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Magnetism

When you’re done with this unit you should be able to answer MCAS questions on magnetism.

What is a compass? A magnetized metal needle that it can spin freely, and which is oriented by the planet’s magnetic field.

You can make one by magnetizing a needle, placing it carefully on a slice of cork, and letting the cork float in a tray of water.

Left to its own, the needle turns until one end points towards the magnetic north pole and the other to the south magnetic pole.

You can figure out which end is which from the position of the Sun in the sky: the Sun rises in the east and sets in the west. So if you’re looking down on the floating needle at about noon, with the eye on the left and the point on the right, and the Sun in front of you, you know the point is indicating north.

http://www.explainthatstuff.com/how-compasses-work.html

Using a compass and map together.

Magnetic Poles

http://www.school-for-champions.com/science/magnetism.htm#.VSQjudzF9xs

Bar magnets Poles

Magnetic fields

We are surrounded by magnetic fields all the time, but our nerves don’t detect magnetism. So how can we see these invisible lines of force?

Bar magnet surrounded by compasses

Using iron filings to visualize the field.

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“A magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. Examples include the spin of a proton and the motion of electrons through a wire in an electric circuit.”

PowerPoint for chapter 36 Magnetism PPT – Hewitt Conceptual Physics

We’ll be mentioning an electromagnetic field. What exactly is a “field” anyways? See here: What are fields?

Electric charges are surrounded by an electric field.

That is a law of nature: a property of the universe that we always observe to be so.

By similar observations we learn this:

A moving electric charge is surrounded by a magnetic field. That is a basic law of nature – a property of the universe that we always observe to be so.

In fact, charges in motion always have both an electric and a magnetic field.

But where in a magnet is there moving electric charge? It’s just a piece of metal, sitting there.

The magnet as a whole may be stationary, but it is composed of atoms – and the electrons are in constant motion about atomic nuclei.

This moving charge constitutes a tiny electric current and produces a magnetic field.

Hewitt

Hewitt

Let’s make an analogy: To keep things simple, electrons can be thought of as spinning billiard balls – they spin about their own axes like tops.

Caution analogies

A spinning electron creates another magnetic field.

An orbiting electron creates a different magnetic field.

In most materials, the field due to spinning predominates over the field due to orbital motion.

So every spinning electron is a tiny magnet!

A pair of electrons spinning in the same direction makes up a stronger magnet.

Electrons spinning in opposite directions work against one another.  Their magnetic fields cancel.

Most substances are not magnets because the various fields cancel one another due to electrons spinning in opposite directions.

In materials such as iron, nickel, and cobalt, however, the fields do not cancel one another entirely.

An iron atom has four electrons whose spin magnetism is not canceled.

Each iron atom, then, is a tiny magnet.

The same is true to a lesser degree for the atoms of nickel and cobalt

http://www.scientificamerican.com/article/what-exactly-is-the-spin/

Caution: the above is an analogy.
Electrons are not really small billiard balls; they don’t spin
– rather, they have magnetic moments as if they spin.

What exactly is the ‘spin’ of subatomic particles? Scientific American

Earth’s magnetic field

Where does our planet’s magnetic field come from? How do they function?  And we know that the magnetic field of the Earth is not stable; it has flip-flopped throughout geologic time. How does that work?

Earth’s magnetic field

Maritime magnetism: How binnacles on ships work

Magnetic domains

We can make non-magnets, into magnets.

“Permanent” magnets are made by simply placing pieces of iron – or certain iron alloys – in strong magnetic fields.

Ferromagnets

“(a) An unmagnetized piece of iron (or other ferromagnetic material) has randomly oriented domains. (b) When magnetized by an external field, the domains show greater alignment, and some grow at the expense of others. Individual atoms are aligned within domains; each atom acts like a tiny bar magnet.”

Why do magnetic fields have the symbol B?

The origin of B was James Clerk Maxwell himself. See the article by Ralph Baierlein in the American Journal of Physics, v68, n8 (Aug 2000), p.691. In his text, “A Treatise on Electricity and Magnetism“, Maxwell presents a list of the vector quantities he will be dealing with. He then labels them in alphabetical order.

  • Electromagnetic momentum at a point: A (now called vector potential)
  • Magnetic induction: B (usually called magnetic field)
  • Total electric current: C
  • Electric displacement: D
  • Electromotive force: E
  • Mechanical force: F
  • Velocity at a point: G
  • Magnetic force: H (usually called magnetic intensity)

The use of A, B, D, F, and H has lived on, but C and G have been abandoned.

E for EMF has been replaced by the Greek letter epsilon ɛ , or ℰ (script capital E, Unicode U+2130).

So the letter E is now ‘electric field.’

https://www.harker.org/cfhttp.cfm?script=extensions/includes/resource/resourcecontent.cfm&rid=268&pageid=962

 

Where does magnetism come from?

I’ve heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, how does this work?

Luboš Motl, a Czech theoretical physicist, replies:

Special relativity makes the existence of magnetic fields an inevitable consequence of the existence of electric fields. In the inertial system B moving relatively to the inertial system A, purely electric fields from A will look like a combination of electric and magnetic fields in B. According to relativity, both frames are equally fit to describe the phenomena and obey the same laws.

So special relativity removes the independence of the concepts (independence of assumptions about the existence) of electricity and magnetism. If one of the two fields exists, the other field exists, too. They may be unified into an antisymmetric tensor, FμνFμν.

However, what special relativity doesn’t do is question the independence of values of the electric fields and magnetic fields. At each point of spacetime, there are 3 independent components of the electric field E⃗ E→ and three independent components of the magnetic field B⃗ B→: six independent components in total. That’s true for relativistic electrodynamics much like the “pre-relativistic electrodynamics” because it is really the same theory!

Magnets are different objects than electrically charged objects. It was true before relativity and it’s true with relativity, too.

It may be useful to notice that the situation of the electric and magnetic fields (and phenomena) is pretty much symmetrical. Special relativity doesn’t really urge us to consider magnetic fields to be “less fundamental”. Quite on the contrary, its Lorentz symmetry means that the electric and magnetic fields (and phenomena) are equally fundamental. That doesn’t mean that we can’t consider various formalisms and approximations that view magnetic fields – or all electromagnetic fields – as derived concepts, e.g. mere consequences of the motion of charged objects in spacetime. But such formalisms are not forced upon us by relativity.

Does special relativity make magnetic fields irrelevant? Physics StackExchange

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Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

7.MS-PS2-5. Use scientific evidence to argue that fields exist between objects with mass, between magnetic objects, and between electrically charged objects that exert force on each other even though the objects are not in contact.

7.MS-PS3-2. Develop a model to describe the relationship between the relative positions of objects interacting at a distance and their relative potential energy in the system. {Examples could include changing the direction/orientation of a magnet.}

HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a mathematical model describing change in motion (the acceleration) of objects when acted on by a net force….{forces can include magnetic forces}

HS-PS3-5. Develop and use a model of magnetic or electric fields to illustrate the forces and changes in energy between two magnetically or electrically charged objects changing relative position in a magnetic or electric field, respectively.

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