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Energy is the ability to do work.

Energy makes things happen. It makes a car move, a person talk, alight bulb shine, a cooker cook food.

Forms of energy

  • Energy exists in many forms or types, these are:
  • Heat (thermal energy) energy
  • Movement (kinetic)energy
  • Light energy
  • Sound energy
  • Stored (potential) energy
  • Electrical energy

Potential/stored energy can be stored as:

  • Chemical energy e.g. in food, batteries, dynamite
  • Elastic or strain energy e.g. in a stretched elastic band, a compressed spring
  • Gravitational potential energy e.g. in a raised object, water falling down a waterfall, a rock suspended at the top of a mountain.

Now try to identify the forms of nergy in each of these:

Conservation of energy

We often say that energy cannot be created nor destroyed (you can't create new energy or destroy energy). You can only change it from one form to another. This is called the Law of Conservation of Energy.

This leads us to the concept of energy transformations (transfers)

Energy transfers

A light bulb transfers electrical energy into light and heat energy

Electrical energy (energy in) Light energy + Heat energy (Energy out)

We write these as Energy transfer diagrams:

Electrical energy —» Light energy + Heat Energy

Now write down energy transfer diagrams for these:

1. ........................................2.



Useful and wasted energy- Sankey diagrams

For each of these devices, one form of energy (out) is useful and the other is wasted (not useful), for example, for the light bulb, we need the Light energy (useful energy), not the Heat energy (wasted).

We use a Sankey diagram to show these. A Sankey diagram is a scale diagram showing how much of each we get.


The total Energy out is Equal to the Energy in- No energy is lost or created.

Energy efficiency

Efficiency is how good something is at doing its job, for a light bulb, this is how good it is at producing light.

A more efficinet light bulb produces more light and less heat.

Energy efficiency = Useful energy out/Total Energy in x 100%

So for the bulb above Efficiency = 20/80 X 100%


Energy saver bulbs are said to be more Energy efficient. What does this mean in terms of how much Light they produce compared to Heat energy?