WJEC GCSE Physics

Circuit symbols

Circuits are drawn with standard symbols so that any diagram can be read and built by anyone.

Two or more cells joined end to end make a battery.

Current and potential difference

An electric current is a flow of electric charge. In a metal the charge is carried by electrons, which are free to drift through the lattice of positive ions. Current is measured in amperes (A), and it can only flow round a complete loop, so opening a switch anywhere in a series loop stops the current everywhere in it.

Potential difference (p.d., also called voltage) is what pushes the charge round. It is measured in volts (V) and describes how much energy each unit of charge transfers as it passes between two points, for example across a lamp. A cell or power supply provides the p.d.; the components in the circuit transfer that energy onwards.

Conventional current is drawn flowing from the positive terminal of the cell round to the negative terminal, even though the electrons in a metal move the other way.

Measuring current and p.d.

An ammeter is connected in series with the component, so that the current passes through the meter. A voltmeter is connected in parallel across the component, because it measures the difference in potential between the two ends. A good ammeter has very low resistance and a good voltmeter has very high resistance, so that neither changes the circuit it is measuring.

Resistance and the relationship between I, V and R

The resistance of a component is a measure of how strongly it opposes the flow of current. Electrons drifting through a metal collide with the vibrating ions, which transfers energy to the lattice and heats the material. The larger the resistance, the smaller the current for a given p.d. The unit is the ohm ($\Omega$).

Here $I$ is the current in amperes, $V$ is the potential difference in volts and $R$ is the resistance in ohms. Rearranged, $V = IR$ and $R = V/I$.

Worked example

A lamp has a resistance of 24 $\Omega$ and is connected across a 12 V supply.

If a resistor carries 0.15 A when the p.d. across it is 6.0 V, its resistance is $R = 6.0 / 0.15 = 40\ \Omega$.

Current and p.d. in the two kinds of circuit

In a series circuit there is one path for the current, so the current has the same value at every point. The p.d.s across the separate components add up to the p.d. of the supply.

In a parallel circuit the current has a choice of paths, called branches. The p.d. across every branch is the same as the supply p.d. The currents in the branches add up to the current drawn from the supply.

A series circuit is simple, but one broken component switches everything off. In a parallel circuit each branch works independently, which is why the lights in a house are wired in parallel: one failed bulb does not darken the rest.

Worked example

Three lamps are connected in series across a 9.0 V supply. The p.d. across the first is 2.5 V and across the second 4.0 V. The third must therefore take $9.0 - 2.5 - 4.0 = 2.5$ V.

Two resistors are connected in parallel to a cell. The ammeter in the main wire reads 0.80 A and the ammeter in one branch reads 0.30 A. The other branch carries $0.80 - 0.30 = 0.50$ A, and both resistors have the same p.d. across them.

How adding components changes the total resistance

Each extra component in series gives the charge another obstacle in the same path, so the total resistance goes up. Each extra component in parallel gives the charge another path to take, so the total current from the supply rises and the total resistance goes down.