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Teach Electricity in eight steps.

For each idea: show it, ask before you tell, then say one line.

Gather paper, a pencil, a disconnected torch cell marked 1.5 V and a cold, unplugged kettle.

Common misconceptions & worked visual for this chapter · 15 practice questions with answers

Why it matters

  • Switching off one appliance need not switch off another.
  • Electric irons turn electrical energy into heat for pressing clothes.
  • An appliance's running time affects how much electrical energy it uses.

What to teach

Current, voltage and resistance

Electric current and circuit diagramsCurrent measures how much charge passes a point each second.Potential differenceEnergy transferred for each coulomb of charge.Ohm's law and V–I graphsVoltage and current rise together at constant resistance.Resistance and resistivityMaterial, length and thickness determine how strongly a wire resists current.

Connecting resistors

Resistors in seriesOne path carries equal current through successive resistors.Resistors in parallelBranches share voltage, while their currents add.

Heating, power and energy

Joule's law and heating materialsCurrent produces heat; suitable materials withstand it.Electric power and energyPower measures energy transfer per second; duration determines total energy.

How to teach it

Current, voltage and resistance

Circuit symbols show a complete loop, current direction and an inset with an open switch.

1. Electric current and circuit diagrams

Current measures how much charge passes a point each second.

Show
Trace the diagram's arrows from the cell. Find the open switch.
Ask
Will opening this switch let the bulb stay lit?
Say
Current needs a complete path, so opening the switch will not let the bulb stay lit. Outside the cell, conventional current flows from positive to negative.
Used in
An ammeter measures current when placed in the component's path.
If your child says…“Current still flows through a break.”
Ask
Where could charge cross the gap?
What’s true
A break interrupts sustained charge flow.
A disconnected torch cell with its voltage marking facing the camera.

2. Potential difference

Energy transferred for each coulomb of charge.

Show
Find the 1.5 V marking on a disconnected torch cell.
Ask
At this voltage, would twice the charge transfer twice the energy?
Say
A 1.5 V marking means 1.5 joules transferred per coulomb. Two coulombs transfer 3 joules, while the voltage stays unchanged.
Used in
A voltmeter connects across a component to measure its voltage.
If your child says…“The voltage number is the total energy.”
Ask
Does voltage alone tell us how much charge moved?
What’s true
More charge transfers more energy at unchanged voltage.
Pencil graph showing voltage rising evenly with current at constant temperature.

3. Ohm's law and V–I graphs

Voltage and current rise together at constant resistance.

Show
Find the current and voltage axes in the diagram. Locate the three marked points and the straight line. Find “constant temperature” and “2 Ω”.
Ask
If voltage doubles from 2 V, what will happen to current?
Say
Here, doubling voltage doubles current.
Used in
Voltage and current measurements reveal a component's resistance.
If your child says…“Every device always obeys Ohm's law.”
Ask
Does a warming filament keep its temperature unchanged?
What’s true
Heating can change resistance.
Three wires compare reference dimensions with doubled length and doubled cross-sectional area.

4. Resistance and resistivity

Material, length and thickness determine how strongly a wire resists current.

Show
In the diagram, find length L, cross-sectional area A and resistance R. All wires have the same material, temperature and resistivity ρ. Compare the middle wire's length and the bottom wire's end area.
Ask
Can these wires have different resistances even though their material is the same?
Say
Resistance equals resistivity times length divided by cross-sectional area: R=ρL/AR=\rho L/A. The longer wire has double resistance, while the thicker wire has half. Resistivity stays unchanged.
Used in
Low-resistivity copper or aluminium gives transmission wires less resistance.
If your child says…“Longer wire has greater resistivity.”
Ask
Do the top and middle wires differ in material or only in length?
What’s true
Resistivity depends on material and temperature.

Connecting resistors

Two series resistors share one current path, with brackets marking their voltage drops.

5. Resistors in series

One path carries equal current through successive resistors.

Show
Follow the current arrows through both resistors in the diagram. Find each resistor's voltage bracket, then the battery's voltage bracket.
Ask
What happens to current if another resistor joins this path at unchanged battery voltage?
Say
The same current passes through both resistors because there is no branch. Their voltage drops add, so their resistances add. At unchanged supply voltage, adding series resistance reduces current.
Used in
A series fuse breaks the appliance's current path when excessive current melts it.
If your child says…“Each resistor uses up some current.”
Ask
Where would charge collect if less left each resistor than entered?
What’s true
In a steady, unbranched circuit, equal charge passes every point each second.
Parallel resistor branches share junctions, with arrows showing current splitting and rejoining.

6. Resistors in parallel

Branches share voltage, while their currents add.

Show
Find branch resistances R₁ and R₂ in the diagram. Follow the arrows splitting at A and rejoining at B. Call their combined resistance Rₚ.
Ask
How will adding the lower branch change the resistance across A and B?
Say
Both branches span the same voltage, while their currents add. An added path lowers combined resistance, found by adding reciprocals: 1Rp=1R1+1R2\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}.
Used in
Parallel connections let appliances operate independently at their intended voltage.
If your child says…“I add parallel resistances directly.”
Ask
Would an extra conducting path make total current smaller at unchanged voltage?
What’s true
Combined resistance falls below either branch's resistance.

Heating, power and energy

H=I2RtH=I^2Rt

H1=(1 A)2(5 Ω)(10 s)=50 JH_1=(1\,\mathrm{A})^2(5\,\Omega)(10\,\mathrm{s})=50\,\mathrm{J}

H2=(2 A)2(5 Ω)(10 s)=200 JH_2=(2\,\mathrm{A})^2(5\,\Omega)(10\,\mathrm{s})=200\,\mathrm{J}

7. Joule's law and heating materials

Current produces heat; suitable materials withstand it.

Show
Write these lines for heat H, current I, resistance R and time t. Say that heat equals current squared times resistance times time.
Ask
How many times the first heat does the doubled current produce?
Say
The second current produces four times the heat because current is squared.
Used in
Heating alloys let electric irons produce useful heat without melting or reacting quickly with air.
If your child says…“Double current means double heat.”
Ask
What happens when the doubled current is squared?
What’s true
With resistance and time unchanged, doubling current gives fourfold heat.
A cold kettle with its power rating visible and its cord unplugged.

8. Electric power and energy

Power measures energy transfer per second; duration determines total energy.

Show
Set down a cold, unplugged kettle. Read its power rating.
Ask
If it runs twice as long at this power, will it use twice the energy?
Say
Power tells us how much energy transfers each second. At unchanged power, twice the operating time uses twice the energy.
Used in
Appliance ratings give intended voltage and power, helping calculate current.
If your child says…“Kilowatt-hours measure power.”
Ask
What extra information does the hour add?
What’s true
A kilowatt-hour measures energy.

A real problem

A thin filament helped make indoor electric lighting practical.

Edison demonstrated his incandescent lighting system publicly in December 1879.

He recognised that a very thin filament with high resistance could make a practical lamp.

His approach used electric current to heat a thin filament.

The lamp could give light dim enough for indoor use.

National Park ServiceSmithsonianSmithsonian

A lit incandescent bulb with its glowing filament visible.

Concept Lab · Common misconception

The tempting mistake in Electricity

Open standalone misconception page →

Does a larger series resistor use up more current?

Wrong

Less current flows just after a 4 Ω resistor than just after a 2 Ω resistor in the same series loop.

Why it feels right. I treat current like something the components use up.

Right

In a steady series loop, the same charge per second passes each point. The potential drops can differ.

Worked example

The series loop has 1 A on both sides of the 4 Ω resistor. Its voltage drop is larger than the 2 Ω drop.
  1. With an ideal 6 V supply and 2 Ω and 4 Ω in series, ammeters on either side of the 4 Ω resistor both read 1 A.
  2. The drops across the resistors are 2 V and 4 V.

Ask your child eight quick questions.

One per step. The answer and the reason are given.

  1. What happens to steady current when the circuit's conducting path breaks?

    It stops. Sustained charge flow needs an unbroken conducting path.

  2. An 8 V source moves 2 C. How much energy does it supply?

    16 J. Multiply energy per coulomb by the charge moved.

  3. Must a filament's current double when its voltage doubles, even while it heats up?

    No. Heating can change resistance, so current need not double.

  4. How does resistivity change when wire length doubles, with material and temperature unchanged?

    It stays the same. Length changes the wire's resistance, not its material's resistivity.

  5. How do current readings compare before and after resistors in one steady, unbranched circuit?

    They are equal. Resistors transfer energy without consuming charge.

  6. How does combined resistance compare with the smallest resistance among parallel branches?

    It is smaller. Extra branches allow more total current at the same voltage.

  7. Current doubles through the same resistance for the same time. How much heat results?

    Four times as much. Heat depends on current squared when resistance and time stay unchanged.

  8. A 2 kW appliance runs for 3 hours. How much energy does it use?

    6 kWh. Energy equals power multiplied by operating time.

15 more questions with answers