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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.
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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.
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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 indefinately.
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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 Dialectric The formula for capacitance is: |
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