Power and domestic electric appliances
All electrical devices in the home should have labels on them that show the power rating of the device.
The label will include:
- the required to make the device work correctly (≈230 V in the UK)
- the frequency of the supply - how often the supply changes direction (50 Hz = 50 times per second)
- the power rating in Watts (Joules per second)
2,000 W means that the kettle 2,000 J of energy per second from one store to another.
Other electrcial transfers in the home may include:
| Appliance | Power in W | Power in kW |
| Clock | 10 | 0.01 |
| Lamp | 50 | 0.05 |
| Drill | 800 | 0.8 |
| Iron | 1,250 | 1.25 |
| Kettle | 2,400 | 2.4 |
| Hot water heater | 3,000 | 3 |
| Electric oven | 12,000 | 12 |
| Appliance | Clock |
|---|---|
| Power in W | 10 |
| Power in kW | 0.01 |
| Appliance | Lamp |
|---|---|
| Power in W | 50 |
| Power in kW | 0.05 |
| Appliance | Drill |
|---|---|
| Power in W | 800 |
| Power in kW | 0.8 |
| Appliance | Iron |
|---|---|
| Power in W | 1,250 |
| Power in kW | 1.25 |
| Appliance | Kettle |
|---|---|
| Power in W | 2,400 |
| Power in kW | 2.4 |
| Appliance | Hot water heater |
|---|---|
| Power in W | 3,000 |
| Power in kW | 3 |
| Appliance | Electric oven |
|---|---|
| Power in W | 12,000 |
| Power in kW | 12 |
Example
How much energy is transferred each second when a 50 W lamp is turned on?
50 W is equivalent to 50 joules per second so a 50 W lamp transfers 50 joules every second.
Question
How much energy is transferred when a 50 W light bulb is turned on for 10 minutes?
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Energy, voltage and charge
When a charge moves through a potential difference, electrical is done and energy transferred. The energy transferred can be calculated using the equation:
energy = charge flow × potential difference
\(\text E = \text Q \times \text V\)
This is when:
- potential difference (\(\text{V}\)) is measured in volts (V)
- energy transferred (\(\text{E}\)) is measured in joules (J)
- charge flow (\(\text{Q}\)) is measured in coulombs (C)
One volt is the potential difference when one coulomb of charge transfers one joule of energy.
Example
What is the potential difference between two points if 2 C of charge shifts 4 J?
\(\text{V} = \frac{\text{E}}{\text{Q}}\)
\(\text{V} = \frac{4}{2}\)
\(\text{V}\) = 2 V
Question
How much energy is transferred when 3 C of charge moves through a potential difference of 6 V?
