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Basic Wireless Communication for Microcontrollers
Chapter 1 - Electricity and Magnetism
Fundamental Concepts: E and B Fields
To the typical electronics experimenter, the fundamental concepts of E&M probably seem to be voltage and current, because these are the most easily manipulated. A somewhat better understanding can be gained, however, if we look deeper, to what voltage is and what causes current to flow. This unseen influence is one of the four known forces of nature, the electromagnetic force, which is manifested by both the Electric (E) and Magnetic (B) fields.Fields in General
A field is simply a quantity which has a value at every point in space and time. The density of a smoke cloud, for example, is a field, because at any point in space you could measure how dense it is and form a 3-D (4-D if you include time) graph of the value you measure. We will look first at static E and B fields, that is, ones which do not change with time. We will look at the more interesting case of changing (or dynamic) fields later in the chapter.Static E Fields
A static E field is defined by the fact that it produces forces only on charged particles. The phrase "an E field" is really meant to indicate a region of space which has a non-zero electric field value. If you place an electron, for example, in this E field, it will experience a force as evidenced by the fact that it accelerates. E fields are often indicated on paper by arrows, which indicate the direction in which a positive charge would be accelerated. Negative charges, like electrons, would be pushed in the opposite direction. These arrows are called vectors, and an E field is known as a vector field because the "value" at each point in space is really both a strength (or magnitude) and a direction.Static B Fields
When charges move (whether accelerated by an electric field or by other causes, such as your picking up a charged object and carrying it) they produce a B field. A B field can be described exactly like an E field (it is also a vector field, with both magnitude and direction) except that it is produced differently and a static B field only produces a force on moving charges, not stationary ones. Because of this, the direction of a B field's vectors is not the direction of the force applied to a moving positive charge. Instead, the force of the charge depends not only on the B field direction and magnitude but also on the charge's speed and velocity vector. If you take your right hand and point your four main fingers in the direction of the particle's velocity, keeping your thumb straight, then curl the fingers along the B field direction, the direction of your thumb is the direction of the force (figure 1)
Figure 1 - Right Hand Rule for determining the direction of the force on a moving charge due to a B field
B fields are produced by two things : moving electric charges and changing E fields. Notice the partial symmetry here: E fields can produce B fields and vice-versa. The only break in the symmetry is the fact that there are no known particles which produce magnetic fields without moving (even permanent magnets rely on constantly moving subatomic particles).The Electromagnetic Force
Modern physics considers all forces to originate from only four forces: the strong nuclear force, the weak nuclear force, gravity, and the electromagnetic force. This implies that, at some level, E and B fields are one and the same thing. Einstein's theory of Special Relativity in fact shows how the effects of the magnetic field can be explained by only a combination of electric field theory and relativity, without needing to resort to another type of field. Stated very simply, according to this theory, the electric field affects objects differently depending on the relative velocity between the source and the object. Because relativity is not an easy thing to think about or use, the concept of the magnetic field was retained as a simpler way to express the same effect. There are also some situations in which it seems as though the magnetic field has some physical significance apart from the electric field(particularly involving the magnetic vector potential as applied in quantum mechanics). This tutorial will use both concepts (E and B field) because it is easier and because of convention. So, when you see reference to the B field in this tutorial, you can consider it either as a separate entity which creates forces only on moving charges, or you can just consider it to be a relativisitc electric field effect. For more information on this topic, please see the bibliography at the end of the chapter.