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Course: Science — Third Preparatory

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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 2 — Electricity and Magnetism

Science — Third Preparatory

Unit 2 — Electricity and Magnetism

1.1 Free electrons and electric current

1.1 Free electrons and electric current

Metals such as copper and aluminium are good conductors because each of their atoms loses some of its outer electrons.
These free electrons are not attached to any one atom; they wander at random inside the metal (Figure: random motion), and since they move in all directions equally, there is no net flow of charge.

An analogy helps.
Imagine a bicycle chain.
When you press the pedals, all the links of the chain move together; the chain carries motion from the pedals to the rear wheel; and if something rubs against the chain, it slows down.
In a simple electric circuit:

Key points

the cell (source) is like the pedalsit provides the push;

the wires are like the chainthe path the charges follow;

the bulb is like the resistance of the rear gearit opposes the flow and turns electrical energy into light and heat.

When a metal wire is connected to a cell to form a closed circuit, the cell pushes the free electrons so that they drift in one direction along the wire.
The ordered flow of electric charges through a conducting material is called electric current.
Two conditions are needed for a current to flow:

an electric source to push the charges (cell, battery, generator);
a closed circuit, a complete path with no breaks (this is why a switch can turn a lamp on or off).

Direction of current.

By convention, current flows outside the source from the positive terminal (higher electric potential) to the negative terminal (lower potential).
Before the electron was discovered, scientists chose this direction.
In fact the free electrons move the opposite way, from the negative pole to the positive pole.
In this unit we always use the conventional direction.

Unit 2 — Electricity and Magnetism · 2 / 25