Electrical power
Electrical energy itself is not a useful form of energy for us, but it supplies energy to our appliances which transform it into other forms of energy which are useful.
An electric fire, a cooker, a toaster and an electric kettle all convert electrical energy into heat energy.
The The energy transferred each second, measured in watts (W). Power = work done ÷ time taken. of an appliance is the rate at which energy is transferred.
- \(Power = \frac{{energy}}{{time}}\)
- \(P = \frac{E}{t}\)
- The symbol for power is \(P\), it is measured in The unit of power.\((W)\)
- The symbol for energy is \(E\), it is measured in joules \((J)\)
- The symbol for time is \(t\), it is measured in seconds \((s)\)
Appliances in the home have their power ratings stated on their base plates:
Typical power ratings include:
| Appliance | Power rating \((W)\) |
| Kettle | 2000 |
| Microwave oven (cooking power of \(800\)) | 1000 |
| Toaster | 900 |
| Hair dryer | 1200 |
| Fridge | 135 |
| Appliance | Kettle |
|---|---|
| Power rating \((W)\) | 2000 |
| Appliance | Microwave oven (cooking power of \(800\)) |
|---|---|
| Power rating \((W)\) | 1000 |
| Appliance | Toaster |
|---|---|
| Power rating \((W)\) | 900 |
| Appliance | Hair dryer |
|---|---|
| Power rating \((W)\) | 1200 |
| Appliance | Fridge |
|---|---|
| Power rating \((W)\) | 135 |
Example
Question
Calculate the power rating of a hair dryer that is used for 2 minutes and uses 100 kilojoules of energy.
To calculate the power rating you need to take the following steps:
\(power = energy \div time\)
\(= 100000 \div (2 \times 60)\)
\(= 830 watts\,(830 W)\)
\(Energy = 830 W\)