# Homework Help: LR Circuit (Calculus)

1. Jun 25, 2013

### will.i.am2

A coil connected across a d.c. supply will cause a spark to occur across a switch when turned off. The current flowing in the circuit at this moment is given by the equation i = IetR/L amps. Where I is the maximum circuit current, R the resistance of the coil and L the inductance of the coil.

(a) For a coil of resistance = 10Ω and Inductance 1.2 henrys through which the maximum current flowing was 5 amps, plot the graph for values of t between 0 and 0.6 seconds.

(b)From the graph determine its gradient at t = 0.2 seconds.
26/0.2 = 130???

(c) Using calculus determine the gradient of the curve and from it the actual gradient when t = 0.2 seconds.

So, any help would be appreciated.
thanks

****work is attached in form of images please don't remove it this time*****

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• ###### part c.png
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2. Jun 25, 2013

Remember when taking the derivative of an exponential, you just get an exponential with some constants out front. That might help in drawing your graph =)

3. Jun 25, 2013

### Staff: Mentor

Your derivative in part c is incorrect, as it was in the other problem you posted.

What is this derivative?
$$\frac{d}{dt}e^{tR/L}$$

BTW, it's reasonable to post images of tables and graphs, but you should post the work you did right here in the text pane, not as an image.
If you're referring to your post for the CR problem, the images are still there. No one removed them.