Basic Electrical Units
Methods for Producing Voltage
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Electromotive Force (EMF) and Potential Difference
If there is a surplus of electrons at one end of a conductor and a deficiency at the other end, a current flows. Certain devices create this difference in charge so current flows. These devices are sources of electromotive force.
EMF is defined as:
The energy transferred per unit when one type of energy is converted into electrical energy.
Potential difference is another term that is almost the same but has a subtle difference.
Potential difference is defined as:
The energy transferred per unit when electrical energy is converted into one other type of energy.
We will discuss this in more depth in a bit.
The six most common types of EMF are:
- Friction
- Chemical
- Pressure
- Heat
- Light
- Magnetism
Friction EMF
When two dissimilar materials are rubbed together, one material can transfer some of its electrons to the other.
This is the triboelectric effect, which is similar to what we discussed in the atomic structure chapter when electrons transfer, causing one object to become negatively charged (a surplus of electrons) and the other to become positively charged (a deficiency of electrons). This situation can result in an electrostatic discharge when the force of attraction becomes so great that the electrons are attracted to the positively charged object.
This attraction creates an arc most commonly seen in lightning. The clouds build up a charge as the water droplets move around. The charge is then attracted to the positively charged earth and ZAP!
Of course, it is much more fun to put your cat in a box full of styrofoam chips and watch them stick.
Chemical EMF
This is the principle upon which batteries work.
Without getting too deeply into the chemistry, basically, batteries work on the process of ionization.
What is an ion? It is a particle that has an electron added or removed (positive or negative).
In ionization, chemicals are added that have particles with a negative charge and particles with a positive charge. Metal plates that take on these charges are added. This allows a difference and creates an EMF.
Pressure EMF
This process is also known as piezoelectricity.
Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure.
When pressure is applied to some objects, the pressure displaces the positive and negative charges in an otherwise neutral object.
The best-known application is the electric cigarette lighter: pressing the button causes a spring-loaded hammer to hit a piezoelectric crystal, producing a sufficiently high voltage electric current that flows across a small spark gap, thus heating and igniting the gas. The portable sparkers used to ignite gas stoves work the same way, and many types of gas burners now have bu