A coil stores magnetic energy and opposes sudden changes in current.
If the Capacitor opposed changes in Voltage, the Coil (or Inductor) is the component that opposes changes in Current.
It is, arguably, the component with the most “character” in electronics. If you try to force it, it kicks back. Let’s see why.
What is a coil
Constructively, it is ridiculously simple: it is a conductive wire wound around a core (which can be air, iron, or ferrite).
By winding the conductor, we take advantage of several electromagnetic phenomena studied, among others, by Ørsted and Faraday:
- Every electric current creates a Magnetic Field around it.
- By winding the wire, we concentrate that magnetic field in the center of the coil.
- That magnetic field stores energy.
Inductance is measured in henries (H), although we commonly use millihenries (mH) or microhenries (
The Inertia of Current
To understand the coil, let’s set electricity aside for a second and think about mechanics.
Imagine a heavy flywheel (a giant stone wheel) that you have to move by pushing.
- At first, it’s very hard: You apply a lot of force, but the wheel barely moves.
- Then it picks up speed: Once it’s spinning, maintaining it is no longer difficult.
- Finally, it’s hard to stop: If you stop pushing, the wheel keeps spinning due to inertia. If you try to stop it abruptly by standing in front of it, it will run you over.
The coil is similar to that, but for electrons.
The coil provides a kind of electrical inertia to the circuit: its current cannot change instantaneously.
Behavior in DC: The Transient
Let’s see what happens when we connect a coil to a battery (DC):
When connecting
We close the switch. The current wants to go from 0 to Maximum. But the coil says: “Hey, hold on there!”. A counter-electromotive force (a reverse voltage) is generated that fights against the battery.
- The current starts rising very slowly.
- The coil is storing energy in its magnetic field.
In steady state
Once the current stabilizes and is constant, the magnetic field stops changing. And if the field doesn’t change, the coil stops opposing.
- In direct current and once the transient is over, an ideal coil is equivalent to a short circuit.
- A real coil retains the resistance of the winding and losses in the core.
When disconnecting
Here comes the fun (and dangerous) part. If we suddenly open the switch, we are trying to make the current go from Maximum to 0 in an instant. The coil, which has a lot of energy stored in its magnetic field, refuses to let the current stop. “You’re cutting the wire? I don’t care, I’m still pushing.”
The coil will use all its energy to generate a gigantic voltage spike (hundreds or thousands of volts) capable of creating a spark (electric arc) across the air gap of the switch to keep the current flowing for one more millisecond.
The inductive spike: This phenomenon is what destroys switches, relays, and transistors if they are not protected. That is why, whenever you see a coil or motor controlled by a transistor, you will see a flyback diode (in antiparallel) to recirculate that energy safely when turning off.
Relationship between Voltage and Inductance
The voltage across a coil depends on the speed at which the current changes:
Notice the term
- If the current is constant (DC),
(Short circuit). - If we try to cut the current instantaneously, the time is nearly zero (dt ≈ 0). By dividing by something very small, the voltage VL tends to infinity. Pure math explaining the spark.
Summary of Passives (R, L, C)
With this, we complete the trio. Let’s compare them to have the final picture:
| Component | Symbol | Property | Stores Energy in… | In DC it behaves like… | Mechanical Analogy |
|---|---|---|---|---|---|
| Resistor | R | Resists current | Does not store (Dissipates heat) | Constant brake | Friction / Drag |
| Capacitor | C | Opposes change in Voltage | Electric Field (Volts) | Open Circuit | Spring / Reservoir |
| Coil | L | Opposes change in Current | Magnetic Field (Amperes) | Short Circuit | Mass / Inertia |
