MAG - Magnets and Magnetic Fields

Magnets and Magnetic Fields

Magnet Basics

Our knowledge of magnets began thousands of years ago with rocks that were found to attract each other at a distance. Our understanding has since grown considerably, taking us into the quantum realm to explain the finer properties of magnets. In this unit we'll focus on the basics that allow you to describe magnets and how they interact with one another.

Poles

Regardless of a magnet's shape, it has two ends called poles. We label one a north pole and the other a south pole. As you should remember from elementary school science:

Like poles repel each other & opposite poles attract.

It's just like the forces between like and opposite electric charges (which is just the first of the similarities between magnets and electric charges). One major way magnets and electric charges differ is that electric charges can be isolated. You can have an electric monopole (one positive or one negative charge alone). However, you will always find magnets with two poles, one north and one south. This fundamental form of a magnet is known as a magnetic dipole ("di" meaning two poles). A consequence of this is that if you were to break a magnet in half, each half would still have a north and south pole. No matter how many times you break the magnet, it will always be a magnetic dipole, even once you get down to an individual atom. The magnetic properties come from the moving electrons in the atoms themselves. This will be covered more in the video below.

Magnetic Fields

The cause of the attractive or repulsive force between magnets is the interaction of magnetic fields surrounding each magnet. These fields are very similar to the electric fields surrounding charged particles as discussed in the Electrostatics unit. Magnetic fields are represented by field lines that point from magnetic north poles to magnetic south poles. The direction of a magnetic field can be defined by the direction the north pole of a compass (a magnet free to rotate) would point when in the field. Like the field lines between electric charges, the field lines between magnetic poles will connect between opposite poles and repel between like poles. Unlike electric charges, the fields don't start and stop at magnetic poles. The magnetic field continues through the material completing a closed magnetic loop.

The following images show the magnetic fields of various magnet configurations through the use of iron filings. These slivers of iron tend to align themselves with the field lines, like mini compasses.

Domains

Not all materials are magnetic or can be attracted by magnets. To become a strong magnet, a material must be ferromagnetic. This term comes from the Latin for iron "ferrum," which was the first element discovered to have this property. Ferromagnetic materials become magnets themselves when exposed to a strong, external magnetic field. Iron, cobalt, nickel, gadolinium, and some of their oxides and alloys are the most common ferromagnetic materials. Ferromagnetic materials have regions where the magnetic properties of the atoms align locally to produce a similar magnetic field. These regions are known as domains. Domains of non-magnetic materials are randomly aligned such that the fields tend to cancel out the effects of each other. When exposed to a strong magnetic field, the domains of ferromagnetic materials align causing the material to take on magnetic properties of its own. To determine if a material is ferromagnetic, see if it is attracted to a magnet. The presence of a magnetic field can cause a temporary alignment of the domains of the material such that it will be attracted to the magnet. This is similar to the principles behind electrically charging an object by induction. In non-ferromagnetic materials, the domains will not align to produce a magnetic field and the object will not be attracted to magnets. For example, steel, being an iron alloy, is ferromagnetic and will be attracted to magnets or may be turned into a permanent magnet itself. Aluminum is non-ferromagnetic. Magnets do not attract aluminum and it can't be easily turned into a permanent magnet.

Earth's Magnetic Field

Earth behaves as if it has a giant bar magnet running through it. This is what produces our planet's magnetic field. It's this field that compass needles align with in order to point to geographic north. Well, almost geographic north. Earth's magnetic north and south poles don't align completely with the planet's axis of rotation. There is about 20 degrees of magnetic declination between the magnetic poles and the geographic poles. So compasses tend to be more accurate near the equator than they are near the poles. When discussing the planet's magnetic poles there's also the issue of which end is up. By that I mean that the north geographic pole is technically Earth's magnetic south pole. Recall that magnetic field lines point from north to south magnetic poles. A compass' north pole, therefore, is pointing to Earth's magnetic south pole. Regardless of this technicality, Earth's magnetic pole in the north is often referred to as the "north magnetic pole" simply because it is located in the north. If you're confused, welcome to the club.

The following video is from the guys at Minute Physics and Veritasium. It does a nice job summarizing what has been presented thus far as well as giving a sneak peak of what is to come.

 

IMAGES  AND VIDEO SOURCED FROM PUBLIC DOMAIN