2.1 Energy and power
One of the most fundamental principles in physics is that of the conservation of energy.
This means that in any process energy cannot be created or destroyed; it can only be transformed from one form into another. The total amount of energy is therefore said to be conserved.
This might or might not be a familiar concept to you, but here are a couple of examples to help explain it.
- Chemical energy stored in food is transformed when you digest what you eat into a form of energy that your body can use as ‘fuel’.
- Solar energy, from the Sun, is used in photosynthesis by plants to create chemical energy that enables the plants to grow and to perhaps produce food for the previous process!
The standard unit that energy is measured in is the joule (J). To get an idea of the size of the joule, look at these examples:
- energy content of a cereal bar: 800,000J
- energy stored in an AA battery: 9000J
- energy required to climb a flight of stairs: 3000J.
These are rough values and may vary a bit in reality, but they can give a general idea. In this course, you’ll be concerned with the transfer of nuclear energy into heat energy and then to electrical energy.
Power is the rate at which energy is transferred. For example, an electric fire transfers energy in the form of heat to its surroundings.
Energy can be related to power and time by the equation:
energy = power × time
power = energy/time
A watt (W) is the unit of power and it corresponds to an energy transfer of 1 joule per second. Many domestic appliances have their power given in thousands of watts, or kilowatts (kW), and electricity power stations normally have their outputs rated in millions of watts, that is, in megawatts (MW). The energy requirement for the UK as a whole is often given in gigawatt (GW). Each gigawatt is 1 billion (1 000 000 000) watts.
Appliances around your home will have power ratings on them which indicate how many joules of energy they transform every second.
For example, an average kettle is rated at 3kW. That means that it uses 3000J every second it takes to heat up and boil water. To make one cup of tea, the amount of water needed takes one minute to boil.
It takes 180,000J of energy to make a cup of tea. This is equivalent to the energy required to climb 60 flights of stairs!
When you plug in a kettle, this 180,000J is provided by the electrical energy supplied from the National Grid. Modern life requires a great deal of electrical energy to be produced and it is sometimes difficult to get a tangible idea of how much energy this is.
In the next section, you will be asked to estimate the energy requirements of appliances found around the home.