AP Magnetism- Short Intro.Pdf

Intro Lesson Magnetism Chapter 19

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Magnets

• Magnets attract iron-containing objects.

• Magnets have two distinct poles called the north pole and the south pole. These names are derived from a magnet’s behavior on Earth.

• Like poles of magnets repel each other; unlike poles attract each other.

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§ Magnets exert forces on each other.

§ The forces depend on the alignment of the poles. Magnets and Magnetic Fields Magnets have two ends – poles – called north and south. Like poles repel; unlike poles attract. Magnets and Magnetic Fields

However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets. Chapter 19Magnetic Fields • A magnetic field is a region in which a magnetic force can be detected. • Magnetic field lines can be drawn with the aid of a compass. Magnets and Magnetic Fields

Magnetic fields can be visualized using magnetic field lines, which are always closed loops. Section 1 Magnets and Magnetic Chapter 19 Fields

The Direction of a Magnetic Field Field lines are conventionally drawn FROM the north pole TO the south pole.

The direction of a magnetic field at any location is the direction that the north pole of a compass at that location would point.

(The arrow head represents the north pole of a magnet and points to the nearest south pole).

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Copyright © by Holt, Rinehart and Winston. All rights reserved. Properties of magnets § Materials that make good permanent magnets are called hard magnets. § Steel, which contains iron and carbon, is a common and inexpensive material used to create hard magnets. § Materials that lose their magnetism quickly are called soft magnets. The Magnetic Field of the Earth

How do we use Earth’s magnetic field to tell direction? The Magnetic Field of the Earth

§ As early as 500 B.C. people discovered that some naturally occurring materials— such as lodestone and magnetite—have magnetic properties.

§ By 1200, explorers from Italy were using a compass to guide ocean voyages beyond the sight of land. The Magnetic Field of the Earth § The Earth’s magnetic poles are defined by the planet’s magnetic field. § That means the south magnetic pole of the planet is near the north geographic pole. § When you use a compass, the north-pointing end of the needle points toward a spot near (but not exactly at) the Earth’s geographic north pole.

Section 1 Magnets and Magnetic Chapter 19 Fields Earth’s Magnetic Field

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Magnetic Fields, continued

• Earth’s magnetic field is similar to that of a bar magnet.

• The magnetic south pole is near the Geographic North Pole. The magnetic north pole is near the Geographic South Pole.

• Magnetic declination is a measure of the difference between true north and north indicated by a compass.

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Copyright © by Holt, Rinehart and Winston. All rights reserved. The magnetic field produced by certain arrangements of bar magnets are represented in the diagrams shown below MAGNETS AND MAGNETIC FIELDS

✔ The magnetic field lines drawn to represent the magnetic field produced by certain arrangements of bar magnets are represented in the diagrams shown below

Notice the similarity between the lines of this slide and the previous slide particle distribution Why & How Does Magnetism Occur at All? The Why & How of Magnets

§ The sources of nearly all magnetic effects in matter are the electrons in atoms.

§ There are two ways in which electrons create magnetism: 1. Electrons’ motion around the nucleus makes the entire atom a small magnet. 2. Electrons themselves act as though they were magnets. The Why & How of Magnets

§ All atoms have electrons, so you might think that all materials should be magnetic, but there is great variability in the magnetic properties of materials.

§ The electrons in some atoms align to cancel out one another’s magnetic influence.

§ While all materials show some kind of magnetic effect, the magnetism in most materials is too weak to detect without highly sensitive instruments. Iron, Cobalt, Nickel

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Magnesium Note: Magnetism is a property not just of the chemical make-up of a material, but of its crystalline structure and microscopic organization. The Why & How of Magnets § In diamagnetic materials, the electrons are oriented so their individual magnetic fields cancel each other out within an atom. § Individual atoms in paramagnetic materials are magnetic but the atoms themselves are randomly arranged so the overall But when paramagnetic materials are placed in a magnetism of a sample is magnetic field, the atoms align zero. so that the material is weakly magnetic. The Why & How of Magnets

§ A small group of metals have very strong magnetic properties, including iron, nickel, and cobalt. § These metals are the best known examples of ferromagnetic materials. § Atoms with similar magnetic orientations line up with neighboring atoms in groups called magnetic domains. Induced Magnetism

Metals that are ferromagnetic have atoms that behave like mini magnets, but on a grand scale they cancel out. When placed in external magnetic field, these can be lined up and the metal then becomes magnetized. Magnetizing Iron

Magnetic domains in iron nails are induced to align by proximity of the strong magnet Each nail becomes itself a magnet, which in turn magnetizes the nail below it, forming a chain. When the strong magnet is removed, most of the domains un-align and the nails lose most of their magnetization. Demagnetizing Iron

Magnetic domains can be scrambled by heating the iron, striking it with great force, or other disruptions of alignment.

Magnetized Demagnetized

Test tube of S un-magnetized iron filings SHAKE S N N Magnetizer Demagnetizing Iron

Iron nail is attracted to the large magnet due to alignment of domains in the nail.

Heat the nail to a high temperature and the domains become randomized so the nail is no longer attracted to the magnet. The Why & How of Magnets

§ Magnetic domains in a ferromagnetic material will always orient themselves to attract a permanent magnet. — If a north pole approaches, domains grow that have south poles facing out. — If a south pole approaches, domains grow that have north poles facing out.

Section 1 Magnets and Magnetic Chapter 19 Fields

Magnetic Domains • Magnetic Domain A region composed of a group of atoms whose magnetic fields are aligned in the same direction is called a magnetic domain.

• Some materials can be made into permanent magnets. – Soft magnetic materials (for example iron) are easily magnetized but tend to lose their magnetism easily. – Hard magnetic materials (for example nickel) tend to retain their magnetism.

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Examples of ferromagnetic materials: iron, cobalt, nickel

Examples of paramagnetic materials: aluminum, manganese, chromium, oxygen, sodium, lithium

Examples of diamagnetic materials: water, pyrolitic graphite, copper, gold, bismuth Diamagnetism • Diamagnetism is the property of an object or material which causes it to create a magnetic field in opposition to an externally applied magnetic field. Diamagnetism • Diamagnetism is the property of an object or material which causes it to create a magnetic field in opposition to an externally applied magnetic field. • Diamagnetism is believed to be due to quantum mechanics (and is understood in terms of Landau levels) and occurs because the external field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz’s law, the field of these electrons will oppose the magnetic field changes provided by the applied field.