Question

An inductor produces a back emf in a dc series rl circuit when a switch connecting the battery to the circuit is closed. We can explain this by.

Answer 1

Hi there!

Let's say that a series RL circuit is constructed, with the switch left open.

When the switch is closed, a current will start flowing in the circuit. The INCREASING current will go through the loops of the inductor, producing an INCREASING magnetic field and consequentially a magnetic flux through the inductor. (Use hand rules).

However, the inductor does NOT want to experience this increasing magnetic flux, so an emf in the OPPOSITE direction of the current will be produced to counteract the increasing flux. (The flux would also be in the opposite direction).

This is expressed with the equation:

`\epsilon = -L (di)/(dt)`

ε = Back emf (V)
L = Inductance (H)

i = Current (A)
t = time (s)

With the negative sign, we can see that if the current of the circuit is INCREASING, the emf produced would be in the opposite direction.