Power in Electrical Appliances
Date: ___________________
Learning Objectives
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Recall and apply the equations to calculate electrical power.
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Describe what is meant by the power ratings for domestic electrical appliances.
Pre-Starter Retrieval Questions
1. Complete the equation: power = ______________ ÷ time.
2. What is the colour of the live wire in a three-pin plug?
3. What energy store changes when a battery is used?
4. Give one advantage and one disadvantage of using nuclear energy as a resource.
5. Discuss: why does a more powerful cyclist move faster as they peddle?
Starter
What is the name of the system of cables and transformers linking power stations to consumers? _______________
Complete the diagram below.
What do step-up and step-down transformers do?
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Calculating Power
Electrical (electrostatic) energy is easy to move from place to place, using wire or cables. However, electrical energy
must be changed into other forms of energy before we can use it. The rate at which energy is transferred (or
changed) is called power.
We already know an equation for power:
Power = energy ÷ time
P=E÷t
There are two more equations for power that you need to be able to recall and apply:
Power = potential difference x current
P=IxV
Power = (current)2 x resistance
P = I2 x R
Example 1
A kettle uses a power of 1800 W and uses 36000 J of energy. Calculate the time that the kettle was left on for.
Example 2
Calculate the power for an electrical appliance that supplies 0.5 V and has 5 kA of current flowing through it.
Example 3
Calculate the current that flows through a 512 W toaster with a resistance of 8 Ω.
Calculation Tips
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Always show your working – you get marks for working!
Remember to give a unit at the end of your answer.
Ensure your variables are in the correct units and covert them if needed
o To change kW into W – multiple by 1000
o To change minutes in seconds – multiply by 60
o To change mA into A – divide by 1000
Core
1. Name two variables that would increase the power of an appliance.
2. A light bulb is connected to a 2V supply and experiences a current of 6.4 A. What is the power of the bulb?
3. A kettle has a power of 1500W. What is the potential difference that it must be supplied with to have a
current flowing through it of 30A.
4. An Xbox uses an energy of 2500 W and is left on for a time of 2500s. Calculate the amount of power used.
5. A student attaches a 10V supply to a bulb with a power of 100W. What is the current running through the
bulb?
6. An electric radiator has a power of 2,000 W, and a resistance of 20 Ω. Calculate the current in the circuit.
7. Calculate the energy transferred by a 3,000 W hairdryer in 60 seconds.
Harder
7. An electric transformer outputs a voltage of 500 kV, and a current of 100 mA is drawn from the circuit.
Calculate the power drawn by the circuit.
8. A microwave has a power of 1 kW, and a resistance of 55 Ω. Calculate the current in the circuit.
9. An iPhone charger outputs a current of 3,000 mA at a power of 50 W. Calculate the voltage given out by the
charger.
10. Calculate the power for an electrical appliance that transfers 5 kJ of energy in 15 minutes.
What is a power rating?
All electrical devices in the home should have labels on them that show
the power rating of the device.
This tells us how fast the electrical appliance transfers energy from one
store to another.
This kettle transfers 1850-2000 J of energy per second from the UK
mains to the _______________ energy store.
Which of these appliances transfers the least energy per second? _______________
An electric heater is very powerful but may not be very efficient. Explain why.
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