# Storing Charge in Capacitors

The instruction guide included with some of the iBotz kits describes how capacitors can be constructed from placing a non-conductive material between two electrically conducting plates.

There is an excellent web-based tutorial on capacitors and how they work at the Molecular Expressions™ website - enter the word capacitance in to the search box, and then click on the link headed: Electricity and Magnetism: Capacitance

# RC (Resistor-Capacitor) Circuits

Capacitors and resistors can be combined in a simple circuit to provide a simple timer. When you connect a resistor and a capacitor together and then wire them both up to a battery, the voltage across the capacitor rises often quite slowly as the capacitor charges up. The time taken for the voltage of the capacitor to reach about two thirds of the voltage of the battery is known as the time constant of the capacitor, and it is found by mutliplying the value of the resistor by the value of the capacitor.

In the interactive animation below, click on the switch labelled S1 to charge the capacitor up. The reading on the dial is the voltage across the capacitor - notice how the reading rises quite slowly and the charge builds up across the capacitor. You should also notice which way the current flows in the circuit (from the positive side of the battery to the negative side).

Animation imported via a live link to http://sauer.at.northwestern.edu/electric/electrics-rc.swf . For more technical information about the resistor-capacitor (RC) circuit, have a look at http://othello.mech.northwestern.edu/ea3/book/elec5/RC.htm which is the original home of the above animation.

If you now release the switch S1, the capacitor will hold (or store) the electrical charge that has built up inside it. You will see the level of the voltmeter remains constant and the charge remains stored in the capacitor.

To discharge the capacitor, click on switch S2. You will see the level of the voltmeter falls and the charge that has built up disappears. This time the current flows in the opposite direction to the way it flowed when the capacitor was charged up.

Go back to the http://robots.open.ac.uk website.