203 lines
8.6 KiB
Markdown
203 lines
8.6 KiB
Markdown
---
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categories:
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- Electronics
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tags: [physics, electricity, electromagnetism]
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---
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# Electromagnetism
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> Electromagnetism is the physical interaction among **electric charges,
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> magnetic moments and the electromagnetic field**.
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For a long time electricity and magnetism were thought to be separate forces. In
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the 19th century Maxwell demonstrated that they were interrelated phenomena then
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Einstein proved with the Special Theory of Relativity that they are aspects of
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one unified phenomenon.
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The core of the relationship is that a changing magnetic field produces an
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electric field and conversely, a changing electric field produces a magnetic
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field.
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## Electric charge
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We know that charge is an innate property of all charged fundamental particles.
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If a particle is charged it has a positive or negative charge. The most common
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charged particles in the universe are negatively charged electrons or positively
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charged protons. When charged particles are moving, they are known as electric
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currents.
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## Magnetism
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> Magnetism is a physical property produced by the _motion_ of electric charge,
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> which of course, is the same thing as
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> [electric current](/Electronics_and_Hardware/Analogue_circuits/Current.md)
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A **magnet** is a material or object that produces a magnetic field. This field
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is invisible but visible by its effects: pulling on other magnetic materials
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such as iron, steel, nickel, cobalt etc and attracting or repelling other
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magnets.
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All magnets have two ends where their magnetic effects are strongest. These
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regions are called the **poles** of the magnet. Materials can be _magnetic_ but
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they are not _magnetized_ until another magnetic material has entered into their
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field. At this point, the attraction and repulsion behaviour can be observed.
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This behaviour is a function of the **magnetic force** which is
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**_transmitted_** via the **magnetic field**.
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### Relation to electrons
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Magnetism, understood as the effect of a magnetic field, arises from the
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properties of the electrons in an atom. We know that atoms 'orbit' the nucleus
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of the atom but as they circle the nucleus they also spin or rotate on their own
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axis.
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As they spin they produce a **magnetic dipole**: the two poles noted above. We
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call this propensity of electrons the **intrinsic magnetic moment** of the
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electron. It is aggregates of these miniature magnetic behaviours that produce
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the overall magnetic property of the material.
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In most materials, equal numbers of electrons spin in opposite directions. As a
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result, their magentic effects are cancelled out. However **in strongly magnetic
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materials an overall majority of electrons spin in one particular direction**.
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This breaks the equilibrium and produces the magnetic field.
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If you have material A where the electrons all spin in one direction and
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material B where the electrons all spin in a direction opposite to A, then B
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will be attracted to A and you can observe the effects of the magnetic force in
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action.
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## Fields and forces
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### What is a force in physics?
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At its most abstract, divorced from the specific types of force, a force is **an
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influence that can change the motion of an object**. It is an external agent
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capable of changing the velocity of a body with mass. A force has both a
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magnitude and a direction.
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### What is a field
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A field is a property of space. It means that a physical quantity is assigned to
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every point in space. This quantity has a numerical value and may vary over
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time.
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## The electric field
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There are different types of field. The electric field is an instance of a
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**vector field**. With vector fields there is more than one number for each
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point in space:
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- a **magnitude**: a size value, i.e. being larger or greater than something
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else
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- a **direction**
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> The value of the electric field at a point in space equals the force that
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> would be exerted on a unit of charge at that position in space.
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Every charged object sets up an electric field in the surrounding space. A
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second charge “feels” the presence of this field. The second charge is either
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attracted toward the initial charge or repelled from it, depending on the signs
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of the charges. Of course, since the second charge also has an electric field,
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the first charge feels its presence and is either attracted or repelled by the
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second charge too. The electric field from a charge is directed away from the
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charge when the charge is positive and toward the charge when it is negative.
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## The magnetic field
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As already noted, the magnetic field is the field created by a magnetic
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material: a material where the spin of the electrons is in disequillibrium. As
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with the electric field, a magnetic field exerts an attraction or repulsion on
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other spinning electrons. This is the **magnetic force**; the force is
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transmitted by the field. Attraction occurs when the two sets of electrons spin
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in opposite directions to each other. Repulsion occurs when the two sets of
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electrons spin in the same direction.
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> Crucially the magnetic force influences only those charges that are already in
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> motion.
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The magnetic field and force is more complex than the electric field/force.
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Whereas the electric field and force point either towards or away from the
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charge, the magnetic field is different:
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- The magnetic field points perpendicular to its source
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- The magentic force points perpendicular to the magnentic field
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This is illustrated below which shows the magnetic field operating at right
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angles to the flow of charge within a wire.
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<img src="/home/thomas/repos/eolas/_img/magnetic_field.png" width="300" />
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## The electromagnetic field
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Although we have described the electric and magnetic fields separately, they are
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in fact a single unified and inseparable field created by charged particles and
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particles with a magentic moment. In describing electricity and magnetism we
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have focusing on different attributes of the same phenomenon.
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The electromagnetic field does not carry charge or magnetic moment, it carries
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energy and momentum. This energy and momentum can be transferred to charged
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particles and particles with magnetic moment.
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### Maxwell's Equations
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The equations express four laws that offer a complete account of the
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electromagnetic field. The equations integrate separate discoveries and laws
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identified by other scientists with a common mathematical representation. This
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is why they are not called 'Maxwell's Laws' since he didn't originate the laws,
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he unified them into a common expression. These collective laws capture
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everything we have discussed so far in relation to electric and magnetic fields.
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1. An electrically charged particle creates an electric field. (Gauss' Law for
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Electricity)
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2. Magnetically charged particles do not exist. (Gauss' Law for Magentism)
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3. A changing magnetic field creates an electric field. (Faraday's Law of
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Induction)
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4. A changing electric field creates a magnetic field. (Ampere's Law)
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## The electromagnetic force
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Maxwell's Equations describe how electromagnetic fields are generated and behave
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but they don't specify how the fields interact with charged particles. An
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account of this is provided by Lorentz's force law which specifies what we have
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already described above:
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> An electric field exerts a forward or backward force on a charged particle and
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> a magnetic field exerts a sideways/perpendicular force on a moving charged
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> particle.
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## Electromagnetic radiation / waves
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> Electromagnetic radiation consists in waves of the electromagnetic field
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> propagating through space carrying electromagnetic radiant energy.
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Electromagnetic waves consist of rapidly changing electric and magnetic fields.
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They are emitted any time an electrically charged particle accelerates. These
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waves are generally referred to as light. However, more accurately, 'light'
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refers to the types of electromagnetic wave that we can see. Electromagnetic
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waves form a spectrum based on their frequency and wavelength. For example,
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'radio waves' are low-frequency / long wavelength electromagnetic waves and
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gamma rays are high-frequency / short wavelength waves:
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The image below shows the propagation of an electromagnetic wave through space.
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We can identify the core components as follows
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- The vector of the magnetic field $B$ propagates outwards along the $z$ axis
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- The magenetic field is perpendicular to the vector of the electric field $E$
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which propagates upward along the $y$ axis
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- The directionality of both waves is forward along the $x$ axis
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## Using magnetism to generate electricity
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## Understand better:
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- How do EM waves relate to simple electrical circuits.
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- Link discussion here to discussion of Hertz
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https://www.britannica.com/science/electromagnetism/Coulombs-law
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