An important point to be made is that in our everyday experience most metals appear to be magnetic because the most widely used metal is steel containing iron. This is made more difficult for students to explore because they must have at least two magnets of comparable strength and many of the familiar advertising fridge magnets used for simple investigations are weak and constructed in a way that they have no identifiable magnetic poles.
The surfaces of these magnets are well protected and will reduce the risk of students accidentally pinching fingers or the magnets shedding fragments if handled roughly.
Ask the students to investigate what they need to do to make the magnets attract and repel each other. Have them identify different ends of each magnet with identifying stickers. How well can the students predict what will happen when the magnets are brought near to each other?
Now encourage students to use masking tape to fix one magnet onto the roof of a toy car. Use the hand held magnet to push the car along without touching it or to attract the car towards it by changing its orientation.
Can students predict if the magnet on the car will be attracted or repelled by the approach of a new magnet? The intention here is for students to recognise that magnets can repel as well as attract each other. At this level it is not considered important for students to be able to recall that like poles repel and unlike poles attract, but to recognise that magnets can repel and attract without making physical contact and that their orientation is important.
Students can be encouraged to investigate if magnetic forces pass through other non magnetic materials. To capture student interest, place a magnet such as a fish tank glass cleaning magnet on a classroom table. Introduce another magnet the other glass cleaning magnet under the table so the two are strongly attracted. Position the magnet so you can move the magnet under the table with your knee or other hand. The magnet on the table top will follow the movement of the magnet below.
Next have them fix a paper clip to a length of cotton with sufficient length to reach from the table top to the magnet. Encourage students to investigate if various materials will stop the magnetic force of attraction when they are introduced between the magnet and the paper clip.
Try sheets of paper, glass, tile, aluminium foil, copper and zinc sheet. Do any of these materials have an effect on reducing the magnetic force?
By not running into each other, the electrons can save a huge amount of repulsive electric energy. To make this discussion a little more qualitative, one can talk about the probability that a given pair of electrons will find themselves with a separation. For electrons with opposite spin in a metal , this probability distribution looks pretty flat: electrons with opposite spin are free to run over each other, and they do.
The size of that hole is given by the typical wavelength of electron states: the Fermi wavelength where is the concentration of electrons. And the only meaningful definition of the electron size is the electron wavelength.
If you plot two probability distributions together, they look something like this:. If you want to know how much energy the electrons save by aligning their spins, then you can integrate the distribution multiplied by the interaction energy law over all possible distances , and compare the result you get for the two cases. This is what drives magnetism. These two features are more-or-less completely generic.
The answer is that there is an additional cost that comes when the electrons align their spins. Specifically, electrons that align their spins are forced into states with higher kinetic energy. You can think about this connection between spin and kinetic energy in two ways. The first is that it is completely analogous to the problem of atomic orbitals or the simpler quantum particle in a box.
In this problem, every allowable state for an electron can hold only two electrons, one in each spin direction. But if you start forcing all electrons to have the same spin, then each energy level can only hold one electron, and a bunch of electrons get forced to sit in higher energy levels.
The other way to think about the cost of spin polarization is to notice that when you give electrons the same spin, and thereby force them to avoid each other, you are really confining them a little bit more by constraining their wavefunctions to not overlap with each other.
This extra bit of confinement means that their momentum has to go up again, by the Heisenberg uncertainty principle , and so they start moving faster. So when you try to figure out whether the electrons actually will align their spins, you have to weigh the benefit having a lower interaction energy against the cost having a higher kinetic energy.
But the basic driver of magnetism is really as simple as this: like-spin electrons do a better job of avoiding each other, and when electrons line up their spins they make a magnet. The simplest quantitative description of the tradeoff between the interaction energy gained by magnetism and the kinetic energy cost is the so-called Stoner model.
I can write a more careful explanation of it some time if anyone is interested. But you will definitely need to think about them if you want to predict the exact strength of the magnetic field in a material. There is a pretty simple version of magnetization that occurs within individual atoms.
If I were a good popularizer of science, then I would really go out of my way to emphasize the following point. The existence of magnetism is a visible manifestation of quantum mechanics. It cannot be understood without the Pauli exclusion principle, or without thinking about the electron spin. I see that Iron and Neodymium both conform roughly to his conclusion.
Is this true of other elements near the center of their electron shell line in the periodic table? Also, when you use the term kinetic energy in reference to electrons, are you using it in the sense of mass in motion? If so to increase the kinetic energy of anything it must either move faster or gain mass. Creating an electro-magnetic field around an iron nail will make it a permanent magnet.
Last, only some materials are attracted to or repulsed by magnetism, iron in particular. Are these materials the same materials that can be made into magnets? This law is part of the Lorentz force law, which describes both electric and magnetic forces. The magnetic force law states:.
This expression implies that a magnetic field exerts a force on a charged particle only when the particle is moving. This expression also shows that the force is largest when the magnetic field is perpendicular to the direction of motion of the charged particle, and there is no force if the field is parallel to the direction of motion.
See also: Motion. The magnetic force law further states that the directionality of the magnetic force is always sideways relative to the motion of the particle.
Therefore, the magnetic force can only change the direction of the particle but cannot accelerate it. As a result, a magnetic field can never do mechanical work directly. For a charged particle moving freely in space, the sideways nature of the magnetic force causes the particle to travel along a helical trajectory around the magnetic field lines.
See also: Ionosphere ; Nuclear fusion ; Particle trap. Because all magnets contain collections of currents, magnets exert forces on each other.
In principle, the force between two magnets can be calculated using the magnetic force law. In practice, however, such calculations are complicated. For simple magnets, an easier rule can be used: Opposites attract and likes repel.
This means that a north magnetic pole attracts a south magnetic pole. It also means that two north poles repel each other and, similarly, two south poles repel each other. Determining the location of the poles of an induced magnet or electromagnet involves additional rules. The force between magnets is the operating principle behind electric motors and audio speakers.
See also: Loudspeaker ; Motor. For mathematical convenience, the total magnetic field B is often separated into the sum of two partial magnetic fields: the magnetizing field H and the magnetization M. Historically, these fields have been called many terms, such as flux density, magnetic induction, and magnetic polarization. Modern scientists refer to these fields by their letter names to avoid confusion. The H field is the magnetic field associated with free currents; that is, the currents in conductors and in free space.
The M field is the magnetic field associated with bound currents; that is, the currents in magnetized materials. The three main types of magnetic materials are ferromagnetic, paramagnetic, and diamagnetic.
See also: Diamagnetism ; Ferromagnetism ; Paramagnetism. A changing magnetic field always produces an associated electric field. This induced electric field is able to generate currents in conductors.
English physicist and chemist Michael Faraday discovered this law in the mid-nineteenth century, leading him to invent the generator. Magnetism is only one component of electromagnetism. Skip to content.
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