Physics 102 - Magnetism

Chapter 24

Horseshoes, anyone?






Legend has it that magnetism was first discovered some 2000 years ago when an elderly shepherd named Magnes was herding his sheep across a rocky valley when the iron nails in the soles of his sandles stuck to the rocks there. We have learned a lot since then.

Motion of charged particles create a magnetic field, and conversely a magnetic field deflects moving charged particles.


Magnetic field affects the moving charged particles, which in turn affect the field,which affects the particles,which affects the field,which affects the particles...




The odd thing about magnetic fields is that the force they exert on a moving charged particle pushes the perpendicular to the direction it was moving, and also perpendicular to the direction of the field. So if a proton was moving in the direction shown, which way would the magnetic field deflect it?



Electrons moving through a wire that is in a magnetic field also feel a force. In that case, the current-carrying wire feels a force exerted on it. If forces from two like magnetic poles are strong enough, they can actually cause levitation. This is the mechanism at work in magnetic levitating trains. They are not in contact with the ground, so there is no friction to slow them down and they can travel at very high speeds, like 250 miles per hour.


Levitron site




Iron filings lie along magnetic field lines. All magnets have a dipolar (two lobed) field. That means that magnets have north poles and south poles. If you cut a magnet in half, each half becomes a magnet with two poles. In the language of physics, we say that the magnetic field is dipolar. There is not such thing as a magnetic monopole. Electric monopoles do exist. A charged particle is an example of an electric monopole.



A magnetized needle floating on water will align itself with the magnetic field of Earth. Earth's magnetic field imposes a torque - a twisting sort of a force - on the needle. The needle rotates until it no longer feels a torque. Is there any net force on the needle? A net force would cause it to accelerate in some direction. Is this similar to or different from the electric field? Would these answers change if magnetic monopoles existed?






The earliest compasses we have found are from China from the 3rd century BC. There is evidence that they may have been in use up to 300 years earlier. Early compasses consisted of a spoon-shaped pointer made of magnetic lodestone placed on a bronze plate.



Magnetic domains

We said that magnetic fields are created by the morion of charged particles. But what about a magnet like the ones that stick to your refrigerator? What is moving there?


Motions of electrons in a magnetic metal create a magnetic field. Electron spins align in regions called magnetic domains. This happens most easily in metals with unpaired electrons, such as iron, giving rise to the term ferromagnets.


Reflectivity is a measure of how much light reflects from a surface. A magnetized surface changes the reflectivity depending on the magnetic domains. The light also changes in polarity. These effects are called the Magneto Optical Kerr Effect (MOKE) and they allow us to photograph the magnetic domains using a special microscope called a Kerr-effect microscope.


MOKE gallery

magnetic domains imaged using a Kerr-effect microscope




The magnetic strip on a credit card is coded by producing a pattern of magnetic domains alternating in direction.



Electric currents and magnetic fields


A magnetic field circulates around a current-carrying wire.

Tiny iron filings align with the magnetic field to show the field lines around these current-carrying wires.



A coil of current carrying wire is called a solenoid. The magnetic field lines circle the wire and cancel in the spaces between the wires. Out side and inside the coil, a strong magnetic field is produced where the field lines get closer together.

solenoid field animation





By placing ferromagnetic material inside a coil of wire, we can make the magnetic field inside the solenoid even stronger. When the current is turned on, this configuration becomes an electromagnet.



Electric motors



A magnetic field exerts a force on a current loop, at right angels to the direction of the field and to the direction of the current.


As the current loop turns, the magnetic field continues to exert a force on it.


When the loop turns far enough, the force would work to stop turning it, so the polarity of the current is switched to run the other way. This makes the force on the loop such that it continues to turn. This is the basic mechanism of an electric motor.



A simple electric motor operates using switching magnetic fields to push and pull a rotator with magnets affixed to it.


DC motor animation



A particle accelerator like the Large Hadron Collider works on the same principle. Magnetic fields switch on and off to accelerate charged particles to high speeds for collisions.


Earth's magnetic field




The magnetic field surrounding Earth is largely a simple dipole field like that of a bar magnet. We know that magnetic fields are caused by moving charged particles. What causes the magnetic field of Earth?




The magnetic field is evident in this photo from the sun's surface. Remember that magnetic fields exert a force on moving charged particles that deflects them perpendicular to the field. This causes them to spiral along the strong magnetic field lines of the sun, mapping out the field as they release light.




The solar wind contains charged particles which distort Earth's magnetic field. The magnetic field of Earth deflects charged particles in the solar wind. Huge magnetic storms on the sun called coronal mass ejections can hurl gigantic clouds of plasma into space. If they happen to come our way, they can cause problems like surges in power grids and breakdown of satellite communication.


coronal mass ejection animation




Charged particles in the solar wind can collide with particles in Earth's atmosphere, especially near the north and south rotational axes. When they do, they excite atoms which then return to ground state, emitting light. We see the eerie streaming flows of color that result. They are called the aurora borealis and aurora australis for the northern and southern phenomena respectively.






By studying magnetic material deep in ocean trenches where the continental plates are drifting apart, we can see that Earth's magnetic field has switched polarity many times. In other words, the north pole and south pole switch places back and forth over time.







This is a Bar-tailed Godwit.




Bar-tailed godwits have the longest nonstop migration of any bird, 7000 miles between Alaska and New Zealand. They find their way across the ocean by sensing Earth's magnetic field.






Some fish have been found to have rows of magnetic crystals in their brains, and in their sides. It is thought that they use these magnets to aid them in swimming in schools, making it easy to become aligned. Similar magnetic crystals have been found in people but it is not clear what function(s) they serve...







1) If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the iron near the south pole of the magnet?


2) Why does dropping a magnet on a hard surface weaken its magnetism?


3) One way to make a compass is to stick a magnetized needle into a piece of cork and float it in a glass bowl full of water. The needle will align itself with the magnetic field of the earth. Since the north pole of this compass is attracted northward, will the needle float toward the north side of the bowl?


4) The north pole of a compass is attracted to the north pole of the earth, yet like poles repel. Can you resolve this apparent dilemma?


5) Cans of food in your kitchen pantry are likely magnetized. Why?


6) We know a compass points northward because the Earth is a giant magnet. Will the northward-pointing needle point northward when the compass is brought to the southern hemisphere?


7) When iron naval ships are built, the location of the shipyard and the orientation in the ship while in the shipyard are recorded on a brass plaque permanently fixed to the ship. Why?


8) A magnetic field can deflect a beam of electrons, but it cannot do work on the electrons to change their speed. Why?


9) Two charged particles are projected into a magnetic field that is perpendicular to their velocities. If the charges are deflected in opposite directions, what does this tell you about them?


10) If you had two bars of iron--one magnetized and the other not--and no other materials at hand, how could you tell which bar was the magnet?