As we have studied the effects of electricity flowing through wires, and have discussed resistors, coils, and metering devices. Both resistors and coils have a restricting effect on the flow of current. We also discussed how a coil has more resistance to AC than it does to DC. Another component which has a restricting effect on current flow, but in a different way. This component is called the CAPACITOR.
Once again, resort to our water examples to describe the function of a capacitor, as it is easier to see fluid in motion, when it is water, than when it is electricity.
Examine the example on the left. Here we have 2 tanks of water, equally full. The two tanks are connected in the middle by a pipe or piece of tubing. Let us say now that we have, in the middle of the tubing, a thin rubber membrane. The membrane would keep the liquid in the two tanks from ever coming into contact with each other. We could further illustrate this by adding food coloring to one of the tanks of water.
If we now take a plunger, and apply pressure to the tank on the left, it will push the water downward, and try to push it out the tube and into the other tank. However, the membrane will not allow the water to actually exit the tank, and enter the second tank. While the two systems are sealed off from one another, the rubber membrane would flex, and allow the EFFECT of movement, in that it would push the water level of the second tank higher, in direct proportion to the movement in the first tank. For instance, assuming both tanks are of equal diameter, if the first tank went down 2 inches, the second tank would rise 2 inches.
Now, if we should reverse the action, and push the plunger down in the second tank, it would move the membrane in the opposite direction, also moving the water within the tanks in the opposite direction, but AT NO TIME would the water flow from one tank into the other tank. It would have the effect of movement from one tank to the other, without actually having done so.
This is basically the same operating principle behind another of electronics most important components – the capacitor. The capacitor appears to have the effect of passing alternating current, while actually not passing anything. At the same time, it blocks the flow of direct current. Just as in the water circuit, the water flow in either given direction is blocked by the membrane, if we should push the water pressure, and hence the membrane back and forth, it would appear as if the membrane weren’t there at all, except that the food coloring would not pass from one container to the other.
In its most basic form, a capacitor is made up of 2 plates of conducting material (for instance copper, aluminium, iron), divided by a piece of insulative material (for instance mica, air, or plastic). When we apply an electric potential to the two plates, electricity will want to attract and flow from one plate to the other, but the insulator will act as the membrane, and block the flow of electricity. For this reason, a capacitor blocks the flow of DC. As power is applied, a certain number of electrons on one plate will be attracted to the positive side of the battery. These electrons, leaving the plate will leave it with a deficiency of electrons, and the plate will be positively charged.
At the same time, electrons from the negative side of the battery will see the positive charge on the plate, and want to move toward it. As these electrons leave the negative side of the battery, they will pass through our light bulb, and light it. We will notice, however that it only lights for a moment…. just a split second! Why? Because between the two plates of the capacitor is an insulator, and while the electrons on the negative side may be attracted to the plate on the positive side, the electric current can’t pass through the insulator. So our light flashes for just a second, and then goes out.
Now if we reverse the polarity of the battery, we see that the same thing happens again, only in reverse. As power is applied, the electrons on the now negativly charged plate of the capacitor will be attracted to the positive side of the battery. As these electrons now leave the plate, it will leave a deficiency of electrons, and the formerly negative plate will become positively charged. At the same time, the electrons from the negative side of the battery will move toward the positively charged plate until the positive plate swings negative.
Note that in the examples, the schematic symbol for the capacitor is very similar to that of the battery. There is good reason for this. In a battery, we have 2 (or more) conductive plates divided by some kind of dialectric material (usually an acid). In a capacitor, we have 2 (or more) plates divided by some kind of dialectric material – an insulator. A battery has the ability to generate electricity chemically, and can store energy for long periods of time. While a capacitor does not “generate” electricity, it does have some amazing “storage” capabilities, as we will discuss now.
Recall that when we applied power to the capacitor/lamp circuit, electric current flowed for an instant from one side of the battery and lit the lamp for a moment, but then the light went out? What took place, was while the electric current was flowing, a potential was being built up on the surface of the plates of the capacitor. As long as the potential kept building, current continued to flow, and the light remained lit. At some point, however, the capacitor reaches its maximum CAPACITY to hold an electric potential. In other words, it reaches its peak voltage limit, and we say the capacitor is fully charged. If at this time, we were to remove the battery from the circuit, the capacitor, in theory, would remain at full charge indefinitely.
If at this time, we shorted the wires between the capacitor and lamp, such that it formed a complete circuit, the lamp would light for just a second….. WITHOUT THE BATTERY. Where does the energy come from to light the lamp if the battery is not connected? The answer lies in one of the magical properties of the capacitor….it can STORE energy! When energy is stored in a capacitor, we say it is charged. When a capacitor releases it’s energy, we say it is discharging.
The symbol for Capacitance is C. The unit of capacitance is the FARAD. The symbol for Farads is F. The Farad is an extremely large quantity, so we typically speak of microfarads ( mf ), nanofarads ( nf ), and picofarads ( pf ). * Note: in some older texts, the term micromicrofarad ( mmf ) is used in lieu of pf. The Capacitance (the amount of energy a capacitor can store) of a capacitor depends on 3 factors:
The Area of the plates
The Distance between the Plates
The Type of Dielectric
The formula for capacitance is:
Where C is the capacitance in picofarads, A is the area of one of the plates in square inches, and K is the dielectric constant of the insulative material separating the plates.
