History of Magnets
• More than 2000 years ago, rocks called lodestones were found in the region of Magnesia in Greece. • In the 12th century, the Chinese used them for navigating ships.What are magnets?
• Most materials – Have paired up electrons moving in opposite directions. – The field created by one moving charge is canceled by the other. – No magnetic field is created.What are magnets?
• Any charges in motion produce a magnetic field. • Some materials like Iron, Nickel, or Cobalt – Have a single electron or paired electrons spinning in the same directions. – The magnetic field created by one electron is not canceled by the other. – An atomic sized magnet is created.Why is Fe magnetic and Al not?
• What makes a good magnet? – Every spinning electron is a tiny magnet. – A pair of electrons spinning in the same direction is a stronger magnet. – A pair of electrons spinning in opposite directions work against one another; the magnetic fields cancel. • Fe has two unpaired electrons spinning in the same direction. – Cobalt has 3. – Nickel has 4. – Aluminum has one unpaired electron.Magnetic Poles
• All magnets have two regions “poles” that produce magnetic forces. • They are named like this because if you take a magnet and suspend it from the middle (so that it can swing freely), it will rotate until the north pole of the magnet points north and the south pole points south. • Like poles repel. • Opposite poles attract.No less than two.
• North and South cannot be separated.
• If a magnet is broken, poles aren’t separated;
Magnetic Domains
• In magnetic materials, neighboring atoms pair up to form large groups of atoms whose net spins are aligned. • These groups are called domains. • When a piece of iron is not a magnet, the domains point in random directions.Magnetic Domains
• If the non‐magnetized iron is placed in a strong magnetic field, the domains will line up in the direction of the field. • In temporary magnets, the domains will return to their random orientation after the field is removed. • In permanent magnets, the domains will remain aligned. • 1 domain = 1 quadrillion (1015) atomsMagnetic Fields
• You have probably noticed that forces between the magnets (both attraction and repulsion), are felt not only when the magnets are touching each other, but also when they are held apart. • In the same way that gravity can be described by a gravitational field, magnetic forces can be described by the magnetic fields around magnets.Magnetic Field Demo
• What kinds of magnetic fields are produced by
Magnetic
Field Lines
The shape of the magnetic field. If you place a compass in the field its arrow will point parallel to the field lines.Magnetic Field Lines
• Magnetic field lines are the same as electric field lines in that both are stronger when lines are drawn closer together. – So the magnetic field is stronger at the poles • The magnetic field lines have arrows going from north to south. • Magnetic field lines do not cross because the magnetic field cannot go in two directions at once.Temporary Magnets
• What happens when you place a magnet next to a nail? • This is because the magnet causes the nail to become polarized; the nail becomes a magnet. • Aluminum and lead are not a magnetic. • This is temporary; if you pull the magnet away, the nail loses its magnetism.Permanent Magnets
• Permanent magnets are produced in the same manner as the nail; however, due to the microscopic structure of the material, the magnetism becomes more permanent. • Most permanent magnets are made of ALNICO, an iron alloy containing 8% Aluminum, 14% Nickel, and 3% Cobalt. altho. Al is not a magnet • Some rare earth elements, such as neodymium and gadolinium, produce strong permanent magnets.Common Uses of Magnets
• Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard disks. • Credit, debit, and ATM cards • Common television and computer monitors • Speakers and Microphones • Electric motors and generators • Compasses • Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose. • Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, steel, and nickel) from non‐magnetic metals (aluminum, non‐ferrous alloys, etc.). • Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains). The maximum recorded speed of a maglev train is 361 mph.How to demagnetize a magnet
• Heating a magnet past its Curie temperature ‐ the molecular motion destroys the alignment of the magnetic domains. – 768°C for Iron • Hammering or jarring – the mechanical disturbance tends to randomize the magnetic domains. • Placing the magnet in an alternating magnetic field.Ferrofluid
• a mixture of tiny iron particles covered with a liquid coating that are then added to water or oil. • Used in car suspensions, cancer detection, loud speakers • videoEarth’s Magnetic Field
• Earth is a huge magnet. • This is possibly due to the molten Iron core. • The magnetic field around Earth is called the MagnetosphereEarth’s Magnetic Field
• Magnetic north pole is different than geographical north pole. • There is about a 25 ̊difference from geographic north pole to magnetic north pole, this is called magnetic declination • In addition, the north pole of a magnet is attracted to earth’s north pole because that is the magnetic south pole. • The south pole of a magnet is attracted to the earth’s south pole because that is the magnetic north pole.Magnetosphere
• Extends several tens of thousands of km into space.
Auroras
• Charged particles from the sun become trapped in Earth’s magnetic field. • This occurs near the magnetic poles. • These charged particles collide with electrons of the atoms in our atmosphere and transfer their energy. • The colors of the lights are determined by the type of gases in the atmosphere. – O2 releases green light; N2 releases red light • aurora borealis (northern lights); aurora australis (southern lights)Magnetic field of Earth is not stable
• The magnetic poles of Earth wander up to 15 km every year. • Based upon the study of lava flows throughout the world, Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. • The last reversal is theorized to have occurred 780,000 years ago.Animal Migration
• Some animal species do have the ability to detect the magnetic field, & they use it to make their migrations. • Bats and sea turtles use magnetic information to find their way. • We're not 100 percent sure how animals detect the magnetic field, but small particles of magnetite have been found in the brains of some species. Those particles may be reacting to the magnetic field and activating nerves in such a way as to send orientation information to the animal's brain.Moving Charges
• A moving charge produces a magnetic
field.
• Many charges in motion – an electric
current – also produce a magnetic field.
• This was discovered in 1820 by Hans
Oersted. He ran an electric current
through a wire and positioned compasses around the wire.
The Right Hand Rule
• Which way is the magnetic field pointing? 1. Hold your hand as if grasping the wire. 2. Point your thumb in the direction of the current. 3. The magnetic field will is in the direction that your remaining four fingers point. • The right hand rule can also be applied to a loop of wire.Solenoids
• A solenoid combines loops of wire to increase magnetic field strength. The greater the number of loops, the greater the field strength. • The field of a solenoid can be increased further by inserting a ferromagnetic rod in the center. This is called an electromagnet.Another Right Hand Rule
• Which way does the force push the charge? 1. Put your index finger in the direction of the charge’s velocity (current). Index = I = current 2. Point your middle finger in the direction of the magnetic field. Middle = M = magnetic 3. Your thumb will point in the direction of the magnetic force.Magnetic Force on Wires
• The force experienced by a current‐carrying wire is the sum of all of the individual forces of the electrons moving within it. F = BIl • The parallel wires near each other will experience magnetic force: towards each other if the current is in the same direction and away if it is in opposite directions. • A speaker uses the force on a coil of wire to vibrate a cone that transmits sound.Electromagnetism
• Oersted discovered that a magnetic field can be formed from a current, now we will learn how a current can be formed from a magnetic field • When Michael Faraday made his discovery of electromagnetic induction in 1831, he discovered that a changing magnetic field is necessary to induce a current in a nearby circuit.Creating Current
• Faraday discovered that if a wire is moved in a magnetic field, a voltage is produced, and if there is a complete loop, a current will flow. This is how electricity is generated. • This is electromagnetic induction3 Ways of Inducing a Current
1. A circuit is moved into or out of a magnetic field. 2. A circuit is rotated in a magnetic field. 3. The intensity or direction of a magnetic field is varied.Why the loops?
• By bending the wire into a loop, the magnetic field lines are bunched up inside the loop. • If you add more loops, the magnetic field will become stronger. – Two loops = magnetic field is doubled. – Three loops = magnetic field is tripled. • Electromagnet – a current carrying wire with many loops.Add a core to the electromagnet
• If a piece of iron is placed inside the coil of the electromagnet, the domains of the piece of iron are forced into alignment, increasing the intensity of the magnetic field. • Electromagnets are strong temporary magnets – they can be turned on and off easily.Generating electricity
In industry, electricity is generated by spinning a coil of wire in a magnetic field. To increase the voltage or current generated: • Spin the coil faster. • Put more loops on the coil. • Use a stronger magnetic field. • Use a coil with a larger area.Applications of Induction
• Generators – A generator is nothing more than the reverse of an electric motor – A motor, you put current (battery) into it , electrical energy, and you get mechanical work out of it (spinning our wire) – A generator, you put mechanical work into it (water running, or falling, steam, etc.) and you get current out • Microphone – A simple microphone is the reverse (symmetry) of a speaker. – A speaker, you put alternating current into it and you get sound out. – A microphone, you put sound into and you get alternating current out.Speakers
• Alexander Graham Bell patented the first electrical loudspeaker as part of his telephone in 1876. • The loudspeakers in most sound systems use magnets to produce sound waves. • One design is a flexible paper cone attached to a coil of wire and a permanent magnet • The current through the wire causes a magnetic force on the coil.Faraday’s Law of Induction
• Michael Faraday calculated the magnitude of electromotive force (emf) caused by induced current. emf = ‐NΔφm/Δt • emf is measured in volts. N is the number of loops in a wire. What factors go into flux?Transformerssssssss!
• Transformers can either step voltage up OR down! · This is the main reason why we use AC circuits! • Notice that the transformer is made with two different wire coils. • The magnetic field of one coil is transferred to the other coil via an iron core. • The difference in coil configuration creates a difference in voltage!• The principal reason voltage is induced in the loops of a generator coil is that loops are
rotating, and changing the amount of
