In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. If you have read How Batteries Work, then you know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on o Contact online >>
In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. If you have read How Batteries Work, then you know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and the other terminal absorbs them when you create a circuit. A capacitor is much simpler than a battery, as it can''t produce new electrons — it only stores them. A capacitor is so-called because it has the "capacity" to store energy.
In this article, we''ll learn exactly what a capacitor is, what it does and how it''s used in electronics. We''ll also look at the history of the capacitor and how several people helped shape its progress.
Capacitors can be manufactured to serve any purpose, from the smallest plastic capacitor in your calculator, to an ultra capacitor that can power a commuter bus. Here are some of the various types of capacitors and how they are used.
Inside a capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper (and some electrical clips). It won''t be a particularly good capacitor in terms of its storage capacity, but it will work.
In theory, the dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor''s function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. Depending on the size and type of dielectric, some capacitors are better for high-frequency uses, while some are better for high-voltage applications.
When you connect a capacitor to a battery, here''s what happens:
Once it''s charged, the capacitor has the same voltage as the battery (1.5 volts on the battery means 1.5 volts on the capacitor). For a small capacitor, the capacity is small. But large capacitors can hold quite a charge. You can find capacitors as big as soda cans that hold enough charge to light a flashlight for a minute or more.
Even nature shows the capacitor at work in the form of lightning. One plate is the cloud, the other plate is the ground and the lightning is the charge releasing between these two "plates." Obviously, a capacitor that large can hold a huge charge!
Here you have a battery, a light bulb and a capacitor. If the capacitor is pretty big, what you will notice is that, when you connect the battery, the light bulb will light up as current flows from the battery to the capacitor to charge it up. The bulb will get progressively dimmer and finally go out once the capacitor reaches its capacity. If you then remove the battery and replace it with a wire, current will flow from one plate of the capacitor to the other. The bulb will light initially and then dim as the capacitor discharges, until it is completely out.
In the next section, we''ll learn more about capacitance and take a detailed look at the different ways that capacitors are used.
A capacitor''s storage potential, or capacitance, is measured in units called farads. A 1-farad capacitor can store one coulomb (coo-lomb) of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.
A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. For this reason, capacitors are typically measured in microfarads (millionths of a farad).
To get some perspective on how big a farad is, think about this:
If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! It''s impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.
The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. That''s why the electronic flash on a camera uses a capacitor — the battery charges up the flash''s capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous — flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can potentially kill you with the charge they contain.
Capacitors are used in several different ways in electronic circuits:
In the next section, we''ll look at the history of the capacitor and how some of the most brilliant minds contributed to its progress.
The invention of the capacitor varies somewhat depending on who you ask. There are records that indicate a German scientist named Ewald Georg von Kleist invented the capacitor in November 1745. Several months later Pieter van Musschenbroek, a Dutch professor at the University of Leyden, came up with a very similar device in the form of the Leyden jar, which is typically credited as the first capacitor. Since Kleist didn''t have detailed records and notes, nor the notoriety of his Dutch counterpart, he''s often overlooked as a contributor to the capacitor''s evolution. However, over the years, both have been given equal credit as it was established that their research was independent of each other and merely a scientific coincidence.
The Leyden jar was a very simple device. It consisted of a glass jar half-filled with water and lined inside and out with metal foil. The glass acted as the dielectric, although it was thought for a time that water was the key ingredient. There was usually a metal wire or chain driven through a cork in the top of the jar. The chain was then hooked to something that would deliver a charge, most likely a hand-cranked static generator. Once delivered, the jar would hold two equal but opposite charges in equilibrium until they were connected with a wire, producing a slight spark or shock.
Benjamin Franklin worked with the Leyden jar in his experiments with electricity and soon found that a flat piece of glass worked as well as the jar model, prompting him to develop the flat capacitor, or Franklin square. Years later, English chemist Michael Faraday would pioneer the first practical applications for the capacitor in trying to store unused electrons from his experiments. This led to the first usable capacitor, made from large oil barrels. Faraday''s progress with capacitors is what eventually enabled us to deliver electric power over great distances. As a result of Faraday''s achievements in the field of electricity, the unit of measurement for capacitors, or capacitance, became known as the farad.
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Many electronic circuits (like the one shown) are powered by batteries. Increasingly, however, engineers are looking to capacitors as another option for providing energy as needed to all or parts of such circuits.
Energy can be stored in a variety of ways. When you pull back on a slingshot, energy from your muscles is stored in its elastic bands. When you wind up a toy, energy gets stored in its spring. Water held behind a dam is, in a sense, stored energy. As that water flows downhill, it can power a water wheel. Or, it can move through a turbine to generate electricity.
When it comes to circuits and electronic devices, energy is typically stored in one of two places. The first, a battery, stores energy in chemicals. Capacitors are a less common (and probably less familiar) alternative. Theystoreenergy in an electric field.
In either case, the stored energy creates an electric potential. (One common name for that potential is voltage.) Electric potential, as the name might suggest, can drive a flow of electrons. Such a flow is called an electric current. That current can be used to power electrical components within a circuit.
Thesecircuits are found in a growing variety of everyday things, from smartphones to cars to toys. Engineers choose to use a battery or capacitor based on the circuit they''re designing and what they want that item to do. They may even use a combination of batteries and capacitors. The devices are not totally interchangeable, however. Here''s why.
Batteries come in many different sizes. Some of the tiniest power small devices like hearing aids. Slightly larger ones go into watches and calculators. Still larger ones run flashlights, laptops and vehicles. Some, such as those used in smartphones, are specially designed to fit into only one specific device. Others, like AAA and 9-volt batteries, can power any of a broad variety of items. Some batteries are designed to be discarded the first time they lose power. Others are rechargeable and can discharge many, many times.
A typical battery consists of a case and three main components. Two are electrodes. The third is an electrolyte. This is a gooey paste or liquid that fills the gap between the electrodes.
The electrolyte can be made from a variety of substances. But whatever its recipe, that substance must be able to conduct ions — charged atoms or molecules — without allowing electrons to pass. That forces electrons to leave the battery via terminals that connect the electrodes to a circuit.
When the circuit isn''t turned on, the electrons can''t move. This keeps chemical reactions from taking place on the electrodes. That, in turn, enables energy to be stored until it is needed.
The battery''s negative electrode is called the anode (ANN-ode). When a battery is connected into a live circuit (one that has been turned on), chemical reactions take place on the anode''s surface. In those reactions, neutral metal atoms give up one or more electrons. That turns them into positively charged atoms, or ions. Electrons flow out of the battery to do their work in the circuit. Meanwhile, the metal ions flow through the electrolyte to the positive electrode, called a cathode (KATH-ode). At the cathode, metal ions gain electrons as they flow back into the battery. This allowsthe metal ions to become electrically neutral (uncharged) atoms once again.
The anode and cathode are usually made of different materials. Typically, the anode contains a material that gives up electrons very easily, such as lithium. Graphite, a form of carbon, holds onto electrons very strongly. This makes it a good material for a cathode. Why? The bigger the difference in the electron-gripping behavior between a battery''s anode and cathode, the more energy a battery can hold (and later share).
As smaller and smaller products have evolved, engineers have sought to make smaller, yet still powerful batteries. And that has meant packing more energy into smaller spaces. One measure of this trend is energy density. That''s calculated by dividing the amount of energy stored in the battery by the battery''s volume. A battery with high energy density helps to make electronic devices lighter and easier to carry. It also helps them last longer on a single charge.
In some cases, however, high energy density can also make devices more dangerous. News reports have highlighted a few examples. Some smartphones, for instance, have caught fire. On occasion, electronic cigarettes have blown up. Exploding batteries have been behind many of these events. Most batteries are perfectly safe. But sometimes there may be internal defects that cause energy to be released explosively inside the battery. The same destructive results can occur if a battery is overcharged. This is why engineers must be careful to design circuits that protect batteries. In particular, batteries must operate only within the range of voltages and currents for which they have been designed.
Over time, batteries can lose their ability to hold a charge. This happens even with some rechargeable batteries. Researchers are always looking for new designs to address this problem. But once a battery can''t be used, people usually discard it and buy a new one. Because some batteries contain chemicals that aren''t eco-friendly, they must be recycled. This is one reasons engineers have been looking for other ways to store energy. In many cases, they''ve begun looking at capacitors.
Capacitors can serve a variety of functions. In a circuit, they can block the flow of direct current (a one-directional flow of electrons) but allow alternating current to pass. (Alternating currents, like those obtained from household electrical outlets, reverse direction many times each second.) In certain circuits, capacitors help tune a radio to a particular frequency. But more and more, engineers are also looking to use capacitors to store energy.
Capacitors have a pretty basic design. The simplest ones are made from two components that can conduct electricity, which we''ll call the conductors. A gap that doesn''t conduct electricity usually separates these conductors. When connected to a live circuit, electrons flow in and out of the capacitor. Those electrons, which have a negative charge, arestored on one of the capacitor''s conductors. Electrons won''t flow across the gap between them. Still, the electric charge that builds up on one side of the gap affects the charge on the other side. Yet throughout, a capacitor remains electrically neutral. In other words, the conductors on each side of the gap develop equal but opposite charges (negative or positive).
The amount of energy a capacitor can store depends on several factors. The larger the surface of each conductor, the more charge it can store. Also, the better the insulator in the gap between the two conductors, the more charge that can be stored.
In some early capacitor designs, the conductors were metal plates or disks separated by nothing but air. But those early designs couldn''t hold as much energy as engineers would have liked. In later designs, they began to add non-conducting materials in the gap between the conducting plates. Early examples of those materials included glass or paper. Sometimes a mineral known as mica (MY-kah) was used. Today, designers may choose ceramics or plastics as their nonconductors.
A battery can store thousands of times more energy than a capacitor having the same volume. Batteries also can supply that energy in a steady, dependable stream. But sometimes they can''t provide energy as quickly as it is needed.
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