You already know that current flows through a circuit at some voltage and meets some resistance along the way. Ohm's Law ties those three together: V = I x R, so the current through a bulb is its voltage divided by its resistance. Now look at how the parts are arranged. There are two main patterns: series and parallel.
In a series circuit, there is only one path for current to flow. That means the current is exactly the same at every point in the loop. The voltage, though, splits: each part uses some of it, and those voltage drops add up to the battery's total voltage. Break that single path anywhere and everything in the loop stops at once.
In a parallel circuit, the parts sit on separate branches. Each branch connects straight across the battery, so every branch gets the full voltage. The current is what splits this time: it divides between branches, and those branch currents add up to the total current the battery supplies.
Every lamp in your house is wired in parallel, so each one gets full voltage and works on its own path. Turn off one light and the others stay on. Old-style Christmas lights are wired in series instead, so one broken bulb breaks the only path, and the whole string goes dark.
Series shares one path, so current stays the same while voltage splits; parallel gives every branch full voltage while the currents add up.
Series circuits have one path. Current is the same everywhere. Voltages add up across each component. Total resistance is the sum: R_total = R1 + R2 + R3. Parallel circuits have multiple paths. Voltage is the same across each branch. Currents add up. Total resistance is less than the smallest branch. For two resistors: 1/R_total = 1/R1 + 1/R2. Household outlets are wired in parallel. Each appliance gets the full 120 V or 230 V. If one bulb burns out the others stay lit. Old Christmas lights were series so one broken bulb killed the whole string.
In series the current has no choice but to flow through each resistor one after another. The voltage drops across each one. In parallel the current splits. Each branch gets the full voltage. More branches mean more total current from the battery. The battery sees a smaller total resistance. This is why adding more appliances in parallel draws more current. A fuse or circuit breaker protects the wiring from too much current.
Equivalent resistance simplifies a network to a single resistor. For series just add the resistances. For parallel use the reciprocal formula. A combination circuit has both series and parallel parts. Simplify step by step. Replace parallel groups with their equivalent. Then add series resistances. This lets you find total current from the battery. Then work backwards to find voltages and currents in each branch.
Voltmeters measure voltage. Connect them in parallel across the component. They have very high resistance so they draw almost no current. Ammeters measure current. Connect them in series in the circuit. They have very low resistance so they do not affect the current. Never connect an ammeter in parallel it would short the circuit. Multimeters can do both by switching internal connections.
In a series circuit the current is the same everywhere. Measure it anywhere with an ammeter. In a parallel circuit the total current from the supply equals the sum of branch currents. You can calculate branch currents using I = V/R for each branch since voltage is the same. The supply current is the sum. For example two 10 ohm resistors in parallel across 12 V. Each draws 1.2 A. Total is 2.4 A. Equivalent resistance is 5 ohms. 12/5 = 2.4 A matches.
Last time you tied V, I and R together with V = I x R. The class torch runs one bulb on its 6V battery, and the ammeter reads 2A. Zoe wants a second bulb in there so the beam covers more ground, and Omar wires it into the same single loop.
The battery still gives 6V. With both bulbs in that one loop, what does each bulb get?
Optional connections
This opens up
Jobs that lean on this skill. Follow one to see everything it is built on.
14 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.