Overview
What you’ll learn
- State that current is measured in amperes, potential difference in volts and resistance in ohms.
- Describe the heating, magnetic and chemical effects of an electric current.
- Draw and interpret circuit diagrams that use a cell, switch, lamp, resistor, variable resistor, ammeter and voltmeter.
- Compare series and parallel circuits in words.
- Explain common electrical hazards and how fuses, insulation and careful use reduce them.
- State that power is measured in watts and suggest ways to use less electrical energy.
In 30 seconds
- 1Current (ampere, A) is the flow of charge. Potential difference (volt, V) is the push from the cell or supply. Resistance (ohm, Ω) is opposition to the current.
- 2A resistor is a component. Resistance is the quantity, in ohms, that says how strongly that component opposes the current.
- 3A current needs a complete circuit. An open switch leaves a gap, so the current stops.
- 4A current can have a heating effect, a magnetic effect and a chemical effect.
- 5In series there is one path. In parallel each component has its own path, so one lamp can be off while another stays on.
- 6A fuse melts and breaks the circuit if the current is too large. Do not overload sockets.
- 7Power is measured in watts. Switching off unused appliances reduces the energy used.
Notes · 1 of 6
Current, potential difference and resistance
Three quantities, three units. Do not swap them.
| Quantity | What it describes | Unit |
|---|---|---|
| Current | Flow of electric charge | ampere (A) |
| Potential difference | The push that makes charge flow | volt (V) |
| Resistance | How much a component opposes the current | ohm (Ω) |
Current, potential difference, resistance and power are quantities. A cell, lamp, switch, resistor, ammeter, voltmeter, fuse and the wires are components. A cell provides the potential difference. A resistor is the component you add when you want a particular resistance. A larger resistance makes it harder for current to pass. Compare circuits in words: do not calculate one quantity from the others.
Notes · 2 of 6
Effects of an electric current
The same current can heat, magnetise or cause a chemical change.
- Heating: a kettle element or a filament lamp becomes hot.
- Magnetic: a current in a coil can pick up iron. This is an electromagnet.
- Chemical: a current can be used to charge some batteries or to deposit a metal during plating.
These are effects of the current. They are not three different kinds of electricity. Energy is transferred, not destroyed.
Notes · 3 of 6
Circuit symbols and diagrams
The symbol is a drawing of a component. The unit belongs to a quantity.
| Component | How to draw it | Where it goes |
|---|---|---|
| Cell | A long line and a short line. The long line is the positive end. | The source of the potential difference |
| Switch | A break in the line that can be closed | In the circuit. Open means a gap |
| Lamp | A circle with a cross inside | In the circuit |
| Fixed resistor | A rectangle | In the circuit |
| Variable resistor | A rectangle with an arrow across it | In the circuit. The arrow means the resistance can be changed |
| Ammeter | A circle with the letter A | In the circuit, so the current passes through it |
| Voltmeter | A circle with the letter V | Across a component, not in the single path of that component |
To read a diagram, trace the line with your finger. If you can go from one end of the cell back to the other without a gap, the circuit is complete. An ammeter reading is a current in amperes. A voltmeter reading is a potential difference in volts. Neither meter is the component that opposes the current. That job belongs to the resistor, the lamp or the wires, and the quantity is resistance.
Notes · 4 of 6
Series and parallel
Count the paths. One path is series. More than one path is parallel.
In a series circuit the components are in one loop. The same current passes through each of them. If one lamp fails and leaves a gap, the others go out.
In a parallel circuit each lamp has its own loop back to the cell. Opening one branch leaves the other branches complete, so the other lamps stay on. House lighting is wired in parallel for this reason.
Notes · 5 of 6
Hazards and household safety
Too much current heats wires. Water and damaged cables make shocks more likely.
- A fuse is a thin wire that melts if the current is too large, which breaks the circuit.
- Do not replace a fuse with a thicker piece of metal. The wire may overheat before anything breaks.
- Damaged insulation can let the current leave the wire and give a shock. Do not use a cable with bare metal showing.
- Wet hands and water near a socket make a shock more likely, because water can conduct.
- An overloaded socket carries more current than the cable is meant for, so the cable can overheat.
- Switch off and unplug before you change a lamp.
Notes · 6 of 6
Power and using less energy
Power tells you how fast an appliance transfers energy.
Power is measured in watts (W). A 1000 W kettle transfers energy faster than a 10 W lamp. The longer a high-power appliance is on, the more energy it uses. Bills often use the kilowatt-hour as a unit of energy. You do not need to calculate a bill.
Switch off lights and fans that nobody needs. Choose an efficient appliance when you can. Using less electricity saves money and usually means less fuel is burned at the power station.
Revise
Key terms
The words examiners expect you to use precisely.
- Current
- A flow of electric charge, measured in amperes (A).
- Potential difference
- The push that causes current, measured in volts (V).
- Resistance
- The quantity that describes opposition to current. It is measured in ohms (Ω). It is not the name of a component.
- Resistor
- A component that provides resistance. A fixed resistor has one resistance. A variable resistor can be adjusted.
- Complete circuit
- An unbroken path from one terminal of the supply back to the other.
- Series circuit
- A circuit with one path through all the components.
Show 5 more key termsShow fewer key terms
- Parallel circuit
- A circuit in which components have separate paths.
- Fuse
- A thin wire that melts to break a circuit if the current is too large.
- Power
- The rate of energy transfer, measured in watts (W).
- Ammeter
- A meter placed in the circuit to measure current.
- Voltmeter
- A meter connected across a component to measure potential difference.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Measuring current in volts.
The volt is the unit of potential difference.
Instead: Current is in amperes. Potential difference is in volts. Resistance is in ohms.
Mistake: Saying a lamp still lights when its only switch is open.
An open switch is a gap.
Instead: Current needs a complete circuit.
Show 4 more mistakesShow fewer mistakes
Mistake: Saying one lamp failing in parallel always puts the others out.
That is what happens in series.
Instead: In parallel, each lamp has its own path.
Mistake: Replacing a fuse with thicker metal.
The fuse will no longer melt at the safe current.
Instead: Use the correct fuse so the circuit opens before the cable overheats.
Mistake: Answering “resistance” when the question asks for the component, or “resistor” when it asks for the unit.
Resistance is a quantity in ohms. A resistor is the object in the circuit.
Instead: Name the component if the question says component. Name the quantity and the ohm if the question says unit.
Mistake: Saying power is measured in amperes.
The ampere is the unit of current.
Instead: Power is measured in watts.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
State the unit of current
Current is measured in
Ready for more practice on Chapter 5: Electrical Systems?
Generate a fresh 5-question adaptive MCQ drill grounded directly in this chapter’s MOE syllabus key points. Free with your study account.
Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 35 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 5 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 5.
Mark scheme
PROChapter 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What is the difference between current and voltage in Sec 2?
Current is the flow of charge, measured in amperes. Potential difference, often called voltage, is the push, measured in volts. Resistance, in ohms, opposes the current.
Do I need the formula linking volts, amperes and ohms?
No. Describe circuits in words. Say whether the path is complete, and whether the lamps are in series or parallel.
What happens if one bulb blows in series and in parallel?
In series the other bulbs go out because there is one path. In parallel the other bulbs stay on because each has its own path.
Why does a fuse make a circuit safer?
If the current is too large the fuse wire melts and opens the circuit. That stops the cable overheating.
Is electrical systems in the Sec 2 EOY?
Most schools that follow the usual G2/G3 scheme teach this chapter in Sec 2 and include it in the Sec 2 end-of-year paper. A few schools move a topic across the two years, so check your school’s assessment outline.
Up next
Chapter 6: Human Digestive System