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Electric current

When a battery lights a light bulb, electrons flow from one terminal of the battery to the other through wires. Such a flow of charges is called an electric current.

In normal electrical circuits, the charges that flow are electrons.

Current flowing through a wire can be compared to water flowing in a river.

In a river, water molecules flow from a higher region (the source at the top of a mountain) to a lower region (the sea).

In a wire, electrons flow from a negatively charged region to a positively charged region.

An electric current flowing through the filament of a light bulb produces light.
An electric current flowing through the filament of a light bulb produces light.

An electrical circuit is a path along which charges can flow.

A very simple circuit consists of a battery, a light bulb and two wires connecting the light bulb to the two terminals of the battery.

A circuit needs to be complete for charges to flow. If there is a break in the circuit, no charge can flow and there will be no current!

Switches can turn lights off because they cause a break in the circuit.

A direct current (d.c.) circuit is a circuit in which charges flow in only one direction. The charge will flow from the power source, through the wires and components, and then back through the power source again.

Most circuits you will consider are d.c. circuits. These circuits are usually powered by batteries.

The alternative to a direct current is an alternating current (a.c.). The direction of the current in a.c. changes periodically.

The current that enters the house from the public grid is a.c. current.

A simple d.c. circuit.
A simple d.c. circuit.

The current ($$I$$) is defined as the rate of flow of charge. It is equal to the total charge ($$\Tblue{Q}$$) which flows past a stationary point divided by the time taken ($$\Tred{t}$$). $$$I=\frac{\Tblue{\text{total charge}}}{\Tred{\text{time taken}}} = \frac{\Tblue{Q}}{\Tred{t}}$$$

A current of $$5 \text{ A}$$ flows for $$\Tred{ 10 \text{ s}}.$$ The total charge transferred is:$$$\Tblue{Q} = I \times \Tred{t} = 5 \text{ A} \times \Tred{10 \text { s}} = \Tblue{50 \text { C}}$$$

The SI unit of current is the ampere $$(\text{A})$$. It is an SI base unit. One ampere is equivalent to one coulomb (the unit of charge) per second. This means that $$1\text{ A}=1\text{ C}/ \text{s}$$.

A toaster uses about $$ 9 \text{ A}$$ of current, while an average lightning bolt has a current of $$30 \, 000 \text{ A}.$$

Current is not a vector quantity even though we sometimes talk about it flowing in a particular direction.

Current is not a vector because it does not follow the rules of vector addition (it cannot be added graphically).

A microammeter measures the current flowing through it in microamperes $$(\mu\text{A})$$
A microammeter measures the current flowing through it in microamperes $$(\mu\text{A})$$

It is a convention in physics to regard the current as flowing from the positive terminal to the negative terminal of a cell. This is called the conventional current.

A flow of positive ions has the same direction as the conventional current.

By contrast, the direction of electron flow (the flow of particles that carry the charge around the circuit in a wire) is from the negative terminal (where there is a surplus of electrons) to the positive terminal.

A flow of negative ions has the same direction as the electron flow.

The positive terminal of a cell is normally indicated by a long vertical line while the negative terminal is indicated by a shorter one.

  • Current is the rate of flow of charge. It is measured in amperes ($$A$$) and has symbol $$I$$. $$$I = \frac{Q}{t}$$$
  • Potential difference (voltage) is energy that is lost in moving a charge of one coulomb between two points in a circuit. It is measured in volts ($$V$$) and has symbol $$V$$. $$$\text{Energy} = VQ$$$
  • Electromotive force (e.m.f.) is the work done by a power source in moving a unit charge around a circuit. It is measured in volts ($$V$$) and has symbol $$\mathcal{E}$$.
  • Resistance is a quantity that prevents or slows down the flow of charge. It is measured in ohms ($$\Omega$$) and has symbol $$R$$. $$$V = IR$$$
  • Power is the energy lost per unit time. It is measured in joules ($$J$$) and has symbol $$P$$. $$$P=IV$$$