For an electrical system to do anything a current must flow. Electrical current stays inside conductors (such as wires), and thus there is to be a path for the current: a circuit. There must also be a voltage difference across the device; this is the pressure that makes the current go.

In many circuit elements, the current is proportional to the voltage difference, so that Voltage = Current x Resistance.

In this module, we studied more complex circuits, in which all of the current might pass through several different devices in sequence (when they are in series), or in which the current divides and some goes through each device (when they are in parallel).

We have studied several electrical components:

• Wires and conductors. If we wish to deliver a lot of power, we must use wires that are very good conductors.
• Switches. By connecting and disconnecting parts of a circuit, we can affect which parts are working.
• Resistors. These are elements that conduct, but not very well. They limit the current. Resistance is also the mechanism whereby electrical energy is converted into heat.
• Incandescent light bulbs. These are really just resistors. But they deliver power to a tiny wire that gets so hot that it glows.
• Batteries. These are devices for converting chemical energy into electrical energy.
• Light emitting diode. This is a kind of transistor, which turns electrical energy directly into light without having to make something hot. Because they are more efficient (and less bright) than the incandescent light bulb, they require less current to operate.
• Capacitor. This is a device for storing electrical energy. They can't hold as much energy as a battery of similar size, but the are rechargeable indefinitely.
• Buzzer. This is one of several ways to make sound using an electrical signal.
• Motor. This is a device for turning electrical energy into mechanical energy.