Energy transfers in everyday appliances: The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance.

The idea

Energy transferred by an appliance is the product of its power and the time it operates. Power is the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s. A 2000 W kettle switched on for 120 s transfers E = P × t = 2000 W × 120 s = 240 000 J (240 kJ). Doubling either the power rating or the operating time doubles the energy transferred, because E is directly proportional to both P and t. For mains appliances, power ratings are often given in kilowatts (kW), so convert: 2 kW = 2000 W. Time must be in seconds when using P in watts to obtain energy in joules. This relationship explains why a low-power device left on for many hours can transfer more energy than a high-power device used briefly.

What earns the marks

• States that energy transferred E is calculated using E = P × t, where P is power in watts and t is time in seconds.
• Explains that power is the rate of energy transfer, so a higher-power appliance transfers more energy each second.
• Uses a worked example, such as a 2000 W kettle operating for 120 s transferring 240 000 J, to show the effect of both variables.
• Describes the direct proportionality: doubling power or time doubles the energy transferred, assuming the other variable is constant.
• Converts between units correctly, for example 2 kW = 2000 W and minutes to seconds, before substituting into E = P × t.
• Compares two appliances to show that a low-power appliance left on for a long time can transfer more energy than a high-power appliance used briefly.

Common mistakes and how to put them right

• Using time in minutes or hours with power in watts.

  Put it right: This gives an incorrect energy unit. Convert time to seconds first, or convert power to kilowatts and time to hours to obtain kilowatt-hours.

• Confusing power and energy.

  Put it right: Power is the rate of transfer (W), while energy is the total transferred (J). State clearly which quantity is being calculated.

• Assuming a higher-power appliance always transfers more energy.

  Put it right: If it is switched on for a much shorter time, it may transfer less energy than a lower-power appliance left on longer.

Exam tips

• Write the equation E = P × t, substitute values with units, and show the unit of the answer (J or kJ).
• Check unit consistency before calculating: convert kW to W and minutes to seconds if the answer is required in joules.
• For comparison questions, calculate the energy for each appliance rather than relying on power ratings alone.