Following are the important parts of an electric circuit:
1. A source of electrical energy e.g battery or power supply
2. Connecting wires
3. Devices e.g bulb ,cell phone etc
4. A key / switch
When you switch on a light, you probably don't think about what goes on to make the light actually light up. In order for the light to . . . well, light, the four basic parts of an electrical circuit must be in place to provide the power needed to produce light or to power another type of electrical device.
Every electric circuit, regardless of where it is or how large or small it is, has four basic parts: an energy source (AC or DC), a conductor (wire), an electrical load (device), and at least one controller (switch).
Visualize what happens when you switch on a room light. You toggle or push a switch to "turn on" the light. Your action causes electricity to flow (much like water flows through a pipe) through the wires of the electrical system to the bulb, which converts the electrical energy traveling through the wires to visible light. The process is essentially the same for powering up a computer, or turning on a TV, or providing the electrical power needed to start or operate any electrical device.
The Energy Source
In an electrical circuit, the power source provides the voltage (the force that pushes electrons through a conductor -- measured in volts) and current (the rate of flow of electrons -- measured in amperes) to energize a device attached to the circuit.
A voltage power source provides a constant voltage level to the circuit. Examples of voltage power sources are batteries, like the ones in your laptop computer or car, the energy from the solar panels on your roof, your car's alternator, or the energy coming from your local power plant or hydroelectric dam.
On the other hand, a current power source (a.k.a. constant-current source), which is measured in amps, provides a constant current of energy regardless of its voltage. Typically, constant-current circuits are included in a system to protect the device providing the electrical load on the circuit. For example, an LED requires a constant level of current to protect it from burning out or being damaged.