What is Electricity

If you have ever pumped air into a tire, you already understand a lot of what you need to know about electricity. Like air, electricity is a fluid. Like air, it can be pressurized and made to flow from high pressure to low pressure, such as a high-pressure air tank to a deflated tire. The only difference is that electricity flows through wires, whereas compressed air flows through hoses and pipes. It is very tempting at this point to explain why electricity acts like compressed air, but that doesn’t matter. All that matters is that electricity in an electric circuit works like compressed air flowing through pipes and hoses. Everything is pretty much the same. For example, all other things being equal, you can get more air to flow through a large hose than a small hose. Likewise, you can get more electricity to flow through a large wire than a small wire.

Electricity as a fluid

Many systems work through the use of flowing fluids. If you look at any system that works by use of a circulating fluid—from an air wrench to blood circulation—you will see a fluid being circulated through a system by a pump. That system makes use of the fluid as it passes through. The fluid may transfer energy or information or perform a task while passing through the circuit. Many people in the electronics field use water as an analogy for electricity (the hydraulic model). This works to explain many components and circuits. However, in some cases, the water analogy falls short. This is because water is not compressible.[1] Electricity is quite compressible. Therefore, a gas, such as air, makes a better analogy (a pneumatic model). The table below shows some parameters of gases along with their electrical equivalents .

Parameter Pneumatic Units Electrical Unit


Pounds per square inch,
bar, pascal, etc.
Fluid Flow cubic feet per minute
gallons per hour, etc.
Opposition to
Fluid Flow
pipe friction, restrictions

Exceptions to the electrical fluid theory

There are some cases where the gas analogy falls short too. When electricity is flowing, there is a magnetic field surrounding it. When the electricity stops flowing, the magnetic field collapses. There is, of course, no equivalent magnetic field around a pipe that has compressed air flowing in it. This magnetic field is essential to an electrical property called inductance. This will be covered later under Inductors in DC circuits. In a compressed air system, the air around the system (the atmosphere) acts as a reservoir; it is part of the system. The pump takes in air, and then it escapes back to the atmosphere after being used. Electricity is more like a hydraulic system. Most hydraulic systems recirculate the same fluid; the fluid returns to the pump after completing the circuit. Electricity likewise returns to the source after completing the circuit.

Page summary:

Conductors, Resistors and Ohm's Law

1Water is compressible, but it is 20,000 times less compressible than air. Liquids compress so little that they are said to be incompressible.