Voltage, current, power and Ohm's law
Voltage, current, resistance and power explained for makers and software engineers, with Ohm's law, power dissipation and resistor ratings.
Three quantities
Voltage (V, volts) is the electrical pressure between two points. It is always measured between two points, usually a point and ground. Current (I, amperes) is the flow of charge through a path. Resistance (R, ohms) is how much a component opposes that flow.
A useful picture: voltage is the height difference in a water pipe, current is the flow, resistance is how narrow the pipe is. The picture breaks down eventually, but it holds for everything on this page.
Ohm's law
V = I × R. Rearranged: I = V / R and R = V / I. With it you can size a resistor for an LED, find the current through a sensor, or check whether a wire is thick enough.
Power: what makes things hot
P = V × I, or combined with Ohm's law, P = I² × R and P = V² / R. Power is dissipated as heat. A resistor rated 0.25 W will survive 0.25 W, but it runs hot; the rule in professional design is to stay at or below half the rating. The same logic applies to regulators, transistors and traces.
Units you will see
| Prefix | Factor | Typical use |
|---|---|---|
| m (milli) | 10⁻³ | mA of an LED, mV of a sensor |
| µ (micro) | 10⁻⁶ | µA of sleep current, µF capacitors |
| n (nano) | 10⁻⁹ | nF capacitors, ns of signal edges |
| p (pico) | 10⁻¹² | pF capacitors |
| k (kilo) | 10³ | kΩ resistors |
| M (mega) | 10⁶ | MΩ resistors, MHz clocks |
Ohm's law stays true everywhere, but real circuits add temperature, tolerances and time. When a design must survive a temperature range, a battery discharge curve or production tolerances, the calculation becomes a worst-case analysis. That is part of every schematic we design.
Parts and tools to practise
| Item | What to look for | Buy |
|---|---|---|
| Digital multimeter | Auto-range with continuity beeper, CAT III rated. You will use it every day. | View on Amazon ↗ |
| Resistor assortment, 1 % metal film | E24 or E12 values from 10 Ω to 1 MΩ, 1/4 W. Sorted in labelled bags or a box. | View on Amazon ↗ |
| Solderless breadboard kit with jumper wires | Full-size breadboard, pre-cut jumpers and flexible Dupont wires. | View on Amazon ↗ |
| Practical Electronics for Inventors | The friendliest complete introduction. Start here if the other is too much. | View on Amazon ↗ |
Links go to Amazon. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. We only list tools of the kind we use on our own bench.
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