This java applet is an electronic circuit simulator. When the applet starts up you will see an animated schematic of a simple LRC circuit. The green color indicates positive voltage. The gray color indicates ground. A red color indicates negative voltage. The moving yellow dots indicate current.

To turn a switch on or off, just click on it. If you move the mouse over any component of the circuit, you will see a short description of that component and its current state in the lower right corner of the window. To modify a component, move the mouse over it, click the right mouse button (or control-click if you have a Mac) and select “Edit”.

The "Circuits" menu contains a lot of sample circuits for you to try.
Link: Circuit Simulator Applet

Or download the applet and use it offline: http://www.falstad.com/circuit/circuit.zip

Link to lots more Physics/Maths applets: Math, Physics, and Engineering Applets