R-L circuit exponential growth - need help

Thread Starter

_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.
 

Papabravo

Joined Feb 24, 2006
12,767
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
6,954
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.
 
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