An electric circuit provides a complete path for charge to flow. Components can be joined in series (one loop) or in parallel (separate branches). The way they are connected decides how current and voltage are shared.
Series and parallel
In a series circuit the current is the same at every point, while the source voltage is shared between the components. In a parallel circuit each branch has the full source voltage, and the current splits between the branches.
Key idea
Series resistors add: R = R1 + R2. For two parallel resistors: 1/R = 1/R1 + 1/R2. Ohm's law: V = IR.
Common components
- Ammeter — measures current, connected in series.
- Voltmeter — measures voltage, connected in parallel.
- Variable resistor (rheostat) — changes resistance to control current.
- Diode — allows current in one direction only.
- Thermistor (NTC) — resistance falls as temperature rises.
- Light-dependent resistor (LDR) — resistance falls as light gets brighter.
Worked example
Two 4 Ω resistors are joined in parallel. Combined resistance: 1/R = 1/4 + 1/4 = 2/4, so R = 2 Ω. If this pair carries a current of 2 A, the voltage across it is V = IR = 2 × 2 = 4 V.
Remember
- Current is shared in parallel, unchanged in series.
- Ammeter in series, voltmeter in parallel.
- A parallel combination has a resistance smaller than the smallest resistor.