Solving Old Midterm Problem: Voltage Across 9kΩ Resistor

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SUMMARY

The discussion focuses on calculating the voltage across a 9kΩ resistor in an electrical circuit involving an inductor. The user initially assumes that the current through the resistor equals the current through the inductor and attempts to derive the voltage by multiplying the current by the resistor value. However, a more accurate approach is suggested, emphasizing the need to express the inductor current (IL) as a function of input current, inductance (L), and resistances (R1, R2, R3) to simplify the calculation. The correct expression for the current at time t=0 is highlighted as i(0) = i∞ + (i∞ - i0), indicating a potential error in the user's initial formulation.

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gfd43tg
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I am working on the second problem of an old midterm, and I cannot figure out how they got that answer for the voltage across that 9kΩ resistor. I said the current going through the resistor is the same as that going through the inductor, so I figured I would find the current as a function of time, then just multiply by the resistor value.

I posted the solution as well
 

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When you present a symbolic expression as an answer, you have to be sure that all symbols are defined. Also you want each symbol to be a non changing value. IL is a dynamic value. While your answer of Vo=IL*R3 is correct. it not the simplest. Find IL as a function of the input current, L, R1, R2, and R3
 
When t=0 the exp(-t/T) term equals 1.

So the expression you have for i becomes i(0) = i + ( i - i0 )

This does not look right. You have a sign wrong, or something.
 
Last edited:

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