ELC - Circuits with Capacitors
Circuits with Capacitors
Introduction
Like resistors, capacitors can be combined in series and parallel. Combinations of capacitors will change the equivalent capacitance of the circuit.
To get a recap of this information and hear it from a different perspective, what this video explaining combining capacitors in series and parallel.
Capacitance Practice
This presentation has several capacitor practice problems to help you hone your skills.
RC Circuits
Consider a circuit that contains both a resistor and capacitor with a switch. With the switch open, there is no current in the circuit. When the switch is closed current will flow in the circuit, fast at first then decreasing, as charge builds on the plates of the capacitor. As the charge builds the voltage across the plates of the capacitor increases until it reaches a value equal to the voltage of the battery. At this point the capacitor is fully charged and no current flows in the circuit. Current only flows through the resistor R during the charging process.
We can alter the circuit to add a switch so that if S1 is open and S2 is closed the resistor and capacitor are isolated in their own circuit. If we close S1 while S2 is open, current flows through R and charges the capacitor C. If we open S1, the capacitor is isolated and will hold its charge. Closing S2 completes the loop containing R and C and the capacitor will discharge, first quickly and then slowing gradually, sending current through R until the capacitor is completely discharged. A camera flash takes advantage of a similar circuit. When the flash is charging a large capacitor (around 100 μF) is being charged by a battery. When you take a picture, the capacitor is allowed to quickly discharge through the flash.
Safety Note: If you're the kind of person who likes to take devices, like electronics, apart to see how they work, be careful around capacitors. Capacitors can hold a charge for a long period after a device has been unplugged. If you bridge the gap between the terminals of the capacitor, it will discharge through you! If you are playing with the internals of an electric device it is a good practice to use a piece of metal to connect the leads of the capacitors to allow them to discharge before you do too much poking around. Of course, you should also be careful because you might accidentally discharge the capacitor through a part of the circuit not meant to handle the resulting current.
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