Magnetism IGCSE Physics

Unit 7: Electricity and Magnetism Chapter 22 Electric Charges and Forces 22.1 Properties of Magnets 22.2 Magnetic Properties of Materials 22.3 The Magnetic Field of the Earth

Chapter 22 Objectives Describe the forces between two permanent magnets. Sketch the magnetic field of a single permanent magnet. Predict the direction of the force on a magnet placed in a given magnetic field. Explain why ferromagnetic materials always attract magnets of either pole. Describe the theory behind why a compass works. Use a compass to find the direction of true north.

Chapter 22 Vocabulary Terms magnet north pole south pole magnetization demagnetization magnetic field compass magnetic field lines diamagnetic paramagnetic ferromagnetic gauss soft magnet magnetic declination magnetic domain hard magnet permanent magnet

History of Magnets (~800 BC) Ancient Chinese and Greeks discovered that certain stones would attract and magnetize iron. Small slivers of the stone were found to align themselves with the North Pole. Chinese were the first to use magnets for navigation. The orienting properties were used to align streets in cities in the North-South / East-West direction.

22.1 Properties of Magnets Key Question: How do magnets interact with each other? *Students read Section 22.1 AFTER Investigation 22.1

22.1 What is a magnet? If a material is magnetic, it has the ability to exert forces on magnets or other magnetic materials. A permanent magnet is a material that keeps its magnetic properties even when it is NOT close to other magnets.

Poles of a Magnet Magnets have a North and South Pole. Like poles repel. Unlike poles attract. What happens if you break a magnet in half? Will you get two monopoles? No. N S N S +

Oddly shaped magnets still have a north and a south Magnets either attract or repel each other South poles are attracted to north poles

22.1 Properties of Magnets Magnets have two opposite poles. north south Magnets exert forces on each other. The forces depend on the alignment of the poles.

22.1 Properties of Magnets Plastics, wood, and most insulating materials are virtually transparent to magnetic forces. Conducting metals, like aluminum, also allow magnetic forces to pass through, but may change the forces.

Like poles repel Unlike poles attract

22.1 The force between two magnets The strength of the force between magnets depends on the distance between them. The magnetic force decreases with distance much faster than does either gravity or the electric force.

22.1 The force between two magnets Two magnets near each other often feel a twisting force, or torque. This is a result of having two poles. The combination of attractive and repulsive forces on the same magnet creates a torque.

22.1 The magnetic field All magnets create a magnetic field in the space around them, and the magnetic field creates forces on other magnets. The number of field lines in a certain area indicates the relative strength of the magnetic field in that area. The closer the lines are together, the stronger the field. The arrows on the field lines indicate the direction of the force

22.1 The magnetic field The number of field lines in a certain area indicates the relative strength of the magnetic field in that area. The arrows on the field lines indicate the direction of the force. The closer the lines are together, the stronger the field. Magnetic field lines always point away from a magnet’s north pole and toward its south pole.

Magnetic Field Lines vs. Electric Field Lines Magnetic Dipole Electric Dipole

22.2 Magnetic Properties of Materials Key Question: How do magnets interact with different materials? *Students read Section 22.2 AFTER Investigation 22.2

22.2 Magnetic Properties of Materials The sources of nearly all magnetic effects in matter are the electrons in atoms. There are two ways in which electrons create magnetism: Electrons around the nucleus and their motion makes the entire atom a small magnet. Electrons themselves act as though they were magnets.

22.2 Magnetic Properties of Materials 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.

The magnetic North Pole is responsible for more than just the direction a compass points. It's also the source of the aurora borealis, the dramatic lights that appear when solar radiation bounces off the Earth's magnetic field.  This happens at the South Pole as well. In the southern hemisphere, the lights are called the aurora australas.

Source of Magnetic Fields Electrical Charge in motion. Currents occur at the atomic level in atoms due to the orbits of electrons around the nucleus. The intrinsic spin (+1/2, -1/2) is critical in the case of magnetism.

Magnetic Domains A: Iron absent of a magnetic field. B: Iron in the presence of a magnetic field. C: A non-magnetic material.

Magnetic Domains = groups of atoms with aligned poles Magnets can be temporary (like the needle used in the compass). This nail has its atoms aligned, but the effect is only temporary.  You can get this affect by rubbing the nail on a magnet. Neat fact:  Hitting the nail can demagnetize it, you are basically scrambling the atoms.

22.2 Magnetic Properties of Materials In diamagnetic materials, the electrons are oriented so their individual magnetic fields cancel each other out. Individual atoms in paramagnetic materials are magnetic but the atoms themselves are randomly arranged so the overall magnetism of a sample is zero. When paramagnetic materials are placed in a magnetic field, the atoms align so that the material is weakly magnetic.

22.2 Magnetic Properties of Materials 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.

22.2 Magnetic Properties of Materials 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.

Magnetism of Soft Ferromagnetic Materials How does a magnet attract screws, bolts nails, paperclips, etc. when they are not magnetic to start with? Soft ferromagnetic material align their domains in the presence of an external magnetic field creating a magnetic dipole. When the magnetic field is removed, the domains re-randomize resulting in no magnetic attraction. They are temporary Soft ferromagnetic material is attracted to both the North pole and South pole. N S S

Types of Magnets Temporary: When charged particles move through space, they induce a magnetic field (Electromagnets). Permanent: Electrons have an intrinsic magnetic field that may add together in certain matter to create a magnetic field (Speakers). Temporary Permanent

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field.  An everyday example is a refrigerator magnet used to hold notes on a refrigerator door.

Metals that are magnetic: nickel, iron, cobalt   Things that are not magnetic:  aluminum, plastic, glass Ferromagnetic - a substance such as iron in which the magnetic moments of the atoms spontaneously line up with each other, making a large net magnetic moment.  Ferromagnets lose their ferromagnetism when heated above a specific temperature , because the thermal energy melts the magnetic alignment. What else can cause a ferromagnet to lose its magnetism?

22.2 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.

22.3 The Magnetic Field of the Earth Key Question: How do we use Earth’s magnetic field to tell direction? *Students read Section 22.3 AFTER Investigation 22.3

22.3 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.

22.3 The Magnetic Field of the Earth 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. 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.

22.3 The Magnetic Field of the Earth The gauss is a unit used to measure the strength of a magnetic field. The magnetic field of the Earth is very weak (0.5 gauss) compared with the strength of the field on the surface of the classroom ceramic magnets (1000 gauss). Historical data shows that both the strength of the Earth’s magnetic field and the location of the north and south magnetic poles can switch places. Today, the Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years.

22.3 The Magnetic Field of the Earth Depending on where you are, a compass will point slightly east or west of true north. The difference between the direction a compass points and the direction of true north is called magnetic declination. After correcting for the declination, you rotate the whole compass until the north-pointing end of the needle lines up with zero degrees on the ring. The large arrow points in the direction you want to go.

Application: Magnetic Resonance Imaging

Focus on SPEAKERS The loudspeakers in your radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone. By varying the electric current through the wires around the electromagnet, the speaker cone moves back and forth. The resulting vibration of the speaker cone will create sound waves, including that from voice and music.

Maglev Trains Magnetic + Levitation = maglev Maglev vehicles “float” over an electromagnetically powered fixed steel guideway and are propelled by the current with no motors, wheels, moving parts or additional energy sources. The system is environmentally friendly, energy efficient and runs in all weather conditions. http://dsc.discovery.com/videos/extreme-engineering-season-1-shorts-maglev-train.html

Make your own compass http://adventure.howstuffworks.com/outdoor-activities/hiking/compass2.htm

Applications Computer disc drives (hard and floppy) Video Cassette Recorder and cassette tape Credit cards Speakers Motors (Both AC and DC) Speed sensors Solenoids for relays, valves, etc. Magnetos (piston engine aircraft)