Power of Electrical Appliances

• Energy is transferred between stores electrically by electrical appliances
• Energy is transferred from cells and other sources
• A moving charge transfers energy. This is because the charge does work against the resistance of the circuit
• Work done is the same as energy transferred
• Electrical appliances are designed to transfer energy to components in the circuit when a current flows
• Kettles transfer energy electically from the mains ac supply to the thermal energy store of the heating element inside the kettle
• Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan's motor
• No appliance transfers all energy completely usefully. The higher the current, the more energy is transferred to the thermal energy stores of the components (and then the surroundings)
• Energy transferred depends on power
• The total energy transferred by an appliance depends on how long the appliance is on for and its power
• The power of an appliance is the energy that it transfers per second. So the more energy it transfers in a given time, the higher its power
• The amount of energy transferred by electrical work is given by:

  Energy transferred (J) = Power (W) x Time (s)

  > E = Pt
• Example

  A 600 W microwave is used for 5 minutes. How long (in minutes) would a 750 W microwave take to do the same amount of work?

  • Calculate the energy transferred by the 600 W microwave in 5 minutes

  > E = Pt = 600 x (5 x 60) = 180,000 J

  • Rearrange E = Pt and sub in the energy you calculated and the power of the 750 W microwave

  > t = E / P = 180,000 / 750 = 240 s

  • Convert the time back to minutes

  > 240 / 60 = 4 minutes

  = So the 750 W microwave would take 4 minutes to do the same amount of work
• Appliances are often given a power rating - they're labelled with the maximum safe power that they can operate at. You can usually take this to their maximum operating power
• The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use. This helps customers choose between models
• On power rating, the lower the rating, the less electricity an appliance uses in a given time and so the cheaper it is to run
• A higher power doesn't necessarily mean that it transfers more energy usefully
• An appliance may be more powerful than another, but less efficient, meaning that it might still only transfer the same amount of energy (or even less) to useful stores