Wherever there’s an electrical circuit, you can expect to find a capacitor. Some smooth the output of power supplies, stabilise voltage and/or improve the efficiency of power flow – others store energy for voltage conversion or can filter harmonics. In this ABC article we’ll tell you more about how capacitors work.
Capacitor anatomy – conductors and dielectrics
A capacitor basically has one job: to temporarily store electricity in an electrical field. To succeed at this task, the capacitor needs at least two electrical conductors (plates) separated by a dielectric material. The film is usually rolled up to save space. The plates (thin metal films or foils) ensure that the electrical current can flow freely in one or more directions. The dielectric (we use plastic film), on the other hand, is a material with next to zero electrical conductivity. It can also be impregnated with a liquid to improve its properties.
The absence of conductivity means that the dielectric can store and return electrical energy by becoming polarised. What happens is that stationary, slow moving electric charges build up on the surface of the materials, creating an electrostatic field between the plates.
Electrostatic fields and energy storage
The electrostatic field is where the magic happens. It’s the capacitor’s way of storing energy – a bit like a rechargeable energy storage system. If you want to know how well a capacitor is performing, you need to look at what is called capacitance, i.e. the ability to store electrical charge. High capacitance means that the material can hold more electric charge at a given voltage, low capacitance means that the electric charge is weaker at the same voltage.
The electrostatic field is where the magic happens. It’s the capacitor’s way of storing energy – a bit like a rechargeable energy storage system.
In capacitors large surfaces and narrow distances mean greater capacitance. When the conductors are close to each other, their electric fields attract and allow for a higher energy storage in the capacitor than if the conductors were separated. Another important thing to consider is how the dielectric material (e.g. plastic film) can handle the capacitance. An electric field strength limit, also known as breakdown voltage, determines how much the dielectric material can take before the capacitor breaks down. In other words, breakdown voltage determines the maximum energy that the capacitor can store.