Electric Circuit – JSS2 Intermediate Science Lesson Note (Third Term)

An electric circuit is a complete, unbroken path through which electric current flows.

• Race track idea: cars keep moving only if the track is a closed loop, and a gap in the road stops them.
• Same in a circuit: current flows only when the path is complete, and a gap stops it.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define an electric circuit and name its four main parts.
2. Explain the difference between an open circuit and a closed circuit.
3. Describe a series circuit and state its features.
4. Describe a parallel circuit and state its features.
5. Compare series and parallel circuits and explain why houses are wired in parallel.

What are the parts of a simple circuit?

A simple circuit has four main parts: a source, conductors, a load and a switch.

1. Source (cell or battery): gives the “push” (voltage) that moves the charges.
2. Conductor (wires): the “road” the electricity travels on, usually made of copper.
3. Load (bulb or fan): the device that uses the electricity to do work.
4. Switch: the “gatekeeper” that opens or closes the path.

What is the difference between an open and a closed circuit?

A closed circuit is a complete path that lets current flow, while an open circuit has a gap that stops it.

• Closed circuit: the switch is closed, the path is complete and the light is ON.
• Open circuit: the switch is open or a wire is broken, so there is a gap and the light is OFF.

What is a series circuit?

A series circuit is a circuit in which all the parts are connected one after the other in a single loop.

• There is only one path for the electricity to follow.

Features of a series circuit

• Shared current: the same amount of current flows through every bulb.
• Divided voltage: the “push” from the battery is shared between the bulbs.
• Dimmer bulbs: if you add more bulbs, they all get dimmer.
• “All or nothing” rule: if one bulb blows or a wire breaks, the whole circuit stops and all bulbs go off.

Example of a series circuit

• Cheap fairy lights: some cheap Christmas lights are wired this way, so one broken bulb makes the whole string go dark.

Disadvantage of a series circuit

• One fault stops everything: a single broken part switches off all the others.

What is a parallel circuit?

A parallel circuit is a circuit in which the parts are connected side by side, so the current can split into different branches.

• The electricity meets a “fork in the road” and can travel through more than one path.

Features of a parallel circuit

• Independent paths: each bulb has its own direct connection to the battery.
• Full brightness: each bulb gets the full voltage of the battery, so adding more bulbs does not make them dim.
• “Independent” rule: if one bulb blows, current still flows through the other branches and the other bulbs stay on.

Example of a parallel circuit

• House wiring: if the bulb in your bedroom blows, the fan in the sitting room keeps working.

Why are houses wired in parallel?

• Independent use: each device works without depending on the others.
• Full voltage: every device gets the same full supply.
• Separate switches: each device can be switched on or off alone.

How do series and parallel circuits compare?

A series circuit has one path and a parallel circuit has more than one path. Think of a single-lane road and a multi-lane highway.

• Series, single-lane road: a breakdown blocks everyone.
• Parallel, multi-lane highway: cars can drive around a stalled vehicle.

Evaluation

1. What are the four main components of a simple circuit?
2. Why do we prefer to wire our houses in parallel rather than in series?
3. In a series circuit with three bulbs, what happens if the middle bulb is removed?
4. Draw a simple symbol for a battery and a switch (open).

Assignment

Draw a series circuit and a parallel circuit, each with a cell, a switch and two bulbs. Label the parts and write two differences between the circuits.