Total Energy of a System
Energy and Thermal Physics

Words used to describe energy

Teaching Guidance for 14-16 PRACTICAL PHYISCS

The topic of energy needs to be visited many times with a gradual increase in the sophistication and depth of the teaching. As there is no convenient definition of energy for beginners, the concept needs to develop slowly until students can write about energy without making mistakes; putting the right words into the right places. It benefits from a spiral approach to teaching.

As a first introduction you can show some interesting demonstrations concerned with energy transfer, which will prepare the ground for a fuller discussion.

See the experiments:

Jobs needing food or fuel

Moving energy from one thing to another 1

Moving energy from one thing to another 2

Energy should not get a reputation among students as a magic word that will answer any question about why things happen. Energy does not explain 'why' things happen. Students know a lot about food and what it does for them. They are interested in climbing hills, hauling up loads, shoving things along and in engines and what they will do. An informal approach to energy can be made by linking students’ natural knowledge of food and fuels with their interest in those activities. Energy is a way of working out if those activties are possible by doing calculations. Energy is a number that you calculate.

The vocabulary that we use when we discuss energy should reflect this ability to do calculations. This can be done by referring to 'types' or 'forms' of energy, but such names are misleading if the students come away with the idea that 'kinetic energy' is a different thing entirely from 'gravitational potential energy'. Here are some ways to talk about energy stored, all or which can be calculated.

  • ‘Energy stored elastically’ or ‘elastic potential energy’ referring to the energy stored in a stretched spring
  • ‘Energy stored gravitationally’ or 'gravitational potential energy' for the energy stored in a raised brick.
  • 'Energy stored kinetically’ or 'kinetic energy' for the energy stored a moving object.
  • 'Energy stored chemically' or 'chemical energy' for energy released from fuels or food and oxygen.
  • 'Energy stored thermally' or 'thermal energy' for energy stored in hot objects
  • ‘Energy stored vibrationally'
  • 'Energy stored electrostatically' or 'energy stored magnetically' for energy stored in electrostatic or magnetic fields.
  • 'Energy stored in nuclear fuel' or 'nuclear energy' for the energy stored in uranium.

As students progress then these simple descriptors will generalize into two labels: stored or potential energy and moving or kinetic energy.

Energy transfers are often more important than energy itself. If we haul bricks to the top of a building then the useful thing is that we have raised the bricks higher up. Energy stored chemically in a fuel and oxygen is now stored gravitationally in the raised bricks.

The energy can be transferred by working or heating. These are not 'types' of energy but mechanisms or processes for transferring energy.

  • Heating can be done by 'radiation' (of which 'light' is a subset). Radiation transfers energy away from a lamp, and produces a heating effect.
  • Heating can also be done through conduction and convection, such as when a saucepan of water is heated
  • Working can be done by forces, such as lifting a load
  • Working can be done electrically, for example, when a current flows in the wire in a toaster.

Indeed, machines built during Britain's industrial revolution to transfer the energy released by burning fossil fuels more and more efficiently, continue to change newly ‘industrialized’ societies for ever. This is most evident currently with economic growth in India and China.

Total Energy of a System
appears in the relation dU=dQ+dW
is used in analyses relating to Thermal Equilibrium
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