Chapter 18, Problem 36P

### College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300

Chapter
Section

### College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300
Textbook Problem

# The RC charging circuit in a camera flash unit has a voltage source of 275 V and a capacitance of 125 μF. (a) Find its resistance R if the capacitor charges to 90.0% of its final value in 15.0 s. (b) Find the average current delivered to the flash bulb if the capacitor discharges 90.0% of its full charge in 1.00 ms.

(a)

To determine
The resistance R if the capacitor charges to 90.0% of its final value in 15.0s .

Explanation

Given Info: Capacitance C is 125â€‰Î¼F , time t is 15.0â€‰s charge on the capacitor q is 90.0â€‰% maximum charge on the capacitor Q is 100.0â€‰%

Explanation:

Formula to calculate the time constant Ï„ of the circuit

q=Q(1âˆ’e(âˆ’tÏ„))

On rearrange

Ï„=âˆ’tln(1âˆ’qQ)

• C is the capacitance of the capacitor,
• Ï„ is the time constant,
• t is the time
• q is the charge on the capacitor,
• Q is the maximum charge on the capacitor,

Substitute, 15.0â€‰s for t , 90.0â€‰% for q and 100.0â€‰% for Q in the above equation to find Ï„

Ï„=âˆ’15.0â€‰sln(1âˆ’90.0%100.0%)=âˆ’15

(b)

To determine
The average current Iav delivered to the flash bulb if the capacitor discharges 90.0% of its full charge in 1.00ms .

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