What is Electricity? Units of Measure and Formulas

To begin, we need to have a very basic understanding of what an electronic charge is. Without going too far into physics or chemistry, let's talk briefly about atoms, the smallest building blocks of everything around us.

Atoms: Electrons, Protons, and Neutrons

A molecule of a substance is made up of atoms. Think of an atom as a tiny solar system. At the center of our solar system is the sun, and all planets orbit the sun. Atoms are very similar. At the center of an atom is the nucleus, which is made up of protons and neutrons. Neutrons have a neutral charge, which makes them pretty useless in terms of understanding what electricity is, so we're going to ignore them and focus just on the protons and electrons. Orbiting our atomic nucleus are electrons. Protons have a positive charge, and electrons have a negative charge. You remember that opposite charges attract from when you were in school? Well, at the center of our atom, in the nucleus, we have positively charged protons. It is this positive charge that makes electrons orbit the nucleus. The force of attraction between the electrons and the protons keeps the electrons from flying off their orbit out into free space. A molecule of a substance, whether copper, zinc, hydrogen, etc have an equal number of protons and electrons when the atom is stable. That is, if a molecule has 1 proton, it will have 1 electron.

Some elements like silver and copper are considered unstable elements because the electrons are arranged in orbit in such a way that makes it easy for electrons to jump off orbit and move freely. We call these electrons free electrons. Conversely, certain elements like helium are considered stable elements because the electrons are arranged in orbit in such a way that makes it difficult for electrons to jump off orbit. When electrons jump orbit and flow from atom to atom, it is referred to as electric current.

When electrons can move easily from atom to atom in some kind of material, whether an element, or a man-made compound, the material is considered to be a conductor. Conductors allow electric current to flow with minimal opposition. Materials that are made of stable atoms, or those in which the electrons tend to stay in their own orbits are considered insulators. Electricity can not flow through an insulator very easily. Insulators can however, store an electric charge better than conductors can, and are sometimes made specifically for that purpose. An insulator is sometimes referred to as a dielectric, meaning it can store an electric charge. There are some materials, such as silicon and germanium, which conduct less than conductors but more than insulators. These materials are called semiconductors and can be made to have specific electrical properties that make them useful in electronics. Transistors are a semiconductor device.

Charge, Current, and Resistance

So what exactly is an electric charge? How is it different from electric current? Well, consider rubbing a balloon on your clothes. The balloon tends to want to stick to your clothes afterwards. The reason is that when you rubbed the balloon on your clothes, you were transferring electrons from one to the other. Since electrons are negatively charged, whichever item obtained more electrons became negatively charged. The other item would have become positively charged because it has a deficiency of electrons, which is the same as saying a surplus of protons. Since both the balloon and the clothes are oppositely charged, they are attracted to one another. While the surplus or deficiency of electrons exsists in the items, they are said to have a static charge, meaning that the charge is just being stored and sitting there, and is not in motion.

An electric charge is measured in Coulombs. It takes many billions of electrons or protons for most applications of electricity. A coulomb of charge stored in a dielectric like the rubber in the balloon is equal to \(6.25 \times 10^{18}\) electrons or protons. Electric charge is represented by the symbol Q, which stands for quantity since an electric charge is a quantity of electrons or protons. A coulomb of charge is represented by the symbol C and a charge can be either negative or positive, which would be denoted either -Q or +Q. It can be said that \(6.25 \times 10^{18}\) electrons equals -Q = 1C or a negative charge of 1 coulomb.

When an electric charge is static (not in motion), no electricity is being conducted. When some force is applied to the charge to set it in motion, the result is electric current. Current is measured in Amperes, more commonly shortened to Amps. Amperes are a measurment of the flow of charge over time. When one Coulomb of charge (Q), moves past a given point in one second, the current is 1 ampere. Amperes are represented by the symbol A. Therefore, \(I = \frac {Q}{T}\) where I is current, Q is charge and T is time.

If charge is considered static until some force is applied which makes the charge flow, what then is the force? Well, as stated before, opposite polarities attract, and equal polarities repel. Consider a battery, which has a positive terminal and a negative terminal. The negative terminal is full of free electrons while the positive terminal has an electron deficiency, or a surplus of protons. This difference in charge is called Potential Difference, meaning it has the possibility to do work. Atoms want to be neutral in charge, so when there is a difference in charge between two points, the electrons want to set things right by moving towards the protons until the number of protons and electrons are the same, and the charge is neutral (neither positive nor negative).

Potential difference is the relationship of charges between two points. If a positive battery terminal has a charge of +2C, and a negative terminal has a charge of -2C, the potential difference between the positive and negative terminals is 4.

The Volt is a measure of the amount of work, or energy required to move an electric charge. This energy is measured in Joules (J). When it takes 1 Joule of energy to move \(6.25 \times 10^{18}\) electrons between two points, the potential difference is one Volt. Remember that \(6.25 \times 10^{18}\) electrons is equal to 1 Coulomb of charge, so: \(1V = \frac {1J}{1C}\).

Any charge can do work in moving another charge either by attraction or repulsion. If we were to hook a conducting wire between a positive battery terminal and a negative battery terminal, the electrons in the conductor would be attracted to the positive terminal where a surplus of protons exist, and repulsed by the negative terminal where a surplus of electrons exist. Because electrons repel electrons, and protons attract electrons, it is considered that current normally flows from the negative terminal to the positive terminal, and not the other way around as is commonly thought.

Resistance is present in all materials, some more than others, as evidenced by the generation of heat when current flows. Resistance is opposition to current, which is to say that resistance fights against current flow. Resistance (R) is measured in Ohms and denoted by the greek letter omega \(\Omega\). Conductors have low resistance, and insulators have high resistance. Assuming a constant voltage supply, the higher the resistance, the less current flows in a circuit. This relationship is defined by Ohm's law.

The opposite of resistance is Conductance (G), and the unit is the Siemens (S). Conductance basically, is how well something conducts, and because it is the opposite of resistance, it can be said that the lower the resistance, the higher the conductance and vise-versa. Conductance is the reciprocal of resistance, that is: \(G = \frac{1}{R}\) and \(R = \frac{1}{G}\).

Related Formulas

\(1C = 6.25 \times 10^{18} electrons\)
\(V = \frac{W}{Q}\) where V= Volts, W= Work in Joules, and Q= Charge in Coulombs
\(I = Q/T\) where I= Current in Amps, Q= Charge in Coulombs, and T= Time in Seconds
\(Q = I \times T\)
\(R = 1/G\) where R= Resistance in Ohms and G= Conductance in Siemens
\(G = 1/R\)

References:

Mitchel E. Schultz, Grob's Basic Electronics, McGraw-Hill, 2011, ISBN 978-0-07-351085-9

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