# How do we extract energy from capacitors?

I have an intuitive reasoning for the use of capacitors. I think that it is used to store electrical energy. Without going in much detail in engineering and being at an introductory level of electrodynamics,how do we extract energy from capacitors?

• Justlike you extract energy from a cell – Aditya Garg May 23 at 7:44
• Some circuits use ultra-capacitors instead of batteries. While they don't store as much energy as batteries, they can be charged and discharged at much higher rates. See for example maxwell.com/products/ultracapacitors – hdhondt May 23 at 9:58

You're probably familiar with the analogy in which electrical circuits can be imagined as being like water flowing in pipes. In this analogy, voltage is water pressure, current is flow-rate and so on. You can extend this to capacitors by thinking of a capacitor as a broad piece of pipe with a rubber membrane across its cross-section.

In this model, as water flows into the capacitor, the membrane bulges and stretches (storing energy). It creates a rising back pressure that opposes the flow. The flow reduces and finally stops (capacitors are open circuits). If you turn off the pump and allow the capacitor to discharge, the stored energy in the stretched membrane will push water back out.

An important point to note, is that capacitors are not really useful in DC circuits - they are usually employed in oscillating circuits. If you have an oscillating current flowing into a capacitor, it will pass through the membrane because as it swings back and forth, it will transmit the motion to the other side.

You have to think of a capacitor as a dynamic component for use in an oscillating circuit.

[Ok pedants; go for your life!]

This is a very basic description.

A charged capacitor has electrical potential energy stored in its electric field equal to

$$E=\frac{CV^2}{2}$$

This is because there is excess electrons on one plate, making it negatively charged, and a deficit of electrons on the other plate, making it positively charged. If there was a conducive connection between the plates instead of the dielectric (insulating material) charge would move from one plate to the other due to the attraction between them.

Connecting a circuit to the capacitor terminals provides the means for charge to move from one plate to the other through the circuit (current flow). The discharge converts the potential energy of the capacitor to kinetic energy for use by the circuit.

Hope this helps.