What is electricity?
Our society is powered by electricity. Just have a look around: transportation by electric cars, street lighting, mobile phones and the internet. Our homes have wall sockets to connect us with the power supply. A comfortable life without electricity is unthinkable.
But what is electricity? To give answer to that question, we go back to the four fundamental laws in nature:
● The first; is the strong nuclear force that keeps the atoms together;
● Then there is the weak nuclear force, also called “van der Waals” force holding the molecules together;
● The gravitational force which keeps our feet on the ground, holds the earth in its orbit around the sun and makes the universe stay together;
● And last but not least: The electromagnetic force.
The weak and strong nuclear force work at very short distances. The gravitational force and the electromagnetic force act over large distances. In electrical engineering we control the electromagnetic force and that is rather special when we realize that we cannot control the gravitational force.
The electromagnetic force has been discovered relatively late. In the middle ages beautiful cathedrals were being build and two thousand years ago the Romans erected impressive aqueducts. Yet, the electromagnetic force was only discovered until the nineteenth century.
An often-heard description of electricity is a flow of electrons inside a wire. This is, however, only a part of the story. The electric energy is stored in the electromagnetic field outside the wire. But electrons do play an important role. In the beginning of its discovery, electricity was assumed to be a type of liquid and that is why today we speak of current, or in German of Strom and in French of courant.
Electrical energy is transferred by the electromagnetic field. What precisely is a field? A field is a state of the space and a field possess momentum, contains energy and a field has waves. We should not try to visualize an electromagnetic field, but scientists like Ampère, Faraday and Maxwell have given us, based on experiments, the mathematical expressions to describe the electromagnetic field.
And here the electrons come in. Electrons are a source for the electric field. Positive and negative electrical charges experience a force when they are placed in an electromagnetic field.
A person travelling with the same speed as the electrons in the wire observes an electric field. At standstill, and the electrons are passing by, the person observes not only an electric field but also a magnetic field. Both fields are perpendicular to another and interact continuously. Here we see the connection with Albert Einstein’s theory of relativity.
The Maxwell equations describe the physical processes of electromagnetic fields in detail, but finding a solution for the mathematical equations, is far from easy.
When we connect a voltage source to the two ends of a wire, the difference in potential creates an electric field that exerts a force on the free moving electrons in the material and they start to move through the wire. At the same time an electromagnetic field is created in the space around the conductor.
For the 50- and 60 Hertz power frequency the electromagnetic field stays close to the surface of the wire. For the higher frequencies the field is more distant from the wire and for the very high frequencies the field parts from the conductor and travels through space. In that case we call the wire an antenna.
However, when we consider the capacitor being the storage element for the electric field, the inductor the storage element for the magnetic field and the resistor taking care of the losses we can, for low frequencies, fruitfully use linear circuit theory to model the electrical phenomena.
But how can a current flow through a capacitor, while a capacitor consists of two plates with an insulating medium in between? When a charged capacitor discharges, the discharge current through the conductor crosses surface 1 and a magnetic field is present along path 1. With Ampère’s law we can calculate the magnetic field around the conductor. But when we apply Ampère’s law to surface 2, that is also bound by path 1, we encounter a problem because the discharge current does not pass through surface 2 and Ampère’s law tells us that there is no magnetic field present. But in reality, a magnetic field does exist. Surface 2 is also crossed by the electric field from the changing charge on the plates of the capacitor.
Maxwell extended Ampère’s law with an additional term to include the influence of the electric field. So not only a current through a wire but also an electric field that changes over time can be the source for a magnetic field.
Let us go back to what happens inside a conductor. The atoms have bands filled with electrons. The highest energy band that contains electrons is called the valence band. This band may be completely filled with electrons or only partially. If the band is only partially full, the electrons in the band can easily be raised to a higher energy level by an electric field and they can freely travel from one atom to another.
The electric and magnetic fields that are created by the travelling electrons are perpendicular to another and together they form an electromagnetic wave, travelling along the wire at a speed a bit less than the speed of light. There is a continuous energy-exchange between the electric and the magnetic field component.
Light is an electromagnetic wave and it is because of this energy exchange between the field components that light is able to travel through space.
In electrical engineering we make fruitful use of the conducting and insulating property of materials, the storage of electromagnetic energy in capacitors and in inductive components and we make the electromagnetic force to do as we want. This is the beauty of electrical engineering.