Energy & Power in Appliances (Oxford AQA IGCSE Combined Science Double Award)

Power & Energy in Appliances

• The power of an appliance is defined as: 

The rate at which energy is transferred by an appliance

• Power can be calculated using:

• Where:

  • P = power measured in watts (W)

    • The watt is equivalent to joules per second (J / s)

  • E = energy transferred measured in joules (J)

  • t = time measured in seconds (s)

• This equation can also be written as

• Where:

  • W = work done measured in joules (J), which is equivalent to energy transferred

• Time is an important consideration when it comes to power

• Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance

• If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time

Two cars with different amounts of power

• Two electric motors:

  • lift the same weight

  • by the same height

  • but one motor lifts it faster than the other

• The motor that lifts the weight faster has more power

Two motors with different amounts of power

• Power ratings are given to appliances to show the amount of energy transferred per unit time

• Common power ratings are shown in the table below:

Power Rating Table

Power, Current & Potential Difference

• The power of a device depends on:

  • The potential difference across the device

  • The current flowing through the device

• The power transferred to an electrical component (or appliance) is given by the equation:

• Where:

  • P = power measured in watts (W)

  • V = potential difference measured in volts (V)

  • I = current measured in amps (A)

Energy, Potential Difference & Charge

Extension Tier only

• The potential difference across a component is defined as:

The energy transferred per unit charge

• In equation form, the energy transferred can be written as:

• Where:

  • E = energy transferred measured in joules (J)

  • V = potential difference measured in volts (V)

  • Q = charge measured in coulombs (C)

• This can be combined with the power definition to produce the electrical power equation:

• Recall that power is defined as:

• Substitute the equation above into this definition

• This is equivalent to

• Finally, recall the definition of the current

• Substituting this into the previous equation gives the electrical power equation