Explaining the Relationship of an Electromagnetic Field

The electromagnetic field is a formation of magnetic lines of force surrounding a conductor, which is created by electrical current in the conductor. Electric motors, such as vehicle starter motors, use this magnetic field interaction to convert electrical energy into mechanical energy. The mechanical result is work being done as a high-torque motor. All ignition systems use electromagnetic induction to produce a high-voltage spark from the ignition coil.

Let’s walk through these concepts one by one. We will start at the atomic level by considering the electric charges in the atoms that make up matter.

To review, atoms are composed of three types of subatomic particles:

  1. Electron: Has a negative charge (e) and orbits the nucleus, which contains the proton and the neutron.
  2. Proton: Has a positive charge (p+) and is attracted to the electron because of the electron’s negative charge.
  3. Neutron: Has no charge, but can be thought of as both (+/-) together. It is electrically neutral.

Figure 1. Diagram of the atom. Electrons orbit the protons and neutrons contained in the central nucleus. CC BY 3.0 by Fastfission.

We see that electrons and protons attract one another because of their complementary charges: one is negative, the other positive.

Now let’s consider magnets. As you remember, a magnet has a north pole and a south pole. The magnetic field is the force field radiating from the north pole of a magnet to its south pole. Magnetic flux linesrepresent these invisible lines of force and are drawn from the north pole to the south pole of a magnet.

Two magnets arranged north pole-to-north pole (or south pole-to-south pole) will repel each other, as in Figure 2. Conversely, two magnets arranged north pole-to-south pole will attract one another as in Figure 3. Another way to describe this is “like charges repel, unlike charges attract” – just as in the atom.

Figure 2. Photograph of the magnetic field of two bar magnets with like poles close together and thus repelling. The magnetic field was made visible by a layer of iron filings on a piece of paper laid on top of the magnets. The iron filings are shaped like long thin needles, and orient themselves parallel to the magnetic field lines at each point. Public Domain by Alexander Wilmer Duff, 1916.Figure 3. Photograph of the magnetic field of the same two bar magnets with unlike poles close together and thus attracting. Public Domain by Alexander Wilmer Duff, 1916.

Now let’s combine the concepts of electric charge and magnetic field to discuss the electromagnet. An electromagnet is a magnet that uses (and depends upon) electric current to create its magnetic field. Unlike a permanent magnet, which is magnetic all the time, an electromagnet loses its magnetic field when the current turns off; thus, it requires a continuous supply of current. Another way to think of an electromagnet is as a conductor placed in a magnetic field.

Electromagnets have an important advantage over permanent magnets: increasing the electric current increases their magnetic field. The electromagnetic field is a formation of magnetic force lines surrounding the conductor. These flux lines are created by current flow and are proportional to the amount of current in the circuit. As current increases, more flux lines are created and the magnetic field expands. The amount of current flow (measured in amperes) determines how many flux lines there will be and how far out they extend from the surface of the wire.

Figure 4. A simple electromagnet, consisting of an insulated wire wound around an iron core. When an electric current is passed through the wire, the iron core becomes a magnet, with a north pole at one end and a south pole at the other. Public Domain by Berserkerus as modified by Chetvorno.

Figure 4 depicts an electromagnet. Its conductor is composed of coiled wire that conducts the current flowing through. Its core is a ferromagnetic (easily magnetized) material like iron, nickel, or cobalt. Remember that electrons, being negatively charged, seek a positive potential. In a conductor, only the electrons move; atoms do not move. Electrical pressure is needed to dislodge a valance electron from its atom and cause it to move through the conductor from the most negative point to the most positive point. When electrons move through the conductor in this way, it is called current. For the current (electrons) to flow, there must be a source of electromotive force (emf) or voltage.

Voltage can be generated by various methods, one of which is induction. Electromagnetic induction uses flux lines (the force lines in a magnet) in conjunction with a conductor to generate a voltage. It is the ability of magnetic force lines to create a voltage on a metallic wire or surface (i.e., the conductor) due to movement of either the magnetic field or the conductor. If both are stationary, then nothing happens; only when the magnetic field is altered do the force lines move. The quantity of induced voltage is dependent on the speed of the wire/field. The amplitude of induced voltage is dependent upon the number of positive peaks. The direction of induced voltage and the direction in which current flows is called polarity.

Applications of Electromagnetism

You have learned about the theory of electromagnetism; now let’s look at some of its applications to electric vehicle technology. From above, you know that voltage can be electromagnetically induced and can be measured. Induced voltage creates current.

First, the principle of electromagnetic induction is used in powering generators. Between the poles of a magnet or electromagnet, a mechanically powered source causes a coil to spin, converting mechanical energy into electrical energy. Enormous generators driven by steam or water turbines are used to supply our communities with electricity. A generator may be designed to produce alternating current (AC) or direct current (DC) electricity. Automobiles use AC generators, called alternators, as a main source of voltage when the engine is running. The AC generated is changed or rectified into DC for use in the vehicle.

Second, the principle of electromagnetic induction is used in powering AC induction motors. Motors, in contrast to generators, convert electrical energy into mechanical energy. Electric motors are perfect for vehicles due to AC induction, which produces maximum torque at low speeds. They are the perfect power source to get a vehicle moving from a stop. Watch this brief video to see the AC induction motor at work:

The AC induction motor contains electromagnets that are wound on a frame; the electromagnets consist of current-carrying coils of wire placed in a magnetic field. Electromagnetism produces motor torque. Alternating current consists of fluctuating voltage and reversing polarity. The interaction between the two magnetic fields causes a bending of the lines of force. The attraction and repulsion of these magnetic poles caused by the reversing polarity produce the desired rotary movement. In an electric motor, the rotating part, usually on the inside, is called the rotor. The rotor bars carry large amounts of induced electrical current. The magnetic fields around wire coils make up stator poles that interact magnetically with the rotor to generate rotor motion and torque.

Last, the principle of electromagnetism is used in regenerative braking. When an AC motor is used for regenerative braking, it acts a generator and produces an alternating current. The AC needs to be converted through rectification to DC to go into the batteries. Regenerative braking varies by speed, similar to the accelerator pedal being used to adjust the speed. Because the cylinders have nothing to compress, the engine does not cause any engine braking, and therefore allows more of the inertia of the moving vehicle to be converted to electrical energy through regenerative braking.

This video shows you how regenerative braking works:

In conclusion, we have discussed electric charge, magnetic fields, electromagnetism (how electric charge and magnetic fields combine), and three applications in electric vehicle technology.

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