# Capacitor questions

1. Apr 24, 2006

### Agent

Hey, just started this topic and I could use some help. Can someone explain the results? I got a bunch of equations and my head is starting to hurt. :grumpy:

First question.
Find the ammount of energy stored in a 1,000-microfarad capacitor that has been charged to 18 volts.

And
A 220-microfarad capacitor is being charged through a 330-ohm path from a 21 volt battery. Calculate the time required for the capacitor voltage to reach 14 volts after the final circuit connection is made.

2. Apr 24, 2006

### Ouabache

Welcome Agent to Physics Forums!! I see this is your 7th post. Feel free to browse around, it is a fun way to learn things.

Since this is a homework question, the best place to post would be: <Homework & Coursework Questions> .. your question can fit in either subtopic <Introductory Physics> or <Engineering, Computer Sci and Tech>

3. Apr 24, 2006

### Agent

4. Apr 24, 2006

### Staff: Mentor

And to get you started, Agent, can you tell us...

-1- What equation relates the charge on a capacitor to the capacitance and the voltage? What similar-looking equation relates the energy stored on the capacitor to the capacitance and the voltage?

-2- What can you tell us about how exponential decay (and exponential rise) equations are used in R-C circuits? What is the definition of the RC time constant? What is the equation that relates the voltage on a capacitor that is charging through a resistor from a constant voltage source? How do you solve that equation for a time, when you are given the other values?

5. Apr 24, 2006

### Staff: Mentor

Okay, this thread didn't get moved yet, so let's stop posting here, and move to the new thread started by Agent in the Homework area....

Last edited: Apr 24, 2006
6. Apr 24, 2006

### Staff: Mentor

Agent, you need to show *your* work, not the example work of your teacher. Can you answer some of these leading questions to get you started?

-1- What equation relates the charge on a capacitor to the capacitance and the voltage? What similar-looking equation relates the energy stored on the capacitor to the capacitance and the voltage?

-2- What can you tell us about how exponential decay (and exponential rise) equations are used in R-C circuits? What is the definition of the RC time constant? What is the equation that relates the voltage on a capacitor that is charging through a resistor from a constant voltage source? How do you solve that equation for a time, when you are given the other values?

7. Apr 24, 2006

### Agent

That was my work, I just didnt go through the steps because its hard to write it out here. Anyways, heres the first.

1.
C=Q/V

ES=
1
- CV^2
2

ES=energy stored
C=1,000mF
V=18
so then its 162? I'll be back with the 2nd question.

Last edited: Apr 24, 2006
8. Apr 24, 2006

### Ouabache

Good job Agent, on your equations.. Here is another reference you may want to keep handy for future questions like this.

Here is an example of how to write equations.
(select it and see the simple coding I used)
$$U = \frac{1}{2} CV^2$$

Be careful with the math (double check your decimal places). And what units is your answer in?

Last edited: Apr 25, 2006
9. Apr 26, 2006

### Ouabache

On your RC circuit question, tell us what you understand about this kind of circuit. Have you learned about the behavior of this kind of circuit with respect to time? After you give some of your thoughts we can help steer you in successful direction.