Electromagnetism Basics

Have you ever wondered how motors work? Have you ever wondered how a relay works? There are many applications where electromagnetism is a key factor in electrical design. This post is to give a basic understanding of how fields form around conductors. I won’t go into the calculations since that is more advanced, but these examples should give a good starting point for understanding behavior of technologies that use electromagnetism.

Electromagnetism

Current passing through an conductive material produces a magnetic field perpendicular to the direction of the current flowing in a specified direction according to the “right hand rule”. There are two “primary” shapes of conductors that have different explanations, one is a straight wire while the other is a coil of wire.

Straight Wire

Straight Wire Visual Examples



If you were to “hold” a wire with your right hand and imagine the current flowing in the direction of the thumb, the electromagnetic fields are forming around the conductor in 3D space; the direction and shape of the field is how the other four fingers are curling around that wire. Here is a short animation I made that shows the effect which may help further:
fieldAnimation
Not only do the fields curl around the wire in a particular direction, but the field also “expands” from the center with decreasing strength. If there was a ferromagnetic needle where the gray plane was, the “north” pole of the field would be attracted to the south pole of the Earth.

Note: The fields propagate all around the wire as long as current is flowing for the entire wire.

Coiled Wire

Coiled wire is composed of many “loops” going either clockwise or counterclockwise. Loops are particularly useful in making electromagnets because the current going in one direction is coming out the other direction which produces fields in opposite rotational directions. When the fields interact, they increase the magnetic field in the center because both fields are pointing in the same relative direction. Here is an illustration to better explain this phenomenon:
LoopField
Notice that the fields arrows meet up in the center going the same relative direction on the gray plane. Since these fields are close enough, the resultant field in the center is of greater strength. This behavior is much stronger with several loops such as coiled wire found in technologies like inductors, mechanical relays, and transformers.

Coiled Wire Visual Examples



Coiled wire is usually wrapped around a ferromagnetic core or material that has high magnetic permeability. This time, the depiction is as if you were holding the solid material that the wire is wrapped around. The four fingers around the “rod” represent the coils and the direction the current is flowing. The thumb is now representing where the north pole is as well as which direction the field is coming out of the end closest to the thumb. Here is a more detailed example showing a clockwise coil and counter-clockwise coil.

The blue lines are the smaller fields around each bend of the coils. Notice that the clockwise fields are all “pointing in” to the center of the coil going from the top to the bottom. The opposite is true for the counter-clockwise coil, the fields are pointing in to the center from the bottom up. Again, the fields are around the whole coil 360 degrees in reality. They also radiate outward with decreasing strength. Here is another animation showing behavior:
CoilAnimation

When you reverse the current direction, the polarity of the magnetic field switches:

Here are four examples of technologies that involve electromagnetism.

Transformers

Transformers, in particular, use the combination of a different number of loops (turn ratio) as well as the magnetic field produced by one side. Since alternating current fluctuates between positive and negative 60 times a second (US) or 50 times a second (Europe), the magnetic fields produced on one side is always changing polarity as well as increasing/decreasing in strength. The changing field reaches the other coil and “induces” a voltage at some different level based on the number of turns in that coil.

Mechanical Relays

Mechanical relays use the polarization of magnetic fields in a very direct manner. Since the north pole of a coil is very directional based on the current direction and winding direction, this field is used to pull down a ferromagnetic contact that usually conducts far more current than the coil can (the current applied to the coil and the current across the contacts are isolated). This is why there are almost always drawings of which side of the coil should be energized (for DC relays) as current direction is critical for proper actuation.

Inductors

One behavior of inductors, which is critical, is kick back. The fields generated in and around an inductor hold energy and will release that energy in the form of a very high voltage on a connected circuit if de-energized rapidly. Refer to this post for more detailed information: Inductor Behavior : Why Orientation Does Not Matter

Motors

Motors in general (both AC and DC) use generated fields for motion. In brushed-dc motors, the stator (part that doesn’t move) has contacts called brushes that provide current to a rotor (part that moves) that has coils. The fields generated by the electricity are repelled by an internal permanent magnet causing motion. There are a few methods for AC motors, but in general the concept is the same where the fields generated by electricity are being repelled by other magnets.

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