# Defibrillators : Capacitator Charge, Negative Exponential, Capacitance and Charge Time

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Background Information:

The paddles of a defibrillator in an ambulance or the emergency room of a hospital are actually the two plates of an electronic device called a capacitor. A capacitor is built from two conducting plates that are attached to a voltage source.

Due to the voltage source, electrical charges move through the wires and begin accumulating on the plates of the capacitor: positive charges on one plate and negative on the other. The longer that the capacitor is attached to the voltage source, the more charges are stored on the plates of the capacitor.

Energy is therefore stored by the capacitor and can be recovered if the capacitor is allowed to discharge. In the case of a defibrillator, this discharge occurs when the paddles are used on a patient.

The amount of charge on a capacitor, q, is related to the capacitance of the capacitor, C, and the size of the voltage source, V, by this equation: q = Q (1 - e'^1/RC), where Q represents the maximum charge of the capacitor (amount of charge on the plates when the capacitor is fully-charged), R is the total of any electrical resistance in the circuit, and t is the amount of time that the capacitor is wired to the voltage source.

1. Using the information in the background reading, how is it possible that the defibrillator in an ambulance can give such a large "zap" when it is only being charged by the car battery in the ambulance?

2. Due to the presence of the negative exponential in the capacitor charge equation, what can be said about the amount of time required for a capacitor to fully charge?

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##### Solution Summary

Capacitance and defibrillators and recharge time are discussed and the solution details are presented.

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1. Using the information in the background reading, how is it possible that the defibrillator in an ambulance can give such a large "zap" when it is only being charged by the car battery in the ambulance?

As the time goes on, the charge builds. The larger the voltage, the faster the defibrillator will charge, but the maximum charge it can hold is not related to the ...

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