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.
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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So, we understand how capacitors work with DC currents. But, as I'm sure some of you know, that's not the only kind of current we've got. There's also alternating current, an electrical current that periodically switches the direction of flow, also called AC current. Since the direction of flow systematically switches, the capacitor cannot continually charge. When the current is flowing in to the capacitor, it charges, but as soon as the current switches direction, the capacitor starts discharging. Capacitors in AC current are not fully charged, so they never fully block the flow of electricity through the circuit. The amount of current that is able to flow through is based on not only the capacitance of the capacitor, but also the frequency of the AC current, or the rate of oscillations between direction of flow. As capacitance and frequency increase, so does the amount of current. Capacitors are used to smooth out AC currents, which can get seem jumpy at times since the current changes direction, while they are used in DC currents to store energy and block the flow of current. So, different currents means different needs for capacitors, and using them correctly means no circuits blowing up. I call that a win.
One of the major components of most electrical circuits is the capacitor, an electrical component with two terminals that stores energy. Capacitors are composed of two plates that are electrical conductors separated by a dielectric, an electrical insulator. When a direct current, a unidirectional electrical charge is applied, the capacitor stores energy at a constant rate and blocks it from passing through the circuit. Once the power source is disconnected, the capacitor discharges. However, when applying an alternating current, an electrical current that periodically switches the direction of flow, this changes. The capacitor charges when then current flows in, then discharges when the direction flips. Since it never fully charges, the capacitor does not completely block the current. Capacitors are useful with both currents, and using them correctly ensures that the only thing to explode will be people's minds when you show off your mad engineering skills.
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