Electric charge, current and circuit structure

Electric current is the rate of flow of charge: the amount of charge passing a point in a circuit each second. Charge is measured in coulombs (C) and current in amperes (A), often called amps.

A current of 1 A means that 1 C of charge passes a point every second. A larger current means more charge passes each second, not simply that more charge is present in the wire.

In metals, some electrons can move through the material. Current in a metal is a flow of electrons; the protons and neutrons in the nuclei do not travel around the circuit.

For a steady current, the charge transferred is:

$$Q = I \times t$$

Here, $Q$ is charge in coulombs, $I$ is current in amperes and $t$ is time in seconds. The same relationship can be rearranged to give $I = Q/t$ or $t = Q/I$.

For instance, if 4 C passes a point in 500 s, the current is $4/500 = 0.008\text{ A}$, or 8 mA. Using the charge equation in reverse, $0.008\times500 = 4\text{ C}$. This is the total charge that passes that point during the time interval.

What makes a circuit work?

A working circuit needs a source of potential difference, such as a cell or battery, and a closed conducting path. The source provides the electrical driving effect that makes charge flow through the circuit.

Closing a switch completes the path. Opening it makes a gap, so current cannot flow around that path. In a simple lamp circuit, the lamp lights when the switch is closed and goes out when it is opened.

Circuit diagrams use conventional current, directed from the positive terminal to the negative terminal through the external circuit. Because electrons are negatively charged, their flow in metal wires is in the opposite direction: from negative to positive.

Reading and drawing standard circuit symbols

A circuit diagram shows electrical connections, not the physical positions or appearance of the equipment. Wires are drawn as lines, and each component has a standard symbol. You can move a symbol on the page without changing the circuit, provided its connections remain the same.

The symbols can be recognised by their distinctive features:

To draw a simple switched lamp circuit, place a cell, switch and lamp along one loop, then join their terminals with wires. Make the switch closed if the diagram is intended to show current flowing. Trace the path from the cell’s positive terminal through the components and back to its negative terminal to check that the loop is complete.

A dot where wires meet marks an electrical junction. When reading a diagram, follow the actual connections rather than judging whether components look close together.

Series circuits: one path for current

In a series circuit, components are connected end to end. There is only one path, so the charge passing through one component must also pass through the others.

The current is the same at every point in the loop. If an ammeter reads 0.5 A before the first lamp, the current between the lamps and after the second lamp is also 0.5 A. The lamps transfer energy; they do not use up current.

A break anywhere in this single path stops current throughout the circuit. A switch in the loop therefore controls all the series components together. The supply’s potential difference is shared between the components.

Parallel circuits: separate branches

In a parallel circuit, components are connected in separate branches between the same two junctions. Charge has more than one possible route through the circuit.

Current splits at the first junction and recombines where the branches join. The potential difference is the same across each branch connected across the supply. The branch currents need not be equal.

Opening a switch in one branch stops current in that branch but leaves the other complete branches working. A switch in the shared part of the circuit can still turn all branches off.

Measuring current with an ammeter

An ammeter must be connected in series with the component whose current you want to measure. This means inserting it into the same path so the current through the component also passes through the meter.

For a series circuit, an ammeter anywhere in the loop measures the current through every component. In a parallel circuit, its position matters: an ammeter in one branch measures that branch’s current, whereas an ammeter in the shared wire next to the supply measures the total current.

Current is conserved at junctions

Charge does not disappear or build up at a junction during steady operation. The rate at which charge arrives must equal the total rate at which it leaves. Therefore:

$$\text{total current entering a junction} = \text{total current leaving it}$$

For two outgoing branches, this can be written as $I_{\text{total}} = I_1 + I_2$.

In the illustrated parallel circuit, 0.30 A reaches the junction. One branch carries 0.10 A and the other carries 0.20 A, so $0.30 = 0.10 + 0.20$. Where the branches rejoin, their currents add back to 0.30 A. The total is conserved even though the split is unequal.