- #1

sagda_m2

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**the self induction**:

The resistance of a coil is 15 ohm, itself induction is 0.6 Henry, it is connected to a source of direct current which gives 120 volts. Calculate the rate of growth of the current at the moment when the current reaches 80 % of its maximum value.

My Answer:

First I calculated the maximum current:

I= V/R = 120/5 =8 Ampere

Therefore: the 80 % of the maximum current =6.4 A

Then the electromotive force will be:

V= I * R = 6.4 * 15 = 96 volt

And since: the induced e.m.f in the coil = -L * (dI/dt)

Therefore: 96 = 0.6 * (dI/dt)

Therefore: dI/dt = 96/0.6 = 160 A/s

But…..

I found the answer in my book as follows:

Since e.m.f = -L*(dI/dt)

Therefore: (120-96) = 0.6 * (dI/dt)

Therefore: dI/dt = 24/0.6 = 40 A/s

So, why did he substitute the e.m.f in the equation with the difference in volts (120-96)? Why didn't he substitute with the (96 volts)?

Thanks in advance,