Magnetism Unit 12

Magnets Magnet – a material in which the spinning electrons of its atom are aligned with one another 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 Bar Magnet – Straight piece of metal Horse Shoe Magnet – Curved piece of metal Horse Shoe Magnet – Curved piece of metal

Poles Poles of a magnet are the ends where objects are most strongly attracted Poles of a magnet are the ends where objects are most strongly attracted Two poles, called north and south Two poles, called north and south Like poles repel each other and unlike poles attract each other Like poles repel each other and unlike poles attract each other Similar to electric charges Similar to electric charges Magnetic poles cannot be isolated Magnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole This differs from electric charges This differs from electric charges

Magnetism Magnetism – is the force of attraction or repulsion between magnetic poles 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 An unmagnetized piece of iron can be magnetized by stroking it with a magnet Somewhat like stroking an object to charge an object Somewhat like stroking an object to charge an object Magnetism can be induced Magnetism can be induced If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized 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 The region where magnetic force exists around a magnet or any moving charged object

Poles North Pole North Pole End of magnet that points to geographic north End of magnet that points to geographic north South Pole South Pole End of magnet that points to geographic south 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 Magnetic Force is the force produced by the motion of charges relative to each other Opposites attract (N-S) Opposites attract (N-S) Likes repel (N-N or S-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. 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. 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 north pole corresponds to a magnetic south pole The Earth’s geographic south pole corresponds to a magnetic north 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 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 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 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 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 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 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 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 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 Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look 1.Form closed loops 2.DO NOT CROSS 3.Exit the (N)orth pole of a magnet and enter the (S)outh pole 4.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 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 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 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 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 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: The most common causes include: moving electrical charges moving electrical charges this is how electromagnets work this is how electromagnets work magnetic dipoles magnetic dipoles how most permanent magnets work how most permanent magnets work changing electrical fields changing electrical fields

Domains Unmagnetized iron Unmagnetized iron Arrows go in all different directions Arrows go in all different directions Not Magnetic Not Magnetic Magnetized iron Magnetized iron Arrows align to go in the same direction Arrows align to go in the same direction North Pole on right end 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 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 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 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 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 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 Using your left – start from the negative end Think of your first finger as the wire Think of your first finger as the wire Follow the wire around – like the wire Follow the wire around – like the wire Which ever direction your thumb is pointing is north 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 When moving through a magnetic field, a charged particle experiences a magnetic force This is a result of two conflicting magnetic fields This is a result of two conflicting magnetic fields The one that is set up by the moving charges and the existing field 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 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 Used to determine the Force Hold your left hand open Hold your left hand open Place your fingers in the direction of magnetic field Place your fingers in the direction of magnetic field Place your thumb in the direction of current or direction of moving charge 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 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 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 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