February 2009

Electrons flow as water through a pipe

By Dave Polka

To understand the operating principles of an electronic drive and motor, it is necessary to understand the basic principles of electricity.

Electricity comes in two forms: alternating current (AC) and direct current (DC). We will first consider the effects of DC on various electrical components and identify the three main characteristics of any electrical circuit.

When we take a close look at nature, we find all matter is composed of atoms. In the basic structure of an atom, we find the nucleus is at the center, surrounded by one or more orbiting electrons. This structure reoccurs millions of times for any material. If the material is an insulator, the orbiting electrons do not move from place to place or from atom to atom.

For the purposes of discussion, we will consider a conductor as an atom with three or less orbiting electrons in the outer shell. An atom with five or more orbiting electrons will be an insulator.

Electrons in the outer orbiting rings find it easy to move from atom to atom (particularly in a conductor like a copper wire) whenever they are forced to do so.

The force that tends to move electrons is voltage (electrical pressure in a circuit). Voltage is basically the force that causes electrons to travel from atom to atom.

As you would expect, the higher the voltage, the more force available to move electrons. Some textbooks use the term electromotive force when describing voltage.

Electrons move from atom to atom to take up a spot vacated by the previous electron. Electrons flow in an orderly manner through a conductor.

Completing the circuit

A typical comparison is to water flow in a pipe with that of electron flow in a conductor. When you turn on a water faucet, a certain amount of water pressure forces water through the pipe and out the end of the faucet.

The exact same phenomena holds true for electricity. When you turn on a light, you allow voltage (force) to push electrons (current) through the wire and cause the light to illuminate.

The obvious question is why is it necessary to move electrons in the first place? The reason is fundamental: Every electrical user (light bulb, TV, toaster, a motor) has resistance, measured in ohms. The user of electricity is an electrical load.

A simple fluid circuit consists of a pump to supply the source of water pressure. The water fountain is the load. The pipes provide the path for the water to flow and also provide a certain amount of resistance to flow.

A simple electrical circuit consists of a source of electrons (battery), a load (light bulb), and conductors (wires) to complete the circuit.





Dave Polka (dave.polka@us.abb.com) has a BS in industrial education. He is the Training Center Manager for ABB Drives & Power Electronics in Wisconsin. His book is Motors & Drives: A practical technology guide, ISA Press, 2003.