RL Circuit Let's begin by looking at an example of an inductorresistance circuit, often referred to as an RL circuit.
When the switch is initially closed, I represents the current in the circuit and the inductor acts as a seat of "back emf."
We can write the following equation
where
is the back emf induced by the coil to oppose the flux being created by the currents flowing through the solenoid.
Time Constant
As we did previously with RC circuits, we will now derive the time constant for this circuit. Since the inductor is intially opposing any changes in current, it will initially act to thwart the current flowing through the circuit, letting I = 0 when t = 0. In our derivaton, a lower case i represents transient current.

The expression L/R is called the LR time constant.

