Electricity & Megnetism

Capacitance of a Capacitor

The defibrillator is essentially a capacitor whose electrodes are placed on the patient’s chest.

Let’s imagine and consider that we have a capacitor charged with charge Q . Suppose then that plate A of this capacitor has electric potential A and plate B has electric potential B . The electrical voltage or potential difference between the plates of the capacitor is represented by U. Therefore, we determine the capacity or capacitance of this capacitor using the following equation:

Capacity depends:

– the insulator between the armatures
– the strength and size of each armature, as well as the relative position between them.

Electric potential energy stored by a capacitor

electrical circuit with capacitor

In order to store energy in a capacitor, it is necessary to do work that is transformed into electrical potential energy . So, let’s consider the figure above, where we have an electrical circuit with a flat capacitor. If we close the circuit switch D , the capacitor will charge. In this way, its capacitance is given by   , which results that the charge of the capacitor is, at each instant, directly proportional to its potential difference.

We can express this result through the Q x U graph shown below. The generator, by charging the capacitor, provided it with electrical potential energy. This energy stored by the capacitor is given, numerically, by the area A represented in the figure below.

Graphical representation of charge versus potential difference (Q x U)

P = shaded area

In summary we have:

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