# Homework Help: Problem with inductance

1. Jan 31, 2016

### crom1

1. The problem statement, all variables and given/known data
Find effective value of voltage between points 3 and 4. The diagram on the right shows behaviour of current i.

2. Relevant equations

$$u_M= M \frac{di}{dt}$$

3. The attempt at a solution
I tried with formula above, didn't work for me. I haven't seen how to solve these kind of problems, so can someone give me a direction or a hint?

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2. Jan 31, 2016

### Staff: Mentor

You need to show what you've tried, not just said you did. How are we to see where you might have gone wrong if we can't see what you've done? Give us some details.

3. Jan 31, 2016

### crom1

I never seen this kind of problem before,so I am not sure what to do. If the above formula is supposed to be used, then on the first part of interval (where i is increasing) i get u_m=5V,and on the second part I get u_m=20V but I guess in different direction?
I am not sure what to do with them.

4. Jan 31, 2016

### cnh1995

Looks correct to me.
Plot the voltage waveform accordingly and find its effective value. Do you know the general formula for finding the rms value?

5. Jan 31, 2016

### Staff: Mentor

You've got the right idea. the quantity $dI/dt$ is a signed quantity. So it'll determine the sign of the voltage at the output. The inductor "dots" are an indication of how the flux linkage between the inductors is oriented with regards to the ends of the inductors. By convention, an increasing current entering into the dot end of one inductor produces a potential across the other inductor which tries to increase the current flowing out of its dot end.

Presumably you're expected to plot the resulting voltage curve?

6. Jan 31, 2016

### crom1

Thanks to both of you. I only have to find effective value, and I get the right solution (10V).