# Homework Help: Replacing a battery with a charged capacitor

1. Feb 26, 2012

### svbalasub

1. The problem statement, all variables and given/known data
A Vbatt = 1.5 V battery stores Ubatt = 2.25 kJ of energy. I want to replace this battery with a charged capacitor that stores the same amount of energy and puts out the same voltage. That is, I want a capacitor such that UC = 2.25 kJ and VC = 1.5 V. What must the capacitance of such a capacitor be? What charge must this capacitor hold? Reasonable capacitors typically have a capacitance of at most one farad. Is this capacitor a reasonable replacement for the battery?

2. Relevant equations
U=1/2 CV^2

3. The attempt at a solution

2. Feb 26, 2012

### cepheid

Staff Emeritus
If you already have the equation for the energy stored in the capacitor, and you know U, and you know V, why can't you just use this equation to solve for C yourself?

One thing that I've heard is a problem with caps as energy sources is that they discharge too quickly, i.e. you get the energy all at once instead of a little bit at a time as needed.