An electric current is a flow of charge, usually electrons in a metal wire. A cell or battery pushes the electrons round a closed loop called a circuit. Current is measured in amperes (A) with an ammeter connected in series.
A cell, a switch, a lamp and an ammeter in one loop: a series circuit.
Circuit symbols
Electricians and engineers draw circuits with standard symbols, so anyone can read them. Learn these. The long, thin line of a cell is the positive side.
Current = charge ÷ time. One ampere is one coulomb of charge per second.
Potential difference and resistance
The potential difference (p.d.), or voltage, is the energy given to each unit of charge. It is measured in volts (V) with a voltmeter connected in parallel across a component. Resistance is how much a component opposes the current. It is measured in ohms (Ω).
The ammeter is in the loop (in series). The voltmeter is on a branch across the component (in parallel).
Ohm's law
For a metal at constant temperature, the current through a conductor is directly proportional to the potential difference across it.
Voltage = current × resistance.
potential difference (volts, V)
current (amperes, A)
resistance (ohms, Ω)
Try itResistors in series
Connect a cell to two resistors one after the other. The total resistance is . A larger total resistance gives a smaller current and a dimmer bulb.
Try itResistors in parallel
Now the two resistors share the voltage side by side. The total resistance is smaller than either one: .
Series (left): add the resistances. Parallel (right): add the reciprocals. Two resistors in parallel always give less resistance than either one.
Worked exampleTwo resistors in parallel
A 6 Ω resistor and a 3 Ω resistor are connected in parallel. Find the combined resistance.
Add the reciprocals.
Turn the answer upside down.
Answer: The combined resistance is 2 Ω, which is smaller than both 6 Ω and 3 Ω.
Worked exampleFind the current
A 12 V battery is connected across a 4 Ω resistor. What current flows?
