# Homework Help: Measuring Capacitance given a voltmeter + ammeter

1. Sep 17, 2009

### mikel2009

1. The problem statement, all variables and given/known data
Given the following:
• a battery,
• a digital voltmeter (which records voltage as a function of time),
• a digital ammeter (which records current as a function of time),
• a battered industrial capacitor,
• two copper plates,
• and several bits of copper wire.

measure the capacitance of the industrial
capacitor

(there's a second segment of the question, but i understand it)

2. Relevant equations
C=Q/V. etc.
I=dQ/dt.

3. The attempt at a solution

I have attempted to write a solution to this, but am only confusing myself.
The voltage is a given, however I'm uncertain how I'm supposed to obtain Q from the dQ/dt graph, in order to determine capacitance.
someone please shed some light -.-

2. Sep 17, 2009

### jk0921

This is just an assumption that I thought: Hope it helps
The ammeter is recording current as a function of time, so the reading is actually I/t.
From here we can calculate Q.
Q = I from ammeter readings . t
the I is the reading from the ammeter, so it becomes Q = I/t . t
the t cancel out so Q = I
Does it makes sense?

3. Sep 17, 2009

### saunderson

the usual way to measure the capacitance works with an ac voltage source!

4. Sep 17, 2009

### Redbelly98

Staff Emeritus
Have you had integrals in calculus class?

Well, not really. The ammeter reading is I, period.

It's not "his way", it's the way given in the problem statement and he is stuck with it.

5. Sep 17, 2009

### deagle2005

You answered this yourself with the provided equations.

Differentiate C = Q/V
Bearing in mind that I = dQ/dt as well as the nature of a capacitance of a capacitor

Then look at what you are provided and it's very simple. Note the function of time parts of the question.

P.S. aren't you leaving this a bit late ;)