Inductance Formula
Learn the inductance formula V = L(dI/dt) and how inductors combine in series and parallel.
Key to understanding electromagnetic circuits.
The Formula
An inductor opposes changes in current flowing through it.
The voltage across an inductor is proportional to the rate at which the current is changing.
The SI unit of inductance is the Henry (H).
Variables
| Symbol | Meaning |
|---|---|
| V | Voltage across the inductor (Volts, V) |
| L | Inductance (Henrys, H) |
| dI/dt | Rate of change of current (Amperes per second, A/s) |
Series and Parallel Rules
Inductors in series (total inductance increases):
Inductors in parallel (total inductance decreases):
Example 1
A 0.5 H inductor has current changing at 4 A/s. What voltage is induced?
V = L × (dI/dt)
V = 0.5 H × 4 A/s
V = 2 V
Example 2
Two inductors, 10 mH and 40 mH, are connected in parallel. Find the total inductance.
1/L_total = 1/L₁ + 1/L₂
1/L_total = 1/10 + 1/40
1/L_total = 4/40 + 1/40 = 5/40
L_total = 40/5
L_total = 8 mH
When to Use It
Use the inductance formula when you need to:
- Calculate the voltage produced by a changing current in a coil
- Design transformers, motors, and electromagnetic devices
- Analyse RL (resistor-inductor) transient circuits
- Build filters and oscillator circuits
Energy stored in an inductor is E = ½LI².
Inductor combination rules follow the same pattern as resistors: they add directly in series and use the reciprocal formula in parallel.