# Homework Help: Ohmic Resistor modelling ?(Check my interpretation of question)

1. May 1, 2012

### sid9221

http://dl.dropbox.com/u/33103477/Resistor.png [Broken]

For the first bit first I solved the equation

$$\frac{dI}{dt}=\frac{U-RI}{L}$$

$$I(t)=\frac{U-C_2e^{\frac{-t}{L}}}{R}$$

Now I put in T(0)=0 to work out the constant and I got

$$I(t)=\frac{U-Ue^{\frac{-t}{L}}}{R}$$

Now here's the dodgy bit, I did not do physics in my final years at school so I no clue what an "Ohmic Resistor" is but here is my interpretation of the question

$$I(t)=\frac{75U}{100R}$$

Working that out I got

$$t = -L log(0.25)$$

Now for the final part:

My interpretation was that the maximum saturation level was U/R=200/50=4

So I worked out:

$$I(t)= 3.5$$

Which gave me t=90.31

Does what I have done make sense cause I don't know much "higher level" physics so am just working from logic.

Last edited by a moderator: May 6, 2017
2. May 1, 2012

### lanedance

an ohmic resistor satisfies U=IR, so what you've done seems reasonable, though I haven't checked your numbers

one thing though, its always good practice to differentiate you solution, in this case I(t), and make sure it satisfies the original DE. I would recommend that check as usually the time constant in the exponential would depend on both R and L

Also you really need to know if they are in series or parallel, though it's implied by the DE..