Back EMF

What it shows:

A sudden change in current in an inductor - resistor circuit produces a very large back EMF. If that resistance is a bulb, it will shine much brighter during the change than during DC flow.

E = -LdI/dt

How it works:

The circuit consists of a 6V bulb connected in parallel with a 10.5mH inductor coil as in figure 1. With the battery connected, the bulb burns at its rated 6V. Disconnecting the battery sends the applied voltage and hence the current to zero. The rapidly collapsing current through the inductor coil produces a very large EMF in the same direction as current flow (it is trying to oppose the change); this momentarily lights the bulb much more brilliantly than does the constant 6V.

figure 1. Back EMF circuit

Setting it up:

The inductor coil and bulb circuit is mounted on a single wooden board. Voltage is supplied by a 6V gel cell. 1 To connect and disconnect the supply, simply touch an alligator clip to the terminal, then lift it away.


Very clearly shows the large EMF generated when you change the current so rapidly.

1 Technacell EP695 rechargeable gel cell 6V 9.5AH. Elpower Corp. Santa Ana, Ca.