Effective Voltage Calculation for Inductance Problem

[crom1]

Homework Statement

Find effective value of voltage between points 3 and 4. The diagram on the right shows behaviour of current i.

Homework Equations

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

The Attempt at a Solution

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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?

 

[gneill]

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.

 

[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.

 

[cnh1995]

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.

Looks correct to me.

Find effective value of voltage between points 3 and 4.

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

 

[gneill]

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.

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?

 

[crom1]

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