Current in Series and Parallel Circuits

Electric current is the flow of electric charge. It is measured in amperes (A). Imagine charge like water in pipes: current tells you how much “water” passes a point each second. The equations are: $I = \frac{Q}{t}$ and, for resistors, $I = \frac{V}{R}$.

Series Circuits

In a series circuit, components are in one single loop. There is only one path for current.

• Current is the same everywhere in a series circuit. The same amount of charge must pass each point because there is only one route.
• Adding more resistors in series increases total resistance, so the current from the source becomes smaller.
• Misconception: current does not get “used up.” Energy is transferred, but the current stays the same around the loop.

Parallel Circuits

In a parallel circuit, components are on separate branches. There are junctions where current can split and rejoin.

• Junction rule: the sum of currents entering a junction equals the sum leaving it: $$I_{\text{in}} = I_{\text{out}}.$$
• The current from the source is larger than the current in each branch. It equals the sum of the branch currents.
• Branches with lower resistance take more current (like a wider pipe carrying more water).
• In parallel, each branch has the same p.d. as the source.

Key Points

• Series: one path, same current at all points.
• Parallel: currents split and rejoin; at any junction, in = out.
• Use $I = \frac{V}{R}$ to find currents when resistances and voltage are given.