Physics of Magnets
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We all know certain situations where magnets are used, hanging things on a refrigerator for example. But other applications are much more useful in our society. They are used in all kinds of speakers, and in many computer parts including hard drives and floppy drives ( for recording and reading purposes). Perhaps a more common use that goes unnoticed is the magnetic strip on credit and debit cards. These have a certain magnetic makeup, that is why you are to keep them away from other magnets. Magnets are also used in many motors, in such items like a dishwasher, DVD and VHS players, and a pager or cell phone vibrator.
Magnets are all dipoles, that is they all have both a north and a south pole. No known magnetic monopoles exist. Looking at magnets from a basic point of view, opposites attract and similars repel. Magnetic field lines always move from the north pole to the south pole, we will discuss this later.
Some of the major contributors to magnets are men like Hans Christian Oersted, James Clark Maxwell, William Scoresby, Michael Faraday and Joseph Henry.
Hans Christian Oersted experiment with a wire carrying a current and a compass led to much of what we know about Magnetic Fields.
James Clark Maxwell discovered relationships between electricity and magnetism many of which are used in the Electromagnetic Theory. More information on the relationship between magnets and electricity can be found here.
William Scoresby used the Earth's magnetic fields to produce powerful magnets.
Michael Faraday and Joseph Henry are reported to have simultaneously discovered electromagnetic induction, which is the effect whereby the relative motion of a magnet and an electric coil produced a current.
There are three types of magnets. Permanent, temporary, and electromagnets. Permanent magnets are the most common ones. Once they are magnetized they stay so (although they can lose much of their magnetic force). They can be metals found in nature. Temporary magnets hold the properties of a magnet while in a magnetic field, but lost these properties once the field goes away. An example of this would be a paper clip that is charged and can act like a magnet for a short while. Electromagnets are wires wrapped around a metal center(usually iron).
The wires have a current flowing through them. An example of this would be a nail with wires wrapped around it and connected to a battery. The nail would then be able to pick up metallic objects.
It is important to note that these magnetic fields always travel from the north pole to the south pole. Magnetic fields always radiate out of the north pole and magnetic fields always go in to the south pole. We define a magnetic field with the symbol B. "The direction of the magnetic field B at any location is the direction in which the compass needle points at that location" (Serway).
With regard to the velocity of a particle moving through a magnetic field, its direction can be changed by the field but speed and kinetic energy of the particle cannot be altered by the magnetic field. It is also important to note that "when a charged particle moves in a magnetic field, the work done by the magnetic force on the particle is zero." We know this" because the displacement is always perpendicular to the direction of the force"(Serway). Think of it as if you were walking. The work done while walking is zero because you are displacing forward and the direction of the force is down (your feet striking down on the ground pushing you up).
If we are dealing with a charged particle in a magnetic field, the particle moves in a circle perpendicular to the magnetic field. It moves in this way because "the magnetic force F sub b is at right angles to v and B and has a constant magnitude qvB" (Serway). The force is deflecting v and F sub b continuously. However, note that again although the direction of the velocity changes, it does not change its magnitude. A really cool webpage related to magnetic forces on moving charges can be found here. Depending on weather the magnetic field goes into or out of the "circle" determines which direction the particle moves. If the field is coming out of the page the particle moves in a clockwise fashion. If the field is going into the page the particle travels counterclockwise. This page provides a real time demonstration of this. Same is true depending on the charge. If the charge is positive(and the force is pointing inward) it would move counterclockwise, and if the the charge is negative (and the force is pointing inward) it would move clockwise.
It is important to note that earth's true magnetic north (the pole that attracts the south pole) isn't the geographical north pole. As http://deeptow.whoi.edu/northpole.html says "Earth's magnetic north pole is where the magnetic field lines are oriented vertically and plunge into the surface of the earth". We know that the field lines travel from north to south, so it must be that the earths geographical magnetic north is true magnetic south. In the picture below the exact location of earths geographical north magnetic pole. Located in northern Canada.
Although this can seem weird at first it makes logical sense to call it the "north pole". Think of a compass for example. The needle "points" to the north pole. What is really happening is the metal in the compass needle is being attracted to the earth's magnetic south pole. It is easier to think of pointing north=north, instead of actually understanding the compass isn't pointing, but instead being pulled.
"The Van Allen radiation belts consist of charged particles (mostly electrons and protons) surrounding the Earth in doughnut-shaped regions" (Serway). These particles are trapped by Earth's magnetic field. The Van Allen belts "spiral around the field lines from pole to pole..."(Serway).The cause for the aurora is particles colliding with atoms in the atmosphere. The reason the Aurora is mostly confined to the polar regions is because this is where the Van Allen belts are closest to the Earth's surface. The Earth's surface is where the field lines enter or exit the planet. It is at this location where the Van Allen belts interact with the Earth's atmosphere causing the collisions discussed above.
As you can now hopefully see magnetic fields not only play a role in physics but are also very applicable to our everyday lives. Magnetic fields are very complex and I've only
touched the surface.
The relationship between magnetism and electricity is very close. This page is dedicated to how magnetism and electricity interact in fields but before we discuss that I feel it is important to know a little bit about the electromagnetic force.
According to this physics page "[the electromagnetic force] one of the four fundamental forces the electromagnetic force manifests itself through the forces between charges (Coulomb's Law) and the magnetic force, both of which are summarized in the Lorentz force law."
Now that we know how closely related the electric and magnetic forces are it is appropriate to discuss further the interaction between electricity and magnetism in fields.
The first interaction is a magnetic force acting on a current carrying wire. This situation acts similar to the situation of a charged particle moving through a magnetic field. There is a force on the wire. It is simply a collection of charges in motion. "Hence the resultant force exerted by the field on the wire is the vector sum of the individual forces exerted on all the charged particles making up the current" (Serway). The total Magnetic force on a wire is equal to the current(I) times L(length of wire) cross B(magnetic field). If we have a closed current loop the net magnetic force acting on it is 0.
Soon after Oersted's discovery that a compass needle is reflected by a current-carrying wire two scientist by the name of Jean-baptiste Biot and Felix Savart performed experiments dealing with the force exerted by a current on a nearby magnet (Serway). Their experiments lead to the Biot-Savart law. "The Biot-Savart Law relates magnetic fields to the currents which are their sources".
There are variants of magnetic fields and wires. Ampere's law can be used in many of these variants. See Ampere's law applications.
The last subject I will touch on here is Electromagnetic Fields. Electromagnetic waves (which naturally involve magnetic fields and electric fields) are predicted by Maxwell's equations. These equations are I. Gauss' law for electricity II. Gauss' law for magnetism III. Faraday's law of induction IV. Ampere's law.
The reason I mentioned the electromagnetic fields is so we could discuss the energy in electric and magnetic fields. We know both fields store energy. Electric fields store energy in capacitors and magnetic fields store energy in inductors. The equations for the energy densities are shown below.
Nature is very symmetric as you may or may not have known. This is why many scientists believe very strongly that there are such things as magnetic monopoles. As to date, however, "a single magnetic pole has never been isolated" (Serway). One of the reasons they are so had to find or make is shown below. As the pictures show if you take a magnet, say a bar magnet, and cut it in half, instead of getting a south pole and a north pole, we are left with two new magnets.
A positive magnetic monopole is an isolated magnetic north pole." This is a good picture to keep in mind because you can have an isolated electric charge that is either positive or negative. This is not the case with magnets.
As we can see the monopole magnets would act like an isolated charge. The fields radiate radially outward from the north pole (positive charge) and go radially in to the south pole (negative charge). Although magnetic monopoles are believed to exist, they have never been observed experimentally. This results in Maxwell's equation.(Click the link to the left for an extensive look at the Maxwell equations). It is predicted, according to grand unified theories(GUT), that "The charge on magnetic monopoles predicted by GUT's is either 1 or 2gD(Jeon and Longo 1995). " and "The upper limit on the monopole mass is 1026 eV, or 0.2ug."
Much time and many resources are given to people trying to find magnetic monopoles as Serway-Beichner says "attempt's to detect [magnetic monopoles] currently make up an active experimental field of investigation". It is important to find these monopoles because it would change the theory around some of the methods solving magnet related problems. For example the Maxwell equations would be shown to be flawed and possibly obsolete.
1. Serway, Raymond A, and Robert J. Beichner. Physics for Scientists and Engineers. N.p.: Thompson Learning, Inc, 2000.
3. http://scienceworld.wolfram.com/physics/- Excellent Source for Information dealing with anything physics related.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html coolest/main page!