21.1 Magnets and Magnetic Fields

21.1 Magnets and Magnetic Fields

We are going to learn… A brief history of magnets How magnetic poles interact How magnetic fields affect a magnet that enters it Why some materials are magnetic 4

Magnets in the Past If you have ever played with a magnet, you will find that either side of the magnet will stick to a refrigerator. Yet, these fun little magnets have a lot of history behind them. 2

Magnets in the Past The most popular legend accounting for the discovery of magnets is that of an elderly Cretan shepherd named Magnes. Legend has it that Magnes was herding his sheep in an area of Northern Greece called Magnesia, about 4,000 years ago. Suddenly both, the nails in his shoes and the metal tip of his staff became firmly stuck to the large, black rock on which he was standing. 3

Magnets in the Past To find the source of attraction he dug up the Earth to find special rocks. This type of rock was subsequently named “magnetite”, after either Magnesia or Magnes himself. Magnetite is now known as a natural magnetic material made of iron oxide. 3

Magnets in the Past After the discovery of magnetite, it was put to use in many different ways. Archimedes is purported to have used magnetite to remove nails from enemy ships thus sinking them. People also realized that magnetite not only attracted objects made of iron, but when made into the shape of a needle and floated on water, magnetite always pointed in a north-south direction. 3

Magnets in the Past This lead to the creation of compasses. This led to an alternative name for magnetite, that of “lodestone” which means "leading stone". For many centuries, magnetite was surrounded in superstition and was considered to possess magical powers, such as the ability to heal the sick, frighten away evil spirits and attract and dissolve ships made of iron! 3

Magnets in the Past However, significant progress was made only with the experiments of William Gilbert in 1600 in the understanding of magnetism. It was Gilbert who first realized that the Earth was a giant magnet and that magnets could be made by beating wrought iron. He also discovered that heating magnetite resulted in the loss of induced magnetism. 3

Magnetic Forces We have just looked at the history behind magnets. Now we are going to look at how magnets behave. If you push two magnets together, they may attract or repel. They will exert a force on each other. 2

Magnetic Forces Magnetic force is the force a magnet exerts on another magnet, on iron or a similar metal, or on moving charges. This is also just one aspect of electromagnetic force, and it acts similarly to electric forces. 2

Magnetic Forces Just like electric forces, magnetic forces act over a distance. The closer the magnets get, the more force they experience. However, electric forces and magnetic forces are different when it comes to their field lines. 3

Magnetic Forces In the 1600s, Gilbert used a compass to map forces around a magnetite sphere. He also discovered that all magnets have two magnetic poles, which are regions where the magnet’s force is the strongest. 2

Magnetic Forces One end of a magnet is its “north pole,” the other end is its “south pole.” This is different from electric forces, which do not have poles and are not stronger on one side of the charge compared to the other. 2

Magnetic Forces Another way magnetic force is similar to electric force is how magnets attract or repel each other. Like magnetic poles repel one another, and opposite magnetic poles attract. 2

Magnetic Forces More things about magnetic poles: If you cut a magnet in half, each half will have its own north and south pole. You can keep on cutting a magnet in half and always get a north and south pole. You will never just have one pole. 4

Magnetic Fields We have just looked at magnetic forces. Now we are going to look at magnetic fields. Just like an electric field surrounds an electric charge, a magnetic field surrounds a magnet and can exert magnetic forces. 2

Magnetic Fields The field lines begin near the magnet’s north pole and extend toward its south pole around the magnet. If a magnet that enters the other magnet’s magnetic field, the magnetic fields from each magnet will either attract or repel one another. 2

Magnetic Fields Around Magnets We can visualize magnetic forces by using iron filings and magnets. There are no iron filings in the gap between like poles. This is due to the fact that the iron filings follow the field lines. 3

Magnetic Fields Around Magnets As you put like poles together, the field lines push each other away. There are very few field lines between like poles. 2

Magnetic Fields Around Magnets There are many iron filings in the gap between unlike poles. This is because there are many field lines between unlike poles. Unlike poles attract. 3

Magnetic Fields Around Earth Even though William Gilbert knew the Earth was a giant magnet in the 1600s, he had little idea why. What we know today is that Earth’s magnetic field is produced by the motion of the hot, liquid iron in its core. The motion of the liquid iron causes electric currents, which give rise to magnetic fields. 3

Magnetic Fields Around Earth Some other things we know about Earth is that the North Pole and Magnetic North Pole are not in the same place! As a matter of fact, the Earth’s magnetic field undergoes sudden reversals every few hundred thousand years. 2

Magnetic Fields Around Earth Also, a compass really points to the magnetic north pole, not the geometric north pole. So compasses will eventually point in the other direction! 2

Magnetic Materials So far, we have learned about magnetic forces and fields and how the Earth is a giant magnet. Now we will look at why certain materials are magnetic and others (like wood and sand) are not. Why are certain materials magnetic? It has to do with the electrons of the materials. 4

Magnetic Materials A weird thing about electrons is that they spin. This electron spin creates magnetic fields. However, in many materials, each electron is paired with another electron. These pairs of electrons spin in a way that cancels each other’s magnetic fields. 4

Magnetic Materials However, if an electron does not have another electron to cancel it out, the material can form “magnetic domains”. Magnetic domains are regions that have a very large number of atoms with aligned magnetic fields. If many of these magnetic domains are aligned in the same direction, the material will be magnetic. 3

Magnetic Materials Some materials are ferromagnetic, such as iron. Ferromagnetic materials contain magnetic domains, which means that it can be magnetized under certain conditions. An iron nail is ferromagnetic, and by itself it does not act like a magnet; it is a “nonmagnetized” material. 3

Magnetic Materials The reason why the iron nail is nonmagnetized is because the magnetic domains in a nail are not aligned; the magnetisms of the domains are cancelled. However, you can easily magnetize a nonmagnetized ferromagnetic material by placing it in a magnetic field. 2

Magnetic Materials If you put a nail in a magnetic field, the nail will turn into a magnet and attract other nails. The magnet that you add causes the magnetic domains to align together. As soon as you take away the magnet from the nail, the magnetic domains go back to aligning randomly, thus no more magnetic effect from the nail. 3

Magnetic Materials However, some materials can keep the magnetic domains aligned together even when the magnet is taken away. These are called permanent magnets. 2-end

21.1 Magnets and Magnetic Fields