Charge, current and voltage: the water park analogy
Electricity starts with charge, a property of tiny particles: protons (+) and electrons (−). Opposite charges pull together; like charges push apart. Current is charge in motion — the flow of electrons through a wire — measured in amperes (A).
Voltage is the push that drives that flow, measured in volts (V) — think of it as electrical pressure. A battery is a pump: it pushes charge out of one end, around the circuit, and back to the other. Resistance (ohms, Ω) is anything that throttles the flow — a thin wire, a resistor, a corroded connection.
- Charge comes in two signs: + (protons) and − (electrons)
- Current = flow of charge, measured in amperes (A)
- Voltage = electrical push or pressure, measured in volts (V)
- Resistance = flow throttling, measured in ohms (Ω)
- Water park: current = flow rate, voltage = pump pressure, resistance = pipe narrowness
Ohm's law: the one equation that runs your phone
Ohm's law links all three: V = IR — voltage equals current times resistance. Bump the voltage with the same resistance and more current flows; add resistance at the same voltage and current shrinks. Rearrange the same equation for any missing piece: I = V/R and R = V/I.
The standard mistake is plugging in mixed units or grabbing the wrong quantity. Convert to volts, amps and ohms first, write the formula, then substitute. That three-second ritual catches nearly every Ohm's law error.
- V = IR · I = V/R · R = V/I
- More voltage (same R) → more current
- More resistance (same V) → less current
- Keep units consistent: V, A, Ω
A 9 V battery drives current through a 3 Ω resistor. I = V/R = 9 ÷ 3 = 3 A. Swap in a 6 Ω resistor and current drops to 1.5 A — double the resistance, half the current, exactly as Ohm's law predicts.
Series and parallel circuits
In a series circuit, one path connects everything: the same current squeezes through each device in turn. More bulbs in series means more total resistance and dimmer bulbs — and one burned-out bulb opens the only path, killing the whole string. That's why old Christmas lights died together.
In a parallel circuit, each device gets its own branch across the same voltage. Every bulb glows at full brightness, one failure doesn't kill the rest, and total resistance drops as you add branches — which is why your house is wired in parallel. Overloaded outlets happen because more parallel branches pull more total current.
- Series: one path, same current through everything
- Series: more bulbs → more resistance → dimmer
- Parallel: branches share the same voltage
- Parallel: one dead bulb doesn't kill the others
- Houses are wired in parallel — every outlet gets full voltage
Key concepts to memorize
🎯 Study tips for this topic
- Anchor V, I and R to the water analogy first — pressure, flow, narrowness — then attach the symbols.
- Drill the Ohm's law triangle: cover the quantity you want; the layout of the other two shows the formula.
- Redraw every circuit as a clean rectangle diagram before solving.
- Practice unit conversion (milliamps to amps) — mixed units cause most wrong answers.
- Predict before you compute ('more resistance should mean less current'), then check; physics gets satisfying when predictions land.
