument called a calorimeter. The change in temperature in the calorimeter is used to calculate how much energy is released from the food sample.

How is heat related to thermal energy?

When two objects at different temperatures comeinto contact, energy as heat flows from the

warmer object to the cooler object. When both objects are at the same temperature, no more energy as heat flows. Though two materials might have the same temperature, their thermal energies might be different. Thermal energy depends on how many particles (particles here is talking about atoms, you will know more about atoms when you resume) are present in the object

How can heat affect the state of an object?

The state of a substance depends on the speed of its particles (you remember state or phase of matter). Adding energy in the form of heat to a substance can result in a change of state (you know what happens when you boil water right?). Removing energy in the form of heat from a substance can result in a change of state (you know what happens when you freeze your water overnight)

Heat transfer

 Energy as heat is transferred in three main ways:

Conduction is the transfer of energy as heat from one substance to another through direct contact. As long as two objects are in contact, conduction continues until the temperatures of the objects are equal. Conductors allow this transfer.

What is conductor?

A conductor is a material that transfers energy as heat very well. Metals are typically good conductors. Unlike insulators

An insulator is a material that is a poor conductor of energy as heat. Wood, paper and plastic are examples of good insulators.


What is convection? Convection is the transfer of energy as heat bythe movement of a liquid or gas.Convection occurs when a cooler, denser mass ofgas or liquid replaces a warmer, less dense massof gas or liquid by pushing it upward.

When water is boiled, the water moves in roughly circular patterns because of convection. This motion is due to density differences that result from temperature differences. The motion is called convection current


What is radiation? Radiation is the transfer of energy byelectromagnetic waves.All objects, including the sun and all living things,emit radiation. When radiation is emitted from one object and isabsorbed by another, the result is often a transferof heat (this is what happens in solar inverter).Radiation can travel through empty space.

Practical Uses of Radiation

A solar cooker is a device that cooks food using mirrors (check the top of your roof) that concentrate radiation from the sun. In parts of the world that are far from electricity and clean water, solar cookers are a useful way to sterilize water for drinking. Many people like to use solar cookers because they do not release harmful emissions.


  1. In your jotter, write, read and reflect on the three modes of heat transfer
  2. Explain the following terms i. Heat   ii.  Temperature   iii. Thermal energy
  3. State the units for measuring the terms listed in question 2