# RL Circuits

## Homework Statement

A coil of resistance 8.0 ohms and self-inductance 4.0 mH is suddenly connected across a constant potential difference of 100 V. Let t = 0 be the time of connection, at which the current is zero. Find the current I and its rate of change dI/dt at times (a) t = 0, (b) t = 0.1 s, (c) t = 0.5 s, and (d) t = 1.0 s.

## Homework Equations

If=E_o / R = 100V/8.00ohms = 12.5A

T(time constant)= L/R = 4.0 mH/8ohms = .500 ms

## The Attempt at a Solution

Substitute If and T to obtain
I = (12.5A) (1-e^(-t/.500ms))

Express dI/dt :

dI/dt = 12.5A(-e^(-t/.500ms)) <--- here is where im stuck. Seems like there should be a numer in the thousands to multiply with units s^-1 to cancel out the other units

which in return would yiels aan anser like:
= (##kA)e^(-t/.500ms)

Then substitute all the reset of the values in.

Not sure what i am missing here Please help me!!!

## Answers and Replies

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anyone?!?!?!?