DP 7-4 A high speed flash unit for sports photography requires a flash voltage

And

The flash unit uses the circuit shown in Figure DP 7-4. Switch 1 has been closed a long time, and switch 2 has been open a long time at t = 0. Actually, the long time in this case is 3 s. Determine the required battery voltage V_B when C = 1/8 F.

Figure DP 7-4

In the steady state condition before t = 0 and the switches change the circuit would look like

The voltage across the capacitor is the same as the voltage across the 1 Ω since they are in parallel. So we can use simple voltage division as the battery, 3Ω and 1 Ω are all in series. So

After the switches are thrown, the circuit becomes

Since cannot change instantaneously, since it was connected to an open circuit before the switches were thrown.

We know for the capacitor

So solving for

With , we have just the right loop as a simple series circuit, so we can get current from

As it should be according to the problem.

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This page last updated on April 14, 2019