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Capacitors: Construction, Charging & Discharging

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  • 0:00 Capacitor
  • 0:46 How it Works
  • 2:08 Charging and Discharging
  • 3:33 Capacitors with AC Current
  • 4:53 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Electrical circuits have several components, each of which must be understood for the system to work. We're going to check out capacitors in this lesson and see how they can be used to control an electric current.

Capacitor

I'm working on creating something that runs on electricity. I've got several electrical currents running through it, and being the perfectionist that I am, I would prefer if it didn't blow up. I'm just picky that way. So, I guess I need to understand the components of my circuits. I've got resistors; I've got inductors; and of course, I've got a capacitor, an electrical component with two terminals that stores energy. Capacitors are used to hold electricity, as well as to block certain currents from passing through the circuit. They are an integral part of most complex electrical circuits. So, I'd appreciate it if we don't blow them up. Let's take a closer look and make sure we know how to use them.

How It Works

So, this is a capacitor. How does it work? Well, let's look at the parts. A capacitor is made of three integral parts. We start with two plates that are electrical conductors. These plates are separated by a dielectric, an electrical insulator. When the capacitor is attached to an electrical source, an electric field develops around the dielectric, causing a positive charge to collect on one plate and a negative charge to collect on the other. What we end up with is a polarized component of an electrostatic field contained between the oppositely-charged plates. This means that the capacitor is capable of storing energy.

The effectiveness of the capacitor is defined by its capacitance, the ability to store an electric charge. Capacitance is measured as a ratio of electric charges of each conductor (Q) to the potential difference between them (V), which as an equation looks like this: C = Q / V. The capacitance of a capacitor is determined by how the capacitor is made; the larger the surface area of the plates and narrower the gap between them equals a greater capacitance. So, it's important to create a capacitor capable of holding the correct amount of energy for the circuit using it.

Charging and Discharging

So, a capacitor can store energy, presumably to be used later. That begs the essential questions of how do we get energy into the capacitor, and how do we get it out? The answer actually depends on the sort of current we're using. Generally, we'll be applying a direct current, a unidirectional electrical charge, also called a DC current. When a DC power source, like a battery, is applied to a circuit with a capacitor, the capacitor begins to store energy. However, once the capacitor is fully charged, not only does it stop storing more energy, it actually blocks the flow of the DC current entirely. Once the DC current source is removed, the capacitor starts discharging that energy. Now, unlike other components, capacitors do not charge and discharge immediately, but do so at a fixed rate called the time constant. The time constant is the time required to charge a capacitor through a resistor and can be calculated through the equation T = RC or time constant equals resistance times capacitance. What all of this means is that capacitors with DC current charge at a constant rate, store energy while blocking the current from passing through the circuit, and then discharge that energy at a consistent rate.

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