HW: What is energy used? What is wattage?

[JerseyGirl]

Cutnell Physics 7th Ed pg 600 # 39

Homework Statement

A camera flash has a capacitor.
Capacitor has a rating of: 850uF
The potential difference between the places is: 280V

a) Determine Energy used to produce flash.
b)If flash lasts 3.9E-9 S, what is the power or wattage of the flash?

Homework Equations

V=C/F
W=VA
A=C/S

The Attempt at a Solution

a) (280V)(850uF)= 0.238C

b) (0.238 C)/(3.9E-3 S)= 61.03A
W=?

The back of the book has a) =33J b) 8500W
Where did I go wrong?

Thanks in advance.

 

[rl.bhat]

Energy = 1/2*C*V^2
Power = E/t

 

[LowlyPion]

Welcome to PF.

What is the energy in a capacitor?

What you found is the charge Q = V*C

But the energy in a capacitor is 1/2 Q²/C

http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/capeng2.html

For 2 ... if Watts are Joules/s and they give you a time ...