Potential difference drives current

The idea

Charges flow in a conductor only if there is a potential difference along it; chemical action in a cell creates this across its terminals, and the cell spends its stored chemical energy to maintain the current.

Nothing moves by itself

A long copper wire is lying on a table. It is full of electrons, but no current flows in it. A cell is then joined across its two ends.

Why does a current flow once the cell is joined?

• The cell creates a potential difference along the wire
• The cell adds new electrons that were missing from the wire
• Joining any two things makes a current flow

Water and wire

Open all three tabs. The first two are about water. The third is the real circuit.

Electric pressure

Water needs a pressure difference to flow along a tube. Charges in a wire need a potential difference, which is a kind of electric pressure. The cell makes it by chemical action, and it must spend its stored chemical energy to keep the current going.

Charges flow in a conductor only if there is a potential difference along it. Chemical action in a cell creates this across its terminals, and the cell spends its stored chemical energy to maintain the current.

Notes

Charges flow in a conductor only if there is a potential difference; a cell creates it by chemical action and spends its stored chemical energy to maintain the current.

Check yourself

Why do charges not flow by themselves in a copper wire?

What produces the potential difference across the terminals of a cell?

What must the cell do to keep a current going in a circuit?

In the water comparison, which plays the part of the potential difference?

• Copper cannot carry a current at all. Copper is a good conductor of electricity. Charges flow in it once there is a potential difference along it.
• There is no potential difference along the wire — correct. Yes!
• Gravity holds the charges still. For charges in a metallic wire gravity has no role to play. The electrons move only if there is a potential difference.
• The wires that are joined to it. The wires are the path for the charges. The potential difference is made by chemical action in the cell.
• The switch of the circuit. A switch only makes or breaks a conducting link. The cell makes the potential difference by chemical action.
• Chemical action within the cell — correct. Yes!
• Spend the chemical energy stored in it — correct. Yes!
• Nothing, because a current keeps itself going once it has started. To maintain the current, the cell has to expend its chemical energy.
• Take in extra electrons from the air. The current is kept going by the cell using its stored chemical energy.
• The tube itself. The tube is like the wire. The pressure difference between its ends is like the potential difference.
• The difference in pressure between the two ends of the tube — correct. Yes!
• The water in the tube. The water is like the charges. What makes it flow is the pressure difference, like the potential difference.