Series circuits
- A series circuit is a circuit in which every component sits on the single loop that the current must travel around
- The same current flows through every component because there is only one path for the charge to take; what enters one end of the loop must come out of the other
- A break anywhere in a series circuit (a blown bulb, a loose wire, an open switch) stops the current everywhere, because there is no alternative route
Current in a series circuit is the same at every point
- An ammeter placed anywhere in a series circuit reads the same value
- The current is set by:
- the total voltage of the supply (cell or battery)
- the total resistance of all the components added together
- Increasing the supply voltage pushes more current around the loop; adding more components in series raises the total resistance and so reduces the current
Parallel circuits
- A parallel circuit contains two or more loops that the current can take, joined at points called junctions
- The wires connecting two junctions are called branches; each branch carries its own share of the current
- Because each branch is independent, a break in one branch leaves the other branches still working, since only the branch with the fault loses its current
Current splits at junctions: conservation of charge
- At any junction, whatever current arrives must also depart, so the sum of branch currents in equals the sum of branch currents out
- This is a direct consequence of conservation of charge: charge can be neither created nor destroyed, so whatever flows in must flow out
- The split between branches is not necessarily equal, because branches with lower resistance carry more current and branches with higher resistance carry less. The two are only equal when the branches have identical resistance
Example — at a junction in a parallel circuit, the supply current flowing in is 18 A. One branch leaving the junction is measured at 11 A. Calculate the current in the other branch.
- Charge is conserved at every junction: current in = current out
- Current in the missing branch = 18 − 11 = 7 A