3.5.1Electricity
Electrical Energy and Electrical Power — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 16
Part 1 of 5Energy Transfers
Electrical appliances change electrical energy into other useful forms.
Heater
- Transforms most of the electrical energy into heat energy.
Lamp
- Transforms electrical energy into light, with some heat produced as well.
Motor
- Transforms electrical energy into kinetic energy, and also produces some heat and sound.
Unit of energy
- Energy is measured in joules (J), and larger amounts are often quoted in kilojoules.
Efficiency idea
- Not all energy becomes useful output, because some is always wasted as heat to the surroundings.
Part 2 of 5Power
Power describes how quickly an appliance transfers energy.
Power
Definition
- Power is the energy transferred each second, so it is the rate of energy transfer.
The watt
- A power of 1 watt means 1 joule of energy is transferred every second, so 1 W = 1 J/s.
Equation
- P = E ÷ t, where P is power in watts, E is energy in joules and t is time in seconds.
Rearranged
- E = Pt finds the energy, and t = E ÷ P finds how long a transfer takes.
Bigger units
- A kilowatt is 1000 W and a megawatt is 1 000 000 W, so power station outputs are given in megawatts.
Common error check
- If an answer for a kettle is only a few joules, the time was probably left in minutes rather than seconds.
| Appliance | Typical power rating |
|---|---|
| LED lamp | About 10 W |
| Television | About 100 W |
| Kettle | About 2 kW |
| Electric shower | About 8 kW |
Part 3 of 5Using P = E ÷ t
A short method prevents unit errors.
1
Convert the time into seconds before starting any calculation.
2
Convert kilowatts to watts by multiplying by 1000, and kilojoules to joules likewise.
3
Write P = E ÷ t in the form that has the unknown quantity on its own.
4
Substitute the values, calculate and include the correct unit.
5
Check that the size of the answer is sensible for the appliance.
Part 4 of 5Worked Example
Question
- A 2.5 kW kettle is switched on for 3.0 minutes.
- Calculate the electrical energy transferred.
- ConvertP = 2.5 kW = 2500 W and t = 3.0 × 60 = 180 s.
- Use E = Pt.
- E = 2500 × 180 = 450 000 J.
Answer The energy transferred is 450 000 J (4.5 × 10⁵ J).
Part 5 of 5Exam Checklist & Tips
3.5.1 – Key Points for the Exam
- Power The rate at which energy is transferred, measured in watts.
- Watt A rate of transfer of one joule per second.
- Energy The ability to do work, measured in joules.
- Power rating The power an appliance transfers when working normally.
Common Mistakes
- Leaving the time in minutes rather than converting to seconds.
- Forgetting that 1 kW is 1000 W.
- Saying that power is the amount of energy rather than the rate of energy transfer.
- Giving energy in watts or power in joules.
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