Ancient trees show when the Earth’s magnetic field last flipped out – This interesting news article from NPR gives some information about the last pole reversal, and how it was detected using trees. I recommend reading it after watching Magnetic Storm.
(Podcast) Magnets: the hidden objects powering your life – This 12-minute podcast gives a helpful introduction to magnets, magnetism, and why magnets are important.
Hello there! Welcome to lecture 24: magnetism.
Magnetism describes the properties of magnetic objects, which are capable of attracting and repelling other magnets. From using a refrigerator magnet to post artwork and photos on a fridge, to navigating with a compass, to the protection we receive from the Earth’s magnetic field, magnetism plays a large role in our lives.
Each of the following concepts will be discussed in this video: magnetism, magnetic fields, types of magnets, electromagnets, the deflecting force, and Earth’s magnetic field.
As we’ve learned in previous lectures, atoms contain a nucleus filled with protons and neutrons. Orbiting around the nucleus are electrons. Those electrons don’t just orbit, they also have a spin. That spin generates magnetism. Most substances, which are not magnetic, have random spin alignment, leading to an overall effect of no magnetism. When all or most of the atoms in a substance contain atoms with the same spin alignment, the effect is a permanent magnet.
Magnetic domains are regions of uniform magnetism within a material. Each domain can be thought of as a tiny permanent magnet. If a magnet is heated, or damaged by getting repeatedly dropped (for example), it is possible for these domains to lose their alignment, and the magnet can become demagnetized.
Gadolinium is a metal that acts as a permanent magnet at temperatures less than about 20 degrees Celsius. At room temperature, we can see that the gadolinium will stick to magnetic surfaces, as we would expect from a permanent magnet. After heating the gadolinium with a heat gun, the magnetic domains become randomized, and the gadolinium is no longer magnetic. The gadolinium loses its magnetic attraction and gravity causes it to fall down. The magnetic domains will become realigned after it cools back down again.
Permanent magnets that become demagnetized from being dropped can be re-magnetized in the presence of a strong magnetic field. Frequently at the College of DuPage, our bar magnets will become demagnetized after lots of heavy use. We have a device we can use to re-magnetize the bar magnets by applying a strong magnetic field.
The magnetic field describes the influence that a magnet has on other magnets and charged particles. We’ve described similar fields for gravity and electric charges. One of the cool things about magnetic fields is that we can visualize them by using iron filings or a compass.
Iron filings, because they are magnetic, will align themselves along the magnetic field lines of a magnet. We can see that for a single bar magnet, the magnetic field loops around the magnet. The convention of a magnetic field is that, outside of the magnet, the field points from north to south pole. Inside the magnet, the field points from south to north.
A compass is a magnet suspended in a fluid. The magnet is free to rotate, and will align itself with the strongest magnetic field in the vicinity. Compasses are used to navigate, as the Earth has a magnetic field. We’ll talk about the Earth’s magnetic field at the end of this lecture video. When a compass is held close to a magnet, the magnetic field of the magnet becomes strong enough to overcome the Earth’s magnetic field, and the compass can be used to point to the north pole. We can see how the alignment of the compass changes as we move the compass around a bar magnet.
Similar to what we learned in lecture 22 with different types of charges, magnetic poles experience two types of interactions. Two like poles (north and north, or south and south) will repel. That is, if I hold two like poles together, they will experience a force that causes them to move apart from each other. Two unlike poles (north and south) will attract. Therefore, if I hold two unlike poles apart from each other, they will experience a force that causes them to come together.
We can use iron filings to see how the magnetic field lines look around attracting poles: a north and south pole. We can see that the magnetic field lines flow from one attracting pole to the next. When using iron filings to visualize the field lines around repelling poles, we can see that the field lines flow away from each other.
All magnets have both a north and a south pole. As far as physicists are currently aware, there is no way for a magnet to have only a north pole, or only a south pole. Magnetic poles exist in pairs. This leads to an interesting question: what happens if we break a magnet in half?
This is a bar magnet that contains a north and a south pole. When I break it into two pieces, those two pieces also have their own north and south poles. We can prove that by using a compass to visualize the poles. We can also see that each piece has its own poles by demonstrating the attraction and repulsion that exists between both magnet halves.
Types of magnets
Permanent magnets are made of different types of materials. Many low-cost magnets are made of iron-oxide ceramics. Rare-earth metals are used to create very strong and small permanent magnets. Magnets also come in many different shapes and sizes. Previous demos in this video have used bar magnets. These are long rectangles with the north pole at one end, and the south pole at the other end.
A horseshoe magnet is essentially a bar magnet that has been bent into a U-shape. The north and south poles are still at opposite ends of the bar, but due to the bent shape, they are now located much closer together than they would be in a bar magnet.
A disk magnet is a short cylindrical magnet. The poles of the disk are located at the top and bottom of the disk. Many rare-earth magnets, so-called because they are made of rare-earth elements, are shaped into disk magnets. These rare-earth magnets are extremely strong, and care should be taken when handling them.
A ferrofluid is made up of liquid that can be attracted to a magnet. This is accomplished by suspending tiny magnetic particles into a fluid. We can see the attraction when holding a disk magnet near the ferrofluid.
Permanent magnets have a great advantage of being magnetic all the time – hence the name: permanent magnet! However, sometimes there is a benefit in being able to control the magnetism of a material.
In the early 1800s, scientists discovered that current flowing through a wire created a magnetic field. In this demo, I connected a wire to a battery through a switch. When the switch is open, no current flows through the wire. The compass points in its natural orientation. When I place the compass above the wire and close the switch, the flow of current creates a magnetic field, causing the compass to indicate the direction of the north pole of that field. When I place the compass below the wire and close the switch, we can see that the north pole points in the opposite direction.
The magnetic field generated by a straight wire takes the shape of concentric circles. This means the field points in different directions above and below the wire.
When the wire is coiled, it increases the strength of the magnetic field created by the current. This is because the coil consists of contributions of current from many hundreds of loops of wire. We can see how this field is stronger than the single straight wire by noticing how much quicker the compass needle orients itself to the magnetic field created in this case.
To create a strong electromagnet, an iron core can be placed into the center of the coil of wire. The current creates a magnetic field that magnetizes the iron core. The magnetism of the iron core then further increases the magnetic field of the electromagnet. The compass reacts to the magnetic field even faster after I place an iron core into the center of the coiled wire.
We can see that an electromagnet can both attract and repel a bar magnet. This verifies the existence of both a north and a south pole. We can determine which is which by seeing which end of the electromagnet attracts the north pole of a bar magnet, and which end of the electromagnet repels the north pole.
One of the major advantages of an electromagnet is that it can be turned on and off. I can demonstrate this by using the electromagnet to pick up paperclips, move them, and then turn the electromagnet off by disconnecting the power source. Electromagnets built using this principle are used to lift and move around objects such as cars or scrap metal.
We saw in these demonstrations that electricity can create magnetism. We’ll learn in lecture 25 that magnetism can create electricity!
The deflecting force
An electric charge moving near a magnetic field experiences a deflecting force. This interesting force is even more evidence linking electricity and magnetism, which we will formalize in the next lecture.
The moving electric charge, which is simply the flow of electric current, will experience a force when the current flow is perpendicular to a magnetic field. This force will act perpendicular to both the direction of current flow and the magnetic field. This is why it is called a deflecting force. This force is different from other forces we’ve learned about so far in this class, forces such as gravity and the Coulomb force that cause motion along the same direction as the field lines.
Because this is a perpendicular force, we need to use something called the “right-hand rule” to determine the direction of deflection. First, we use our index finger to point in the direction of the electric current. Our middle finger will point in the direction of the magnetic field: remember it flows from north to south. Point your thumb, and that shows the direction of the deflecting force.
Let’s say the current points in the direction pointing out of the screen and toward you. If the magnetic field points down, then the deflecting force will point to your right. If the current points out of the screen and toward you, and the magnetic field flips to point upward, then the deflecting force will point to your left. You will explore this deflecting force in more detail in the lab on magnetism and electromagnetic induction.
The magnetic deflection force has many applications. A basic application is in motors. This DC motor contains a battery, a coil of wire, and a magnet. The magnet creates a magnetic field, and the battery and wire generate electric current. The deflecting force causes that coil of wire to rotate, creating mechanical work.
The deflecting force is also used in circular particle accelerators. To cause any object to move in a circular path, there must be a centripetal force. That centripetal force can be caused by this magnetic deflecting force acting on charged particles. Close to home, the Advanced Photon Source at Argonne National Lab in Lemont, Illinois, is a particle accelerator. Over in France and Switzerland, the Large Hadron Collider is the world’s largest particle accelerator, at least as of the time this video was filmed. These particle accelerators are used to carry out advanced physics research and answer questions about how our universe works.
Earth’s magnetic field
As previously mentioned, if you wish to navigate without using GPS, a compass is a wonderful tool. The needle in a compass is a magnet suspended in fluid, that will align itself to the strongest magnetic field in the vicinity. When outdoors, a compass will align itself with the magnetic field of the Earth itself, pointing toward the magnetic north pole.
The Earth’s magnetic field is generated by the currents of molten iron and nickel in the core of the planet. Not only does this magnetic field allow us to navigate by use of compass, but it also protects the inhabitants of our planet from solar winds.
Solar wind consists of charged particles, emitted from the sun, that would cause the Earth’s ozone layer to disintegrate. Without an ozone layer, we would not be protected from the sun’s harmful UV rays, and would become more susceptible to skin cancer.
The Earth’s magnetic field deflects the solar wind, and while doing so, generates spectacular aurora in the process.
Thanks for taking the time to learn about magnetism! Until next time, stay well.