Current, voltage and resistance
Physics · Key Stage 3 · The School
Three different quantities
CURRENT is the rate of flow of charge, measured in amperes with an ammeter placed IN the circuit. VOLTAGE (potential difference) is the energy given to each unit of charge, measured in volts with a voltmeter placed ACROSS a component. RESISTANCE, measured in ohms, is how much a component opposes the current. They are three different things and confusing them is the commonest source of error here.
Series and parallel
In SERIES there is one loop: the same current passes through every component, and the voltage is shared between them. In PARALLEL there are branches: each branch gets the full voltage, and the current divides between them. That is why house lighting is wired in parallel — one bulb failing leaves the rest at full brightness and still working.
Predicting a change
Add a component in series and the total resistance rises, so the current falls and everything dims. Add a branch in parallel and the total resistance FALLS, so more current is drawn overall. The second result surprises people, and it is why overloading a socket with adaptors is dangerous rather than merely untidy.
One cell, two identical lamps in SERIES: the current passes through both, the voltage is shared, and each lamp is dim. Now rebuild with the lamps in PARALLEL: each branch gets the full cell voltage, so both are at normal brightness — and the cell runs down much faster, because it is supplying two lots of current. Now unscrew one lamp. In series the other goes out; in parallel it stays lit, and that is exactly why house wiring is parallel.