What you will learn

• what electric current and voltage mean, and their units,
• how to read and draw simple circuit diagrams with standard symbols,
• the difference between series and parallel circuits,
• how voltmeters and ammeters are connected in a circuit,
• how to use Ohm’s law, $V = IR$, for simple calculations.

1. Current and voltage

Current ($I$) is the flow of charge through a wire. Unit: ampere (A). Measured with an ammeter.

• $1$ A $=$ a lot of electrons passing a point every second.

Voltage ($V$) is the electrical “push” that drives current. Unit: volt (V). Measured with a voltmeter.

• A $9$ V battery provides more push than a $1.5$ V battery.

Resistance ($R$) is how much a component opposes current flow. Unit: ohm ($\Omega$).

• A thin wire or a bulb filament has high resistance; a thick copper wire has low resistance.

2. Circuit diagrams and symbols

Real circuits are drawn as tidy diagrams using standard symbols.

A cell provides one voltage (e.g. 1.5 V). A battery is two or more cells joined together (e.g. a “9 V” battery has six 1.5 V cells in series).

3. Measuring: ammeters and voltmeters

• An ammeter measures current. It is connected in series — the current flows through it. It should have very low resistance so it does not disturb the circuit.
• A voltmeter measures voltage across a component. It is connected in parallel — placed across the component, not inside the loop. It should have very high resistance so almost no current goes through it.

4. Series vs parallel circuits

Series — components are connected end-to-end on the same loop.

• Current is the same at every point: $I_1 = I_2 = I_3$.
• Voltage is shared among components: $V_{\text{total}} = V_1 + V_2 + V_3$.
• If one breaks, the whole circuit is broken (like old Christmas lights).

Parallel — components sit on separate branches.

• Voltage is the same across every branch: $V_1 = V_2 = V_{\text{total}}$.
• Current splits between branches: $I_{\text{total}} = I_1 + I_2 + I_3$.
• If one branch breaks, the others keep working (like house wiring).

5. What components do

• Bulbs convert electrical energy into light and heat.
• Resistors convert electrical energy into heat; they limit current.
• Switches open or close a circuit.
• Motors convert electrical energy into movement.
• Buzzers / speakers convert electrical energy into sound.

Designing a useful circuit means choosing the right components and arrangement for the job.

Practice: Year 8

Challenge