Ohm's law and circuit basics
Every circuit — from a flashlight to a motherboard — comes down to three quantities: current flowing, voltage pushing it, and resistance slowing it. Ohm's law ties them together.
Current: charge in motion
Electric current is the rate of charge flow past a point, measured in amperes (A). One ampere means one coulomb of charge (about 6.24 × 10¹⁸ electrons) passing per second. Current only flows in a closed loop — break the loop with a switch, and the flow stops everywhere in the loop at once.
By convention, current arrows point from positive to negative terminal, opposite to the actual electron drift. Circuit analysis always uses this conventional current direction, and it works regardless.
Voltage and resistance
Voltage is the potential difference between two points, measured in volts (V) — the "push" that drives charge. Voltage is always measured across two points; there is no such thing as the voltage at one point, only relative to another (ground is the chosen zero).
Resistance (ohms, Ω) is how strongly a material opposes current. A resistor's current grows linearly with the voltage across it — the relationship is Ohm's law:
V = I × R
Double the voltage across a fixed resistor and the current doubles. Double the resistance at a fixed voltage and the current halves.
Power: where the energy goes
Circuit elements absorb or deliver power, measured in watts (W). For any element:
P = V × I = I²R = V²/R
Passive sign convention: when current enters the positive voltage terminal, the element absorbs power (a resistor heating up); when it exits there, the element delivers power (a battery). Power balances in every circuit — what sources deliver, the rest of the circuit absorbs.
Flashcard · Unit 1
Ohm's law: voltage across a resistor equals current times resistance.
Check your understanding
A 12 V battery drives a 240 Ω resistor. How much current flows, and how much power does the resistor dissipate?