Electricity Explained

Getting Down To Basics

By Bruce W. Maki

Electro-Motive Force

Static electricity can be interesting, amusing, or annoying, but it has few uses. But the principles behind it are the same as the electricity that homes use every day.

Long ago scientists asked a few questions.

• What makes the electrons move?
• What hidden force drives the electrons from a rug to a person and then to a metal door knob?

Scientists use the term electro-motive force (EMF) to describe the invisible force that seems to push electricity. They noticed that there are several ways to create this mysterious force.

Sources of EMF

Batteries

Long ago scientists noticed that many metals would produce electric charges if they were held in a bath of certain liquids. They noticed that two different metal strips placed in an bowl of acid would create two different charges.

One metal strip would be positive and one would be negative. Scientists noticed that when the metal strips touched, they could get a small spark.

Electrons would be flowing from one metal to the other. They were able to connect the metal strips with conductors (wires) and measure the flow of electrons.

After a while, they noticed that the electrons stopped flowing and that the acid bath was no longer acid, but had been converted to a watery solution.

Sometimes the metal strips had changed, some were eaten away and some had new materials clinging to them.

But with fresh, strong acid and new strips of metal they could create a new battery. The metal pieces had electro-motive force, a force that wanted to push electrons from the negatively charged strip (or terminal) to the positively charged strip.

The chemical action of the acid created the charges in the first place. They discovered that when there was greater EMF there was a corresponding greater flow of electrons.

Scientists discovered that they could connect different types of conductors between the battery terminals and result in different amounts of electron flow.

Some conductors let electrons pass more easily than others. In other words, some conductors had less resistance to electron flow than other conductors.

Thermocouples

Another discovery occurred when two different metals were joined together, such as two strips bonded in a V shape. When the metal junction was heated in a flame, they measured a small amount of electro-motive force at the top ends of the V. The hotter they made the flame, the higher the EMF would be. This device was called a thermocouple, and while it wasn’t useful for generating much electricity it did become valuable for many special purposes.

Today, many gas-fired appliances, such as water heaters and gas fireplaces, use a thermocouple to detect the presence of the pilot flame. The pilot flame keeps the thermocouple (also called a thermo-pile) hot and causes it to generate a small amount of electricity, enough to open or close the gas valve as the thermostat demands.

This means that some gas water heaters, fireplaces and space heaters do not require a source of household electricity in order to operate. These appliances can function during a power failure.

Moving magnetic field: Generators

Without question the greatest discovery came when scientists discovered that a magnet moved across a piece of wire created a tiny and brief amount of electro-motive force.

If the wire was made into a coil, the EMF was greater. If the magnet was stronger, the EMF was greater.

If the magnet was waved back and forth across a coil of wire, the EMF created would occur in short bursts, and as the magnet reversed directions, the charges on the ends of the coil of wire would reverse.

Waving the magnet back and forth created a rapidly reversing EMF in the coil of wire, basically an alternating current.