ELECTRICITY AND MACHINES
ECET 149
LESSON # 18
CHAPTER #
PREPARED BY: LINDSAY STRETCH
OVERVIEW
TIME: 1 HOUR
MATERIALS NEEDED
Electromagnetism introduction
OBJECTIVES
LEAD-I N
What is electromagnetism?
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ELECTRICITY AND MACHINES
WHAT IS ELECTROMAGNETISM?
Electromagnetism is the subfield of classical physics which studies electric charges,
currents, and, most importantly, electromagnetic field. This field permeates space,
carries energy and momentum, and exerts a force on particles that possess electric
charge, while it is in turn affected by the presence and motion of those particles. A
changing magnetic field produces an electric field (this is the phenomenon of
electromagnetic induction, which underlies the operation of electrical generators, induction
motors, and transformers). Similarly, a changing electric field generates a magnetic field.
Because of this interdependence of the electric and magnetic fields, it makes sense to
consider them as a single, theoretically coherent entity—the electromagnetic field. This
unification, completed by James Clerk Maxwell through Maxwell equations, is one of the
triumphs of 19th century physics. Maxwell's equations have far-reaching consequences,
one of which was the elucidation of the nature of light: as it turns out, what is thought of as
"light" is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an
electromagnetic wave. Different frequencies of oscillation give rise to the different forms of
electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at
intermediate frequencies, to gamma rays at the highest frequencies. The connection
between mechanics and electromagnetism is supplied by the Lorentz law giving the force
on a charged (moving) particle. Finally, the theoretical implications of electromagnetism
led to the development of special relativity by Albert Einstein in 1905.
We'll start our discussion with a look at the force of electromagnetism. The theory describing this
force states that there are electric and magnetic fields. Such fields are generated by electrically
charged objects and we can think of them as little arrows at each point in space (or, to be precise,
in spacetime).
This image shows the magnetic field lines of an ideal cylindrical
magnet, which is caused by the electrons inside the magnet.
Mathematically a magnetic field is represented by a little arrow at
each point, which indicates the strength and direction of the force.
Image: Geek3, CC BY-SA 3.0.
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This image illustrates the electric field surrounding a positive
(red) and a negative (blue) charge. Mathematically al electric
field is represented by a little arrow at each point, which
indicates the strength and direction of the force.
Image: Geek3, CC BY-SA 3.0.
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ELECTRICITY AND MACHINES
We feel the presence of these fields by their action on charged particles that enter them. Electric
fields act on charged particles by pushing them along the direction of the electric field. Magnetic
fields only act on moving charges. In the presence of a magnetic field a moving charge feels a force
that acts perpendicularly to the direction of its velocity. So if you were a charged particle moving
forwards in a vertical magnetic field, you would feel a force pushing you to your side. If there were
no other forces, then you would end up moving along a circular trajectory. See the figure below. In
summary, charged particles move in circles around magnetic fields.
(a) An electric field exerts a force on a charged particle along the direction of the field. (b) A magnetic field exerts a force on a
moving charged particle that is perpendicular both to the magnetic field and the direction of motion. (c) The charged particle ends up
moving in a circle around the magnetic field.
These electric and magnetic fields have their own dynamics. We can have electromagnetic waves,
which are interlocked oscillations of electric and magnetic fields. These can propagate through empty
space. They are radio waves, light, X-rays, gamma rays, etc.
All this might be familiar to you. But don't panic if it isn't! You do not need to know the detailed
equations to understand what follows. All you need to know is that there are electric and magnetic
fields, which can exist in otherwise empty space. These fields act on charged particles and affect their
motion. Electric fields push them along the direction of the electric field. Charged particles move in
circles around magnetic fields.
Next page:
0 = 8.85 x 10 farad per meter (F/m) = 8.85418782 × 10 m kg s A
-12
-12
-3
-1
4
2
q = 1.60217662 × 10-19 coulombs and G = 6.67408 × 10-11 m3 kg-1 s-2 = 6.673×10-11 N m2 kg-2
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ELECTRICITY AND MACHINES
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ELECTRICITY AND MACHINES
Frequency is related to wavelength by c=f where c=3x108m/s, f = frequency in Hz and  = wavelength in meters.
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ELECTRICITY AND MACHINES
Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A
particle of charge q moving with velocity v in the presence of an electric field E and a magnetic field B experiences a
force:
𝐹 = 𝑞𝐸 + 𝑞𝑣 × 𝐵
Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called Laplace
force), the electromotive force in a wire loop moving through a magnetic field (an aspect of Faraday's law of
induction), and the force on a charged particle which might be travelling near the speed of light (relativistic form of
the Lorentz force).
Electromagnetism – electricity and magnetism are both
manifestations of the same thing.
Electromagnetic or magnetic induction is the production of
an electromotive force (i.e., voltage) across an electrical conductor due to its
dynamic interaction with a magnetic field.
Michael Faraday is generally credited with the discovery of induction in 1831, and James Clerk Maxwell mathematically
described it as Faraday's law of induction. Lenz's law describes the direction of the induced field. Faraday's law was
later generalized to become the Maxwell-Faraday equation, one of the four Maxwell's equations in James Clerk
Maxwell's theory of electromagnetism.
Electromagnetic induction has found many applications in technology, including electrical components such
as inductors and transformers, and devices such as electric motors and generators.
Lenz's law (pronounced /ˈlɛnts/), named after the physicist Heinrich Lenz who formulated it in 1834, says:
The direction of current induced in a conductor by a changing magnetic field due to Faraday's law of induction will be
such that it will create a field that opposes the change that produced it.
Lenz's law is shown by the negative sign in Faraday's law of induction:
δΦ
𝑑Φ
𝜀=−
= −𝑁
𝛿𝑡
𝑑𝑡
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EMF or voltage is proportional to the rate of change of magnetic flux
and in the opposite direction. If N turns are tightly wound, the voltage
is proportionally greater.
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ELECTRICITY AND MACHINES
CONCLUSION
HOMEWORK
READING
Chapter
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