Electric current and circuit

The idea

A flow of electric charge through a conductor is an electric current; a continuous and closed path of current is an electric circuit, and if the circuit is broken anywhere the current stops flowing.

A torch that goes dark

A torch has cells, a bulb and a switch. It gives light only when the switch is on. Suppose a wire inside the torch circuit snaps at some place.

What will the bulb do?

• It goes dark, wherever the wire breaks
• It goes dark only if the break is right next to the bulb
• It keeps glowing, because the cell is still in the torch

Switch and break

Turn the switch on and watch the charges move. Then break the wire at the bottom while the switch is still on.

A closed path

When the switch was on, charge flowed round the whole loop. Once the path was broken, even at a place far from the bulb, the flow stopped and the bulb went dark.

A flow of electric charge through a conductor is an electric current. A continuous and closed path of current is an electric circuit. If the circuit is broken anywhere, the current stops flowing.

Notes

A flow of charge through a conductor is an electric current; a continuous, closed path of current is an electric circuit, and a break anywhere stops the current.

Check yourself

In a torch the switch is turned off. What has happened to the circuit?

Which arrangement lets a bulb glow?

Which of these describes an electric current in a metallic wire?

Which words describe an electric circuit?

• The current keeps flowing, but the bulb stops glowing. If the circuit is broken, the current stops flowing, and that is why the bulb does not glow.
• The closed path is broken, so the current stops flowing — correct. Yes!
• Nothing has changed in the path, the switch only controls the light. A switch makes a conducting link between the cell and the bulb. Turned off, it removes that link, so the path is no longer closed.
• A cell and a bulb joined by a wire on one side only, with a gap on the other side. A gap breaks the path, so no current flows round and the bulb does not glow.
• A cell, a bulb and a closed switch in a loop where one wire has snapped. A circuit broken anywhere carries no current, so the bulb stays dark.
• A cell, a bulb and a closed switch joined by wires in one continuous loop — correct. Yes!
• A flow of electric charge through the wire — correct. Yes!
• A cell lying on a table, not connected to anything. Nothing flows there. A current is charge flowing through a conductor.
• A bulb that is switched off. A switched off bulb has no flow of charge through it, so it carries no current.
• A path of current with a gap in it. A circuit with a gap is broken, and the current stops flowing.
• A continuous and closed path of current — correct. Yes!
• Only the cell that pushes the charge. A circuit is the whole closed path of the current, not just the cell.