Power: connections to the everyday

Everyday knowledge re-purposed, so start with familiar appliances

Thinking about the learning

Many pupils will have already come across the unit of power, the watt, outside school. This may have been buying light bulbs at the supermarket, choosing an amplifier for a music system, or switching to the correct power setting for a microwave oven when heating up a snack.

These everyday experiences are likely to fit in with the science idea that the power output of an electrical device is a measure of the amount of energy it shifts each second. There's more to measure in a circuit than electrical current.

For example, many pupils will be familiar with the fact that light bulbs come in all shapes and sizes and are sold according to their wattage.

Thus, their bed-side light may use a 40 watt bulb and provide a relatively subdued light (dissipating 40 joule of energy per second). In comparison, the main light in their room might use a 100 watt bulb and be much brighter in comparison (dissipating 100 joule of energy per second).

Making such links to familiar domestic appliances can help pupils to come to understand the usefulness of electrical power, and serve as a motivator for introducing the idea of voltage.

Thinking about the teaching

Start with familiar electrical appliances. To introduce the idea of electrical power, you might demonstrate various electrical appliances with different power outputs and explain that the amount of energy each device gives out per second is called its electrical power.

A desk lamp radiates 100 joule of energy per second.

A microwave oven shifts 850 joule of energy every second as it cooks food.

A kettle shifts 2500 joule of energy every second as it warms up water.

By making comparisons between the appliances, the important idea to get over is that, for example, the kettle shifts energy 25 times more quickly than the desk lamp (2500 to 100).

The important ideas to develop are:

• The energy store is being emptied 25 times more quickly with the kettle.
• The household electricity supply meter spins around 25 times more quickly.
• The costs accumulate 25 times more quickly.

Thank goodness it only takes a matter of minutes to boil a kettle!