MagnetismMagnetism

Unit 12Unit 12

MagnetsMagnets Magnet – a material in which the spinning Magnet – a material in which the spinning

electrons of its atom are aligned with one electrons of its atom are aligned with one anotheranother

Two main types Two main types • Bar Magnet – Straight Bar Magnet – Straight piece of metalpiece of metal

• Horse Shoe Magnet – Horse Shoe Magnet – Curved piece of metalCurved piece of metal

PolesPoles PolesPoles of a magnet are the ends where of a magnet are the ends where

objects are most strongly attractedobjects are most strongly attracted Two poles, called Two poles, called northnorth and and southsouth

Like poles repel each other and unlike Like poles repel each other and unlike poles attract each otherpoles attract each other Similar to electric chargesSimilar to electric charges

Magnetic poles cannot be isolatedMagnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, If a permanent magnetic is cut in half repeatedly,

you will still have a north and a south poleyou will still have a north and a south pole This differs from electric chargesThis differs from electric charges

MagnetismMagnetism Magnetism – is the force of attraction Magnetism – is the force of attraction

or repulsion between magnetic polesor repulsion between magnetic poles An unmagnetized piece of iron can be An unmagnetized piece of iron can be

magnetized by stroking it with a magnetmagnetized by stroking it with a magnet Somewhat like stroking an object to charge Somewhat like stroking an object to charge

an objectan object Magnetism can be inducedMagnetism can be induced

If a piece of iron, for example, is placed near If a piece of iron, for example, is placed near a strong permanent magnet, it will become a strong permanent magnet, it will become magnetizedmagnetized

Magnetic Fields (B)Magnetic Fields (B) The region where magnetic force The region where magnetic force

exists around a magnet or any exists around a magnet or any moving charged objectmoving charged object

PolesPoles North PoleNorth Pole

End of magnet that points to End of magnet that points to geographicgeographic north north

South PoleSouth Pole End of magnet that points to End of magnet that points to

geographicgeographic south south* So what does that really mean * So what does that really mean

about the Earth? *about the Earth? *

Forces between PolesForces between Poles Magnetic Force is the force Magnetic Force is the force

produced by the motion of charges produced by the motion of charges relative to each otherrelative 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)

CompassCompass A compass is a navigational A compass is a navigational

instrument for determining direction instrument for determining direction relative to the Earth's magnetic poles.relative to the Earth's magnetic poles.

• It consists of a magnetized pointer It consists of a magnetized pointer (usually marked on the North end) (usually marked on the North end) free to align itself with Earth's free to align itself with Earth's magnetic field.magnetic field.

Earth’s Magnetic FieldEarth’s Magnetic Field The Earth’s geographic north pole The Earth’s geographic north pole

corresponds to a magnetic south polecorresponds to a magnetic south pole The Earth’s geographic south pole The Earth’s geographic south pole

corresponds to a magnetic north polecorresponds to a magnetic north pole Strictly speaking, a north pole should be Strictly speaking, a north pole should be

a “north-seeking” pole and a south pole a “north-seeking” pole and a south pole a “south-seeking” polea “south-seeking” pole

Earth’s Magnetic FieldEarth’s Magnetic Field The Earth’s The Earth’s

magnetic field magnetic field resembles that resembles that achieved by achieved by burying a huge burying a huge bar magnet bar magnet deep in the deep in the Earth’s interiorEarth’s interior

Source of the Earth’s Source of the Earth’s Magnetic FieldMagnetic Field

There cannot be large masses of There cannot be large masses of permanently magnetized materials permanently magnetized materials since the high temperatures of the since the high temperatures of the core prevent materials from retaining core prevent materials from retaining permanent magnetizationpermanent magnetization

The most likely source of the Earth’s The most likely source of the Earth’s magnetic field is believed to be magnetic field is believed to be electric currents in the liquid part of electric currents in the liquid part of the corethe core Magnetic Fields result from moving Magnetic Fields result from moving

chargescharges

Magnetic Field StrengthMagnetic Field Strength The number of magnetic flux lines The number of magnetic flux lines

per unit area passing through a per unit area passing through a plane perpendicular to the plane perpendicular to the distance of the linesdistance of the lines The closer to the The closer to the endsends of a magnet of a magnet

the stronger the magnetic field the stronger the magnetic field strength isstrength is

Magnetic Flux (Field) LinesMagnetic Flux (Field) Lines Direction is given by the direction a Direction is given by the direction a

north polenorth pole of a compass needle points of a compass needle points in that locationin that location

Magnetic field linesMagnetic field lines can be used to show can be used to show how the field lines, as traced out by a how the field lines, as traced out by a compass, would lookcompass, would look

1.1. Form closed loopsForm closed loops2.2. DO NOT CROSSDO NOT CROSS3.3. Exit the (N)orth pole of a magnet and enter the Exit the (N)orth pole of a magnet and enter the

(S)outh pole(S)outh pole4.4. Closer the lines the stronger the forceCloser the lines the stronger the force

Magnetic Field Lines, Magnetic Field Lines, sketchsketch

A compass can be used to show the A compass can be used to show the direction of the magnetic field lines (a)direction of the magnetic field lines (a)

A sketch of the magnetic field lines (b)A sketch of the magnetic field lines (b)

Magnetic Field Lines, Bar Magnetic Field Lines, Bar MagnetMagnet

Iron filings are Iron filings are used to show the used to show the pattern of the pattern of the electric field lineselectric field lines

The direction of The direction of the field is the the field is the direction a north direction a north pole would pointpole would point

Magnetic Field Lines, Magnetic Field Lines, Unlike PolesUnlike Poles

Iron filings are Iron filings are used to show the used to show the pattern of the pattern of the electric field lineselectric field lines

Magnetic Field Lines, Like Magnetic Field Lines, Like PolesPoles

Iron filings are Iron filings are used to show the used to show the pattern of the pattern of the electric field lineselectric field lines

Magnetic Field Lines, Horse Magnetic Field Lines, Horse Shoe MagnetShoe Magnet

Iron filings are Iron filings are used to show the used to show the pattern of the pattern of the electric field lineselectric field lines

Magnetic Field LinesMagnetic Field Lines

Where do magnetic fields come Where do magnetic fields come from?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 workhow most permanent magnets work changing electrical fieldschanging electrical fields

DomainsDomains

Unmagnetized ironUnmagnetized iron Arrows go in all different directionsArrows go in all different directions Not MagneticNot Magnetic

Magnetized ironMagnetized iron Arrows align to go in the Arrows align to go in the

same directionsame direction North Pole on right endNorth Pole on right end

Magnetism by InductionMagnetism by Induction By bringing a magnet near a By bringing a magnet near a

Ferromagnetic Material the Ferromagnetic Material the domains realign themselvesdomains realign themselves

Current Carrying WireCurrent Carrying Wire

Magnetic Field is Magnetic Field is counter-clockwise counter-clockwise

Magnetic Field Magnetic Field is clockwise is clockwise

ElectromagnetsElectromagnets If a long straight wire is bent into a coil of several closely If a long straight wire is bent into a coil of several closely

spaced loops, the resulting device is called a spaced loops, the resulting device is called a solenoidsolenoid It is also known as an electromagnet since it acts like a magnet It is also known as an electromagnet since it acts like a magnet

only when it carries a currentonly when it carries a current The field lines of the solenoid resemble those of a bar magnetThe field lines of the solenoid resemble those of a bar magnet

Why does iron core increase the Why does iron core increase the strength?strength?

Due to the fact that as the current runs Due to the fact that as the current runs through the wires it causes the domains through the wires it causes the domains to realign and become a magnet itselfto realign and become a magnet itself

Left Hand Rule#2Left Hand Rule#2 Using your left – start Using your left – start

from the negative endfrom the negative end Think of your first finger as Think of your first finger as

the wirethe wire Follow the wire around – like Follow the wire around – like

the wirethe wire Which ever direction Which ever direction

your thumb is pointing is your thumb is pointing is northnorth

Which end is North?

Magnetic Fields - ForceMagnetic Fields - Force When moving through a magnetic When moving through a magnetic

field, a charged particle experiences a field, a charged particle experiences a magnetic forcemagnetic force

This is a result of two conflicting magnetic This is a result of two conflicting magnetic fieldsfields

The one that is set up by the moving charges The one that is set up by the moving charges and the existing fieldand the existing field

The force is zero if the motion of the The force is zero if the motion of the charge is parallel to the magnetic field charge is parallel to the magnetic field lineslines

Left Hand Rule #3Left Hand Rule #3 Used to determine the ForceUsed to determine the Force

Hold your left hand openHold your left hand open Place your fingers in the Place your fingers in the

direction of magnetic field direction of magnetic field Place your thumb in the Place your thumb in the

direction of current or direction direction of current or direction of moving chargeof moving charge

The direction of the force on The direction of the force on a positive charge is directed a positive charge is directed out of your palmout of your palm If the charge is positive, the If the charge is positive, the

force is opposite that force is opposite that determined by the left hand ruledetermined 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. Field - depending on strength 2. Area – the cross sectional area of

the wire3. 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 LawAn induced current is always

in such a direction as to oppose the motion or change causing itBy 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