In this video, electrical engineer Barron Stone explains how capacitors store and release energy from the electrical field between two parallel charged plates to oppose changes in voltage across the capacitor. Barron relates the concept of a capacitor sto
- Objects become positively or negatively charged when they have a deficit or surplus of electrons, and all objects have a limit, or capacity, on the amount of extra electrons they can hold under certain conditions. The capacitance of an object describes its ability to store electrical charge at a certain voltage. All objects that can conduct electricity have some amount of capacitance and some objects are better than others at storing electrical charge based on the structure of the materials.
Capacitors are electronic components that are used to temporarily store energy in circuits and they do so by storing electric charge in two conductive plates. When each plate has a different amount of stored charge, the potential difference between them creates an electric field. The capacitance of the plate will determine how much charge or energy that capacitor can store. To understand how a capacitor stores energy, I like to think of it like a balloon. The air in this room represents the electrons in a circuit.
At the moment, this capacitor is not charged because the concentration of electrons, or air pressure, is the same on the inside as the outside of the balloon. If I use my lungs to apply a voltage to this capacitor, current flows into the capacitor and it fills up with charge. Now the balloon is storing energy because there's a higher concentration of electrons and a higher potential energy on the inside of the balloon than the outside. This balloon has a certain capacity for the amount of charge it can store based on the voltage that's applied to it.
If I blow harder than before to apply an even larger voltage to the capacitor, now it contains even more charge than before. With the balloon pinched off like this, there's not a path for the electrons to flow from the high voltage on the inside of the balloon to the lower voltage outside. If I open my fingers to create a path between the two sides, the capacitor releases that stored energy and the current flows out of it. Although different types of electronic capacitors are produced using a variety of structures and materials, they all work using the same concept.
A capacitor is a two terminal component that consists of two electrically conductive plates that are separated by an insulative material called the dielectric. The two conductive plates are like the air on the inside and outside of the balloon and the dielectric material is the rubber balloon that separates them. When both of the plates are equally charged, there's no voltage or potential difference between them. If the capacitor is connected to a voltage source, current will flow into the capacitor, creating a difference in charge between the two sides.
Now the capacitor is charged and storing energy. When the capacitor is disconnected from the voltage source, those extra electrons remain on the negative side of the dielectric, so there's still a potential difference between the two sides. But those stored electrons can't go anywhere because there's not a path between the two sides. If I connect the two sides of the charged capacitor with a conductive path like wires and a lightbulb, it allows the current to flow. The electrons will move around until both sides are equally charged so the potential difference between them is back down to zero.
Since capacitors are used to store and deliver energy to a circuit, at first glance they seem a lot like rechargeable batteries. But there are a few significant differences between capacitors and batteries. Capacitors store energy in an electric field between two charged plates, whereas batteries use chemical reactions to store and release energy. That gives batteries a much larger capacity and a higher energy density, meaning they can store significantly more energy than a capacitor of that same physical size.
The benefit of capacitors is that they can charge and discharge a lot faster than batteries, which makes capacitors useful when you need a quick burst of power. The schematic symbol for a basic capacitor looks similar to its physical structure. It consists of two parallel lines representing those charged plates with a gap between them, and there's a terminal attached to the plates on either side. The capacitance of a capacitor is measured in a unit called farads, which are abbreviated with a capital letter F.
One farad is defined as the amount of capacitance which stores one coulomb worth of charge across a potential difference of one volt. It's difficult to get a sense of scale from that definition, but one farad is a lot of capacitance. In practice, it's rare to encounter a capacitor with a full farad or more of capacitance. They exist, and they're called supercapacitors, but they're large, expensive, and designed to be used in specific applications. Most of the capacitors you'll use will have a capacitance ranging from a few picofarads on the small end up to several microfarads on the high end, so you should design your circuits to use capacitors within that range.
As I design circuits throughout this course, I'll limit myself to using the list of capacitors shown here, which is based on the values contained in the beginner capacitor kit sold by sparkfun.com. As you build up your own electronics part kit, it's a good idea to gather an assortment of capacitors with these values.
- Reading electrical schematics
- Building circuits on breadboards
- Reviewing types of static and variable resistors
- Reading resistor color codes
- Measuring resistance with a DMM
- Measuring resistive sensors with an Arduino microcontroller
- Making electrical signal measurements with an oscilloscope
- Measuring AC voltage with a DMM
- Understanding the time domain and frequency domain
- Designing passive low-pass and high-pass filters
- Reviewing reactive RC and RL circuits
- The relationship between capacitors and inductors