Inside the resistor
The simplest part in electronics, and the most numerous: a phone has hundreds. A resistor turns electrical energy into heat on purpose, and in doing so sets currents and divides voltages. Every picture below is computed while you watch.
What it is
In a conductor the free electrons move through a lattice of metal ions. A voltage gives them a small push along the wire, but they keep colliding with the vibrating ions and losing what they gained: the energy goes into the lattice as heat. That opposition is resistance.
V = I · R R = ρ · L / A P = I² · R = V² / R
ρ (rho) is the material’s resistivity: copper 1.7 × 10⁻⁸ Ω·m, nichrome about 65 times more, carbon about 2 000 times more. Longer means more resistance; thicker means less.
How it’s made
Inside the conductor: drift and collisions
The electrons zip about at hundreds of kilometres per second in random directions (thermal motion), colliding with the lattice; the field adds a tiny average drift. The resistance comes from those collisions: more vibration (heat) or more disorder (an alloy) means more of them. The drift speed itself is astonishingly slow.
Not to scale: the picture’s drift is exaggerated so you can see it; the readout gives the real one. Carbon’s resistance falls slightly with heat (a negative temperature coefficient); metals’ rises.
Power and heat
All the power a resistor takes becomes heat, P = V²/R, and its body warms until it sheds that heat as fast as it gets it: ΔT = P · Rth, where Rth is its thermal resistance to the air. A bigger body has a lower Rth: that is what the power rating really buys.
At 25 °C in still air; the heating is shown five times faster than real. Typical thermal resistances, not one maker’s datasheet.
The colour code
Bands: two (or three, for 1 % parts) digits, a multiplier, and a tolerance. Pick the bands, or type a value to see its bands.
What goes wrong
- Overheating: run over its rating and it browns, drifts in value, and eventually cracks open. A burnt resistor is usually a symptom: something downstream drew too much.
- Pulse overload: a short spike can melt a thin film even when the average power is low; pulse-rated and wirewound parts handle surges.
- Voltage limit: every resistor also has a maximum voltage (200–500 V for small ones), regardless of power: high-voltage dividers use several in series.
- Noise and drift: carbon parts are noisier and drift more with temperature than metal film; precision circuits use 1 % or 0.1 % metal film.
Use it on the bench
- Ohm’s law — V = I·R with a meter (about it)
- Voltage divider — two resistors dividing a voltage (about it)
- Series resistors — resistors adding in series (about it)
- Parallel resistors — and in parallel (about it)
- Power and power ratings — power, ratings and a resistor too small (about it)
- Wheatstone bridge — the Wheatstone bridge (about it)
Check yourself
Exam-style questions (ECE, EE and NC-II reviewers ask these). Each answer links to the scene that shows it.
1. A copper wire is stretched to twice its length (its volume stays the same). Its resistance:
R = ρL/A: twice the length and, at the same volume, half the cross-section, so 2 × 2 = 4 times. Try length and diameter in the scene. See the scene ▸
2. Why does a metal’s resistance rise when it gets hot?
More thermal vibration of the lattice means a shorter time between collisions. Raise the temperature in the drift scene and watch the ions shake. See the scene ▸
3. The electrons in a copper wire carrying 1 A drift at roughly:
v = I/(n·q·A): copper has about 8.5 × 10²⁸ free electrons per m³, so the drift is tiny, about 0.02 mm/s in 2.5 mm² wire. The signal travels at near light speed; the electrons barely move. See the scene ▸
4. A 100 Ω ¼ W resistor across 9 V dissipates:
P = V²/R = 81/100 = 0.81 W. It will run far above its maximum temperature. Try it in the heat scene. See the scene ▸
5. The bands brown, yellow, red, gold mean:
Brown 1, yellow 4, × red 10² = 1 400 Ω; gold ± 5 %. See the scene ▸
6. An SMD resistor is marked 472. Its value is:
The last digit is the number of zeros: 47 × 10² = 4 700 Ω. See the scene ▸
7. Why do designers keep a resistor’s power under about half its rating?
The rating assumes 70 °C ambient and still air, and the part at its limit runs at 155 °C. Derating keeps it cool and stable for years. See the scene ▸
8. How does a factory get a film resistor exactly to 10 kΩ?
The groove lengthens and narrows the conducting path; the laser stops when the measured value reaches the target. See “How it’s made”. See the scene ▸
Inside the other parts
the capacitor · the inductor · the transformer · the diode and LED · the transistor · the battery · all the parts