What is an electrical circuit

A pneumatic (compressed air) circuit

Electricity is a lot like compressed air. For example, a mechanic uses compressed air to operate tools. A pump sucks air in one side and forces it out the other side under high pressure. A hose carries the air to the tool where the pressurized air is used to turn a spindle (as in an air wrench) or to push a piston (as in a jackhammer). After doing its work, the air returns to the atmosphere. The air makes a circuit from the atmosphere, through the pump, through the hose, through the tool and back to the atmosphere[1].

Pnunamic Circuit
A Pneumatic Circuit

A pump sends high-pressure air through a hose to a pneumatic tool. The path of the air—from the atmosphere, through the pump, the hose, the tool and back to the atmospherecreates a circuit through which the air circulates.

An electrical circuit

In metals, there are gazillions of electrons that are free to move around and, although confined to the metal, act a lot like the air molecules around us. If we make a wire out of metal, that wire becomes a lot like the air hose in the illustration above. If we connect each end of the wire to opposite ends of a battery, the battery will act like a pump. This creates an electrical circuit where electrons are sucked in the positive side of the battery and forced out the negative side. The electrons travel around the wire back to the positive side. The battery acts like a pump and the wire acts like a hose or pipe through which the electrons can flow around the circuit.

An Electric Circuit

A battery acts like a pump and sends high voltage electrons (electrons under high electrical pressure) through a wire. The path of the electrons, from the negative terminal of the battery, through the wire, through one or more devices (such as a motor)  and back to the positive terminal creates a circuit through which the electrons (electricity) circulates.

All we need now is something to pump the electricity through where it can do something useful. For example, we could pass the electricity through a coil of tungsten wire. If we pump enough electrons through that wire, it will heat up until it glows white hot. Put that tungsten wire in an evacuated glass container so that it won't burn up and you have a light bulb.

With a compressed air system, we pump air through tools to do something useful. With electricity, we pump electrons through electrical devices to do something useful. In either case, the fluid (air or electricity) makes a circuit. In a compressed air system a pump circulates the air around the circuit and in an electric circuit a battery pumps electrons around the circuit.

A refrigerations system

Another good analogy of an electrical circuit is a refrigeration system, like the one illustrated below.

High pressure
A Refrigeration System
Low pressure

A Refrigeration System

The pump (4) circulates the refrigerant. The refrigerant backs up when it encounters the restriction (2) increasing the pressure on the left side of the system (1). The high-pressure side of the system gets hot and air is blown across it to remove heat. After the refrigerant passes the restriction, the pressure drops. Expanding refrigerant in the low-pressure side (3) cools. Air is blown across the cool refrigerant tubes to cool the surroundings.

A refrigeration system is an even better analog of an electrical circuit than a compressed air system. A refrigeration system is a closed system. It circulates the same fluid over and over again. An electrical circuit is also a closed system since the electrons cannot leave the system[2].The refrigeration system also clearly shows what happens when you restrict the flow of a fluid. It causes a sharp differential in the pressure from one side of the restriction to the other. It's like blocking a garden hose with your thumb; the pressure increases behind your thumb in the hose. If you partially block the flow of a fluid, you will get a backup of pressure where the fluid encounters the blockage and a drop in pressure on the opposite side of the blockage.

Another electrical circuit

In the following diagram, we have rotated the electric circuit to compare it to the refrigeration system above.

Light bulb acts as a restriction

High voltage
An electric circuit
Low voltage
An Electrical Circuit

The battery circulates the electricity (clockwise). The electricity backs-up when it encounters the light bulb causing a relatively high voltage on the left side of the circuit. After the electricity passes through the light bulb, the voltage drops. The right side of the circuit has a low voltage just as the right side of the refrigeration system has a low pressure.

A light bulb, like any other electrical component, offers resistance to the flow of electricity. When flowing electrons encounter the light bulb in the above circuit, there is a backup of voltage. There is a correspondingly lower voltage where the electrons exit the light bulb (more about this in Resistors). In a complex electronic circuit, there are many components with electrical current flowing through them. Each one has a corresponding voltage differential across it. A large part of the art of electronic engineering is manipulating these changes in voltage and the related electrical currents.

Pneumatic and electrical components

The following table lists several pneumatic devices, what they do and their electrical equivalents. We will discuss these devices in detail later.

Pneumatic Device
Electronic Equivalent


Provides a conduit for fluid to flow through.


Sucks fluid in one end and blows it out the other (circulates the fluid around the circuit).
Battery or Generator


Blocks or unblocks the flow of air.

Check valve

Allows air flow in one direction only

Relay valve

An air-controlled valve. A small volume of air opens the valve, which then passes a large volume of air.

Air tank

Stores compressed air. As more air is pumped into the tank, the pressure increases.
Impedes current flow. A pressure differential develops across the restriction—higher pressure on the side toward the source of the current—lower pressure on the side away from the source of the current.

Page summary:

What is an Electrical Circuit

What is an Electrical Circuit Q and A

Why Don't Batteries Discharge When you Touch Them End to End
1A compressed air system usually has a storage tank, but this is a convenience that stores air under pressure and not an absolute requirement. The electrical equivalent of a storage tank, the capacitor, will be discussed under Capacitors in DC Circuits later.
2A battery only circulates any one electron once, but it appears to recirculate the same fluid over and over so we'll just pretend that it does.