# Inductance and LC circuits

1. Mar 21, 2016

### meaghan

1. The problem statement, all variables and given/known data
In the following figure(Figure 1) switch S1 is closed while switch S2 is kept open. The inductance is L = 0.360 H, the resistance is R = 31.0 Ω , and the emf of the battery is 22.0 V . At time t after S1 is closed, the current in the circuit is increasing at a rate of di/dt = 7.20 A/s.

At this instant what is Vab, the voltage across the resistor?

2. Relevant equations
Emf = L * di/dt
Kirchhoff's law
V= IR

3. The attempt at a solution
I tried to get the emf in the inductor and add it to the 22v given, but that didn't work.
should i add the current (22/31 A) to the change in current integrated? I would get 7.2t + 22/31.

2. Mar 21, 2016

### Staff: Mentor

Can you write a KVL equation for the loop?

3. Mar 21, 2016

### meaghan

V total = 22V + (.360*7.2)
V total = E + L*di/dt

4. Mar 21, 2016

### Staff: Mentor

What is V total?

The loop has three components that exhibit potential changes: The battery, the resistor, and the inductor. Your KVL equation should sum the potential changes for each component as you "walk" around the loop, and that sum should be zero. Since the potential across the resistor is the unknown that you wish to find, let it be represented by a variable.