IGCSE Physics / Current and voltage in circuits
Current in series and parallel circuits revision notes
Edexcel International GCSE Physics (4PH1), chapter 7: Current and voltage in circuits, specification point 2.17, 2.8. Free lesson.
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Current behaves differently in series and parallel circuits. These rules let you work out missing currents and explain them using conservation of charge.
Why current stays the same in series
A series circuit has one loop, so charge has only one path to follow. The current is the same at every point in that loop. Charge is not used up as it passes through components, so moving an ammeter to a different point does not change its reading. In a circuit with a 3.0 V supply, both ammeters read 0.3 A, even though there are two lamps between them. The lamp nearest the battery does not get more current. Identical lamps in series are equally bright.
How current splits in parallel
A parallel circuit has more than one path. Each path is a branch, and the wires split and join at junctions. The current in the main wire is the total current from the battery. It divides between the branches, so the branch currents add up to the supply current. For example, if one branch carries 0.6 A and another carries 0.3 A, the supply current is 0.9 A. The rule can be written as I = I₁ + I₂, where I is the total current and I₁ and I₂ are the branch currents.
Why currents add up at a junction
Current measures how much charge flows each second. A current of 0.9 A means 0.9 coulombs of charge pass each second. Charge cannot be created or destroyed at a junction, so the charge flowing in each second must equal the total charge flowing out. Therefore, the current in equals the total current out. If 0.9 C arrives each second, it could split into 0.6 C and 0.3 C each second. This gives 0.9 A = 0.6 A + 0.3 A.
How to find a missing branch current
Use the total current and the known branch current. The branch currents must add to the total, so subtract the known current from the total to find the missing current.
What changes current in a series circuit
The current in a series circuit depends on the supply voltage and the parts in the loop. For the same circuit, raising the voltage increases the current. Increasing the resistance reduces the current. Adding more cells raises the supply voltage, so the current increases. Adding another lamp in series makes the current fall, so the lamps become dimmer. Replacing a part with one that has a bigger resistance also makes the current fall.