# Capacitor discharge: how is it measured?

1. Mar 12, 2010

### acherentia

This is the problem and it's actually in my calculus book to illustrate the concept of a limit in experiments:

The flash unit on a camera operates by storing charge on a capacitor and releasing it suddenly when the flash is set off. The data in the table describe the charge Q remaining on the capacitor (measured in microcoulombs) at time t (measured in seconds after the flash goes off ).

{Use the data to draw the graph of this function and estimate the slope of the tangent line at the point where t = 0.04. [Note: The slope of the tangent line represents the electric current flowing from the capacitor to the flashbulb (measured in microamperes).]}

All I am wondering is how do they get the tabulated data of the charge on the capacitor at time t? How is that done experimentally?

Thank you

2. Mar 12, 2010

### Feldoh

There are lots of ways to measure the discharge of a capacitor. The easiest way would probably be to measure a voltage which is proportional to the charge on a capacitor.

Does that make sense?

3. Mar 12, 2010

### collinsmark

Hello acherentia,

Well, one can measure the voltage across capacitor's terminals (using a voltmeter, for example), and calculate the charge in the capacitor using

$$Q = CV.$$

This method approximates that the flash unit's capacitor's capacitance is truly constant (like an ideal capacitor), and is not a function of the voltage. Such an approximation should hold pretty well over the rated voltage range of a real-world capacitor. But it does have its limits in extreme conditions.