/sci/ - Science & Math

>>14880034
>>14880027
Fucking confusing, I asked in the last thread as well. It was an exercise about electromagnets. Basically paraphrased:
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A current of I = 1A flows through an electromagnet (coil) with N = 10^3 windings. Its length is l = 0.4m and it has a cross-section of A = 100 cm^2. The whole wire has a resistance of R = 5 Ohm. Inside the magnet, there's an iron core with a magnetic field strength of B = 1T.

Calculate the induced voltage U_ind, when the magnet is turned off for [math] \Delta t = 1 ms. [/math]
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Pic related is the solution, I got the same, except at the end. If you do some proper calculations for L, you get the following, [math] L = \dfrac{NBA}{I} [\math], which they've substituted for the final solution. I just don't get why the current and change in current disappears.

Here's what they get at the end with L subsituted:
[math] U_{ind} = -L \dfrac{\Delta I}{\Delta t} = \dfrac{NBA}{I} \dfrac{\Delta I}{\Delta t} [/math]
and continued from there, somehow the current disappears completely (see pic's solution). How?

>>14858719
Physics question here, I'm getting quite pissed off. So basically the question goes:
"You have a coil with N = 10000 windings, a magnetic field B = 1T inside the windings and a current I = 1A running through it. The cross sectional area of the coil is A = 100 * 10^(-4) m^2 and the coil's length is l = 0.4m.

Calculate the induced voltage U, when the voltage is turned off for deltat = 1*10^(-3)s."

Solution is in pic related. I got as far as the formula. But why the fuck does:
[math] U = -L \dfrac{\Delta I}{\Delta t} =- \dfrac{NBA}{I}\dfrac{\Delta I}{\Delta t} = - NBA \dfrac{1}{\Delta t} = -10 \cdot 10^3 V [/math]
the I disappear here? Why is:
[math] \dfrac{\Delta I}{I} = 1 [/math]
? Or am I missing some shit.