Capacitors, or also known as condensers, are devices that store an electric charge between two parallel metal plates. The plates are separated by a gap or a non-conductive dielectric material.
Although friction methods of producing electric charges have been known for thousands of years, there was no method of storing or accumulating charges for extended periods of time. In 1745, it was commonly believed that electricity was a fluid, and the German experimenter Von Kleist was convinced he could lead electric fluid from his generator and fill a bottle with it. During one of his experiments, he discovered that filling a bottle with a liquid such as water or mercury, placing a wire down the neck, and touching the wire with a friction generator would store an electric charge, but only if he held the bottle. He could carry the bottle around for hours, and from room to room, and still pull a large enough spark to ignite alcohol or cause painful shocks.
Eager to share his discoveries, he wrote to the leading scientists around Europe describing his experiments. In one letter, he warns, “I would not take a second shock for the kingdom of France.” Kleist was not well-known or respected, so those who received this letter did not manage or care to recreate his claims.
The following year, a Dutch scientist, Pieter Musschenbroek independently made the same discovery accidentally while trying to measure the strength of electricity with a suspended gun barrel and a brass wire extending into a jar of water which he held in his hand. To his surprise, he received a strong shock from the jar, which made him believe that he was done for. After further experimenting, he found that metal on the outside of the jar could take place for his hand.
Because of his reputation as a teacher, the science community took notice, and his device was named the Leyden jar after the city in which he taught. Soon after hearing about this effect, Daniel Gralath, a German physicist, found that he could attach multiple jars to increase a discharge which was strong enough to kill small animals and birds.
Curiosity was at an all-time high all over Europe. The well-to-do would gather to experience the effects of electricity. It was common for royalty to line people up hand in hand and watch them jump when discharging Leyden jars across them. Capacitors continued to be studied, and Benjamin Franklin eventually proved that the charge is not stored in the liquid, but on the glass.
Over the last 200 years, capacitors have been refined and come in many forms. A capacitor’s ability to store charge is measured in farads. A 1 farad capacitor will accept 1 amp and change 1 volt over 1 second. The surface area of the metal plates, distance between the metal plates, and the dielectric material are the major factors that determine the farad rating.
In this demonstration, I’ll charge a polarized electrolytic capacitor with a 9-volt battery. You will see the current flow momentarily on the meter, and then stop once the capacitor is fully charged. I’ll then remove the battery clips, and then discharge the capacitor with an LED. Some capacitors may hold a charge for hours or even days. Some of you may have experienced this when working on CRT-based TVs.
Here I’ll demonstrate the relationship of electrode distance and capacitance. The metal surface will be one electrode, and a disk with taped spaces on the edge, the other. The meter is set to nanofarads, and you’ll see the value go up when the electrode distance is very close. The maximum voltage a capacitor can tolerate is mostly determined by the dielectric breakdown voltage. For instance, placing material such as glass or plastic between electrodes will increase the maximum voltage but will reduce the farad rating.
The overlapping surface also plays a role in the farad rating. You can see the value goes down when I slide the upper electrode so there’s less overlap. This technique is commonly used in variable capacitors.
This is a fun demonstration of a switch capacitor circuit that illustrates how charges can be reconfigured to increase or decrease voltage. First I’ll charge up the capacitors in parallel, disconnect the power source, and then connect them up in series, which doubles the voltage across the capacitors. If we put the capacitors back in parallel, we can see their individual voltages have not changed besides the normal discharge rate of the capacitors.
You can also decrease the initial voltage by first charging the capacitors in series and then connecting them in parallel.
It should be noted that changing the configuration of the capacitors increases or decreases the available usable current. I’m demonstrating this by manually moving capacitors, but switching devices, like transistors or relays, would be more commonly used in power supplies.
The last example is DC blocking. Since the two electrodes don’t have direct electrical connection, electrons will not flow from one to the other. This prevents DC current from passing through a capacitor, but alternating current on one electrode will induce the current on the other electrode due to attraction and repulsion of charges. This is useful for changing the DC offset of a signal.
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