# R-L circuit exponential growth - need help

#### _holmesss

Joined Dec 7, 2015
1
The current in an R-L circuit grows by the following equation:

i = 5 [ 1 - e ^(R/L t ) ]

Where I is the current, R the resistance, L the inductance and t the time.

Determine the resistance R, given that the inductance L is 0.4 H and the current I is 4.5 amperes at t = 0.115 s.

#### MrAl

Joined Jun 17, 2014
10,925
Hi,

Start by substituting the known values into your equation, then solve for R.

#### Papabravo

Joined Feb 24, 2006
20,621
Did you drop a minus sign in the exponent, or make some other error? As the time t →∞, -e^((R/L)*t) grows without bound and approaches -∞. This is not physically possible.

#### MrAl

Joined Jun 17, 2014
10,925
Did you drop a minus sign in the exponent, or make some other error? As the time t →∞, -e^((R/L)*t) grows without bound and approaches -∞. This is not physically possible.
Hi,

Good catch. The exponent should show a minus sign.

We dont always have to show the minus sign as in the more general:
1-e^(at)

but when we see R and L or R and C in there we really have to show the minus sign because we normally dont work with negative R or negative C and t itself wont be negative either.