* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Magnets
Electron paramagnetic resonance wikipedia , lookup
History of electrochemistry wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Magnetorotational instability wikipedia , lookup
Maxwell's equations wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Electricity wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Electromagnetism wikipedia , lookup
Electromotive force wikipedia , lookup
Electric machine wikipedia , lookup
Magnetic nanoparticles wikipedia , lookup
Magnetometer wikipedia , lookup
Magnetic field wikipedia , lookup
Hall effect wikipedia , lookup
Magnetic monopole wikipedia , lookup
Superconductivity wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Galvanometer wikipedia , lookup
Scanning SQUID microscope wikipedia , lookup
Magnetic core wikipedia , lookup
Lorentz force wikipedia , lookup
Multiferroics wikipedia , lookup
Superconducting magnet wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
Magnetoreception wikipedia , lookup
Magnetochemistry wikipedia , lookup
Eddy current wikipedia , lookup
Electromagnet wikipedia , lookup
Force between magnets wikipedia , lookup
Unit 12 Magnetism Magnets Magnet – a material in which the spinning electrons of its atom are aligned with one another Two main types • Bar Magnet – Straight piece of metal • Horse Shoe Magnet – Curved piece of metal Poles Poles of a magnet are the ends where objects are most strongly attracted Like poles repel each other and unlike poles attract each other Two poles, called north and south Similar to electric charges Magnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole This differs from electric charges Magnetism Magnetism – is the force of attraction or repulsion between magnetic poles An unmagnetized piece of iron can be magnetized by stroking it with a magnet Somewhat like stroking an object to charge an object Magnetism can be induced If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized Magnetic Fields (B) The region where magnetic force exists around a magnet or any moving charged object Poles North Pole End of magnet that points to geographic north South Pole End of magnet that points to geographic south * So what does that really mean about the Earth? * Forces between Poles Magnetic Force is the force produced by the motion of charges relative to each other Opposites attract (N-S) Likes repel (N-N or S-S) Compass A compass is a navigational instrument for determining direction relative to the Earth's magnetic poles. • It consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth's magnetic field. Earth’s Magnetic Field The Earth’s geographic north pole corresponds to a magnetic south pole The Earth’s geographic south pole corresponds to a magnetic north pole Strictly speaking, a north pole should be a “north-seeking” pole and a south pole a “south-seeking” pole Earth’s Magnetic Field The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior Source of the Earth’s Magnetic Field There cannot be large masses of permanently magnetized materials since the high temperatures of the core prevent materials from retaining permanent magnetization The most likely source of the Earth’s magnetic field is believed to be electric currents in the liquid part of the core Magnetic Fields result from moving charges Magnetic Field Strength The number of magnetic flux lines per unit area passing through a plane perpendicular to the distance of the lines The closer to the ends of a magnet the stronger the magnetic field strength is Magnetic Flux (Field) Lines Direction is given by the direction a north pole of a compass needle points in that location Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look 1. 2. 3. 4. Form closed loops DO NOT CROSS Exit the (N)orth pole of a magnet and enter the (S)outh pole Closer the lines the stronger the force Magnetic Field Lines, sketch A compass can be used to show the direction of the magnetic field lines (a) A sketch of the magnetic field lines (b) Magnetic Field Lines, Bar Magnet Iron filings are used to show the pattern of the electric field lines The direction of the field is the direction a north pole would point Magnetic Field Lines, Unlike Poles Iron filings are used to show the pattern of the electric field lines Magnetic Field Lines, Like Poles Iron filings are used to show the pattern of the electric field lines Magnetic Field Lines, Horse Shoe Magnet Iron filings are used to show the pattern of the electric field lines Magnetic Field Lines Where do magnetic fields come from? The most common causes include: moving electrical charges magnetic dipoles this is how electromagnets work how most permanent magnets work changing electrical fields Domains Unmagnetized iron Arrows go in all different directions Not Magnetic Magnetized iron Arrows align to go in the same direction North Pole on right end Magnetism by Induction By bringing a magnet near a Ferromagnetic Material the domains realign themselves Current Carrying Wire Magnetic Field is counter-clockwise Magnetic Field is clockwise Electromagnets If a long straight wire is bent into a coil of several closely spaced loops, the resulting device is called a solenoid It is also known as an electromagnet since it acts like a magnet only when it carries a current The field lines of the solenoid resemble those of a bar magnet Why does iron core increase the strength? Due to the fact that as the current runs through the wires it causes the domains to realign and become a magnet itself Left Hand Rule#2 Using your left – start from the negative end Think of your first finger as the wire Follow the wire around – like the wire Which ever direction your thumb is pointing is north Which end is North? Magnetic Fields - Force When moving through a magnetic field, a charged particle experiences a magnetic force This is a result of two conflicting magnetic fields The one that is set up by the moving charges and the existing field The force is zero if the motion of the charge is parallel to the magnetic field lines Left Hand Rule #3 Used to determine the Force Hold your left hand open Place your fingers in the direction of magnetic field Place your thumb in the direction of current or direction of moving charge The direction of the force on a positive charge is directed out of your palm If the charge is positive, the force is opposite that determined by the left hand rule Movement of Charge Particle Magnetic Field Force Faraday’s Experiment A current can be produced by a changing magnetic field First shown in an experiment by Michael Faraday A primary coil is connected to a battery A secondary coil is connected to an ammeter – Demo with coils, switch and galvanometer Faraday’s Conclusions Magnetic fields don’t produce current but changing magnetic fields do It is customary to say that an induced current is produced in the secondary circuit by the changing magnetic field Magnetic Flux The induced current is actually induced by a change in the quantity called the magnetic flux rather than simply by a change in the magnetic field Magnetic flux is proportional to both the strength of the magnetic field passing through the plane of a loop of wire and the area of the loop Ways to Change Magnetic Flux 1. 2. 3. Field - depending on strength Area – the cross sectional area of the wire Orientation – depending on the position of the wire (angle) Faraday’s Law The induced electromotion force (EMF) in a circuit equals the time rate of change of magnetic flux through the circuit or The EMF generated is proportional to the rate of change of the magnetic flux. Electromagnetic Induction – An Experiment When a magnet moves toward a loop of wire, the ammeter shows the presence of a current (a) When the magnet is held stationary, there is no current (b) When the magnet moves away from the loop, the ammeter shows a current in the opposite direction (c) If the loop is moved instead of the magnet, a current is also detected Lenz’s Law An induced current is always in such a direction as to oppose the motion or change causing it By opposing the motion it creates the same pole on the side the magnet enters Motional Emf Assume the moving bar has zero resistance As the bar is pulled to the right with velocity v under the influence of an applied force, F, the free charges experience a magnetic force along the length of the bar This force sets up an induced current because the charges are free to move in the closed path ADAM>COACH Application of Faraday’s Law – Motional emf Electrons will be pointed in the downward direction. Making the bottom of the bar negative Left Hand Rule # 4 Only used for finding the direction of induced current All should be perpendicular to each other Direction of Conductor Motion (v) Magnetic Field Electron Motion – Current Flow Generators Converts mechanical energy to electrical energy Uses a wire loop to rotate Alternating Current (AC) generator – Uses a solid ring Direct Current (DC) generator – Uses a ring with slits AC Generators Basic operation of the generator As the loop rotates, the magnetic flux through it changes with time This induces an emf and a current in the external circuit The ends of the loop are connected to slip rings that rotate with the loop Connections to the external circuit are made by stationary brushed in contact with the slip rings DC Generators Components are essentially the same as that of an ac generator The major difference is the contacts to the rotating loop are made by a split ring, or commutator Motors Motors are devices that convert electrical energy into mechanical energy A motor is a generator run in reverse A motor can perform useful mechanical work when a shaft connected to its rotating coil is attached to some external device