An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and winding currents to generate force in the form of rotation. Electric motors can be powered by direct current (DC) sources, such as from batteries, motor vehicles or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates in the reverse direction, accepting mechanical energy (such as from flowing water) and converting this mechanical energy into electrical energy.

 

Most doorbells have what's called a "push-to-make" switch outside the door.  When you prod the button, your finger pushes two electric contacts together to close the circuit; when you release the pressure, a spring moves the button back out again so the circuit is opened.  The circuit itself contains only two basic elements: a battery and something that makes a noise, which is often an electric bell: a little metal bell and a clapper powered by an electromagnet. When someone presses the button, the electromagnet is activated and pulls on the clapper, which strikes the bell. The clapper is actually also part of the circuit. When it flips out to strike the bell, it breaks the circuit at the same time, cutting the power to the electromagnet. A spring attached to the clapper pulls it back to its original position, whereupon it completes the circuit, energizes the electromagnet, and rings the bell once more. This goes on until the batteries run out.