- #1
yosimba2000
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Homework Statement
After a long time, an inductor is disconnected quickly from the battery and then attatched to a resistor.
Homework Equations
V = Ldi/dt
V = ir
The Attempt at a Solution
KVL: voltage across inductor - voltage across resistor = 0
Ldi/dt - ir = 0
di/i = rdt/L
ln|i| from i(t) to i(0) = rt/L + C
i(t)/i(0) = e(rt/L +C)
i(t) = i(0)Cert/L
At time t= 0, i(t) = initial current i(0). Plugging in i(t) = i(0) and t = 0 gives C =1.
Now we have i(t) = i(0)ert/L
The correct equation is i(t) = i(0)e-rt/L, different from what I have; I have a positive time constant r/L.
I know i(t) must be smaller than i(t) when time increases, and usually this is taken care of by a negative constant of integration, but as C = 1, the only way I can account for i(t) < i(0) is to use negative time.
Derivations I have seen started off KVL with -Ldi/di - ir = 0, but I don't understand why my method doesn't also work.
Is my method incorrect?
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