P8.3-25 Consider the circuit shown in Figure P8.3-25a and corresponding plot of the voltage across the 40 W resistor shown in Figure P8.3-25b. Determine the values of L and

Answer: L = 8 H and = 10 W.

Hint: Use the plot to determine values of D, E, F and a such that the voltage can be represented as

(a)	(b)

Figure P 8.3-25

Examining the curve we can determine the equation for v(t). we can see

So

Finally,

Solving for

So the equations for voltage from the plot are

So the circuit at steady state just before the switch opens looks like

The current through the inductor cannot change discontinuously so the current at t<0 is the sum of the current through R₂ and the 40 Ω resistor so .

Now the circuit immediately after the switch is thrown looks like

Since the current cannot change we must have

So we know what the current before the switch thrown was

Solve for R₂

From we can use the relationship

The Thevenin resistance in this case is just the 40 Ω, so solving for L

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This page last updated on July 10, 2019