Power and domestic electric appliances

All electrical devices in the home should have labels on them that show the power rating of the device.

An example UK electrical manufacturer appliance labelThis label is on the bottom of a domestic kettle

The label will include:

  • the potential difference 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 transfers 2,000 J of energy per second from one store to another.

Other electrcial transfers in the home may include:

AppliancePower in WPower in kW
Clock100.01
Lamp500.05
Drill8000.8
Iron1,2501.25
Kettle2,4002.4
Hot water heater3,0003
Electric oven12,00012

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?

\text{E} = \text{P} \times \text t

= 50 × (10 × 60)

= 50 × (600)

\text{E} = 30,000 J

Energy, voltage and charge

When a charge moves through a potential difference, electrical work 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?

\text{V} = \frac{\text{E}}{\text{Q}}

\text E = \text V \times \text Q

\text{E} = 6 × 3

\text{E} = 18 J