# Capacitor discharge: how is it measured?

#### 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

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#### 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?

#### collinsmark

Homework Helper
Gold Member
Hello acherentia,

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?
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.

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