Capacitor or coil: who lets the current through?

Two lamps, one switch, one power box. The back lamp has a capacitor in its path; the front lamp has a coil. Everything drawn in blue can be clicked or dragged: flip the switch, change how the power pushes, and watch which lamp lights up.

Lamp light and moving dots: current flowingBack lane: capacitor (gauge shows how full)Front lane: coil (compass shows its magnetic field)Both lanes get the same push from the same supply through the same switch, and both lamps are the same. The only difference is the part at the end: a capacitor (it stores charge, like a tank filling with water) or a coil (it stores a magnetic field, like a heavy wheel that is hard to start and hard to stop). Real circuits do all this in thousandths of a second, so the figure plays in slow motion (the factor is shown beside the controls).
How to read it: each line is a wire. ⊗ is a lamp. Two parallel plates with a gap are the capacitor: nothing crosses the gap, charge just piles up on the plates (+ on one, − on the other). The bumps are the coil; the loops around it are its magnetic field. The two switch blades are tied together by the dashed line, so they always move together. The little arrows move at the speed of the current, and turn round when it flows backward.

Try this each one sets up the bench and runs it

How it works

The capacitor is a tank it stores charge

A capacitor is two metal sheets with a thin gap between them. Current can't jump the gap, but charge can pile up on one sheet, like water filling a tank. While the tank fills, current flows and the lamp shines. When it is full, the flow stops. So a capacitor lets changes through and blocks a steady push.

The coil is a heavy wheel it stores a magnetic field

A coil is a long wire wound round and round, here on an iron core. Current through it builds a magnetic field (watch the compass turn). The field is like a heavy wheel: hard to get spinning, and hard to stop once it spins. So a coil blocks changes and lets a steady push through.

Switching ON and OFF

ON: the capacitor's lamp flashes bright at once, then fades as the tank fills; the coil's lamp starts dark and slowly gets bright as its wheel spins up. OFF: the capacitor keeps its charge, so the gauge stays lit. The coil's current can't stop at once, so it jumps across the opening switch as a spark, hundreds of volts for a split second.

Wiggles

A wiggling push keeps changing direction. A slow wiggle is almost steady: the capacitor fills and blocks it, the coil keeps up and lets it through. A fast wiggle is all change: the capacitor never gets full, so it lets it through, and the coil's heavy wheel can't turn round fast enough, so it blocks it. The two parts are opposites.

Where real circuits use this

  • Capacitor holds voltage steady. Next to every chip in a phone or computer, a tiny capacitor gives a quick sip of charge when the chip suddenly needs it. Big ones smooth the bumpy power inside chargers.
  • Capacitor passes wiggles, blocks steady. In speakers it sends only the fast, high notes to the small tweeter. In amplifiers it passes the music and blocks the steady voltage.
  • Capacitor stores a burst. A camera flash fills a capacitor slowly, then dumps it all at once into the flash tube.
  • Coil holds current steady. Phone chargers and LED drivers switch on and off thousands of times a second; a coil keeps the current smooth between switches.
  • Coil passes steady, blocks wiggles. In speakers a coil sends only the slow, low notes to the big woofer. The lump on a laptop cable is a coil that blocks fast electrical noise.
  • Coil's kick when switched off. A car's ignition coil uses that spark on purpose to fire the spark plug. Every relay coil in a circuit has a diode beside it to soak up the kick so it can't damage anything.
  • Both together. A capacitor and a coil pass energy back and forth like a swing, at one special speed. Radios use this to pick a single station.

For grown-ups i = C·dv/dt   v = L·di/dt

Back lane: a 10 Ω lamp in series with 1000 µF, time constant τ = RC = 10 ms. Front lane: the same lamp with 100 mH (0.5 Ω of wire), τ = L/R = 9.5 ms. Supply 6 V. Reactances XC = 1/(2πfC) and XL = 2πfL cross at about 16 Hz; "slow" is 5 Hz and "fast" is 100 Hz. The lamps are treated as fixed resistors that light instantly (a real filament takes a few tens of milliseconds to warm). The spark is modelled as a 400 Ω arc. A steady push plays 150× slower than real time; a wiggle is slowed until one wiggle takes about 1.5–3 s (the factor is shown live). The wiggle range is 2–200 Hz.

In short: slow and fast wiggles one picture each

Capacitor = a small balloon in the pipe

Current flows only while the balloon is filling up or emptying.

  • Slow wiggle: each push lasts a long time, so the balloon fills up completely and then stops the flow. Most of the time nothing moves, so the lamp is dim.
  • Fast wiggle: the push flips before the balloon gets full. Air keeps rushing in and out, so the lamp is bright.

Coil = a heavy wheel in the pipe

It's hard to speed up and hard to slow down.

  • Slow wiggle: the wheel has time to get going and turn round with each push. Current flows freely, so the lamp is bright.
  • Fast wiggle: the push flips before the heavy wheel gets moving. It barely turns, so the lamp is dim.

Slow wiggle → the coil's lamp wins. Fast wiggle → the capacitor's lamp wins.

They're opposites because a capacitor likes change and a coil hates change. A fast wiggle is lots of change; a slow wiggle is almost no change.