6.5.2Chemical Reactions
Energy and Power in Appliances — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 21 Unit quiz
Section 1 of 5Definitions
Power
- The rate at which energy is transferred, measured in watts (W), where 1 W is 1 J per second.
Energy transferred
- The total energy transferred by an appliance over a period of time, measured in joules (J).
Power equation
- Power (W) = energy transferred (J) ÷ time (s), written as P = E ÷ t.
Electrical power
- For an electrical appliance the power can be found from P = I × V, with current in amperes and potential difference in volts.
Section 2 of 5Calculation Pathway
1
Identify the power rating of the appliance from its label, or calculate it from the current and the potential difference.
2
Convert the time into seconds, because energy in joules requires the time to be in seconds.
3
Multiply the power in watts by the time in seconds to find the energy transferred in joules, using E = P × t.
4
Check that the unit of the answer is joules and that the value is reasonable for the appliance.
Section 3 of 5Rearranging and Checking
Rearranging and Checking
Energy equation
- Rearranged, the energy transferred is E = P × t, so the energy is the power multiplied by the time.
Units link
- Using kilowatts with hours gives kilowatt-hours, a different unit of energy that is covered in the next lesson.
Common error
- Using minutes directly in E = P × t gives a wrong answer unless the time is first converted into seconds.
Meaning of a rating
- A kettle rated at 2000 W transfers 2000 J of energy every second while it is switched on.
| Quantity | Symbol | Unit |
|---|---|---|
| Power | P | watt (W) |
| Energy transferred | E | joule (J) |
| Time | t | second (s) |
| Current | I | ampere (A) |
| Potential difference | V | volt (V) |
Section 4 of 5Worked Example
Question
- An electric heater takes a current of 5.0 A from a 230 V supply.
- Calculate its power, and the energy it transfers in 10 minutes.
- Power P = I × V = 5.0 A × 230 V = 1150 W.
- Time t = 10 minutes = 600 s.
- Energy E = P × t = 1150 W × 600 s = 690 000 J.
Answer The heater has a power of 1150 W and transfers 690 000 J (690 kJ) in 10 minutes.
Section 5 of 5Exam Checklist & Tips
6.5.2 – Key Points for the Exam
- Power The rate of energy transfer, measured in watts.
- Watt (W) The unit of power, equal to one joule per second.
- Joule (J) The SI unit of energy.
- Energy transferred The amount of energy moved from one store to another.
- Potential difference The energy transferred per coulomb of charge, measured in volts.
Common Mistakes
- Confusing power with energy; power is the rate of energy transfer.
- Forgetting to convert minutes into seconds before using E = P × t.
- Writing 1 W as 1 J per minute; the watt is one joule per second.
- Mixing units, such as using kilowatts with seconds, without converting.
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