MagnetismMagnetism

All of us are familiar with magnets. In a magnet All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the we have magnetic poles – the north and the south pole.south pole.

A North seeking pole is called the A North seeking pole is called the North North PolePole..

A South seeking pole is called the A South seeking pole is called the South South PolePole..

Like magnetic poles repel and unlike Like magnetic poles repel and unlike magnetic poles attract.magnetic poles attract.

A naturally occurring magnet is a mineral A naturally occurring magnet is a mineral called Magnetite (looks like a stone)called Magnetite (looks like a stone)

Every magnet has ends, or poles, about which Every magnet has ends, or poles, about which the magnetic properties seem to be the magnetic properties seem to be concentrated. As this photo shows, more iron concentrated. As this photo shows, more iron filings are attracted to the poles, revealing filings are attracted to the poles, revealing their location.their location.

Magnetic Fields:Magnetic Fields:

A magnet that is moved in space near a A magnet that is moved in space near a second magnet second magnet experiencesexperiences a a magnetic magnetic fieldfield..

A magnetic field can be represented A magnetic field can be represented by field linesby field lines..

The strength of the magnetic field isThe strength of the magnetic field is greatergreater where thewhere the lines are closer lines are closer togethertogether andand weakerweaker where they arewhere they are farther apartfarther apart..

These lines are a map of the magnetic field around These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing will follow the lines, with the north end showing the direction of the field.the direction of the field.

The Source of Magnetic Fields:The Source of Magnetic Fields:

Permanent Magnets:Permanent Magnets:

Moving electronsMoving electrons produce magnetic fields. produce magnetic fields.

In most materials these magnetic fields In most materials these magnetic fields cancel one another and neutralize the overall cancel one another and neutralize the overall magnetic effect.magnetic effect.

In other materials such as In other materials such as iron, cobalt, and iron, cobalt, and nickelnickel, the , the atoms behave as tiny magnetsatoms behave as tiny magnets because of certain orientations of the because of certain orientations of the electrons inside the atom.electrons inside the atom.

These atoms are grouped in a tiny region These atoms are grouped in a tiny region called the called the magnetic domainmagnetic domain..

Our Our EarthEarth is a is a big magnetbig magnet..

The Earth’s magnetic field is thought to The Earth’s magnetic field is thought to originate with moving charges.originate with moving charges.

The core is probably composed of iron and The core is probably composed of iron and nickel, which flows as the Earth rotates, nickel, which flows as the Earth rotates, creating electrical currents that result in creating electrical currents that result in the Earth’s magnetic field.the Earth’s magnetic field.

The earth’s magnetic field is called the The earth’s magnetic field is called the magnetosphere. magnetosphere.

Note that the magnetic Note that the magnetic north pole and the north pole and the geographic North Pole geographic North Pole are not in the same place. are not in the same place.

Note also that the Note also that the magnetic north pole acts magnetic north pole acts as if the south pole of a as if the south pole of a huge bar magnet were huge bar magnet were inside the earth.inside the earth.

You know that it must be You know that it must be a magnetic south pole a magnetic south pole since the north end of a since the north end of a magnetic compass is magnetic compass is attracted to it and attracted to it and opposite poles attract.opposite poles attract.

A bar magnet cut into halves always makes A bar magnet cut into halves always makes new, complete magnets with both a north and new, complete magnets with both a north and a south pole. The poles always come in pairs. a south pole. The poles always come in pairs. You can not separate a pair into single poles.You can not separate a pair into single poles.

Electric Currents Electric Currents

and and

MagnetismMagnetism

Oersted Oersted discovered discovered that a compass needle that a compass needle below a wire below a wire (A) (A) pointed north when pointed north when there was not a there was not a currentcurrent, , (B) moved at right (B) moved at right angles when a angles when a current flowed one current flowed one way,way, and and (C) moved at right (C) moved at right angles in the angles in the opposite direction opposite direction when the current when the current was reversedwas reversed..

(A) In a piece of iron, the magnetic domains have (A) In a piece of iron, the magnetic domains have random arrangement that random arrangement that cancels any overall cancels any overall magnetic effectmagnetic effect (not magnetic). (not magnetic). (B) When an external magnetic field is applied to (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the and those parallel to the field grow in size at the expense of the other domains, and the expense of the other domains, and the iron iron becomes magnetizedbecomes magnetized..

A magnetic A magnetic compass compass shows the shows the presence presence and and direction of direction of the the magnetic magnetic field around field around a straight a straight length of length of current-current-carrying carrying wire.wire.

When a current is When a current is run through a run through a cylindrical coil of cylindrical coil of wire, a wire, a solenoidsolenoid, it , it produces a produces a magnetic field like magnetic field like the magnetic field the magnetic field of a bar magnet. of a bar magnet. The solenoid is The solenoid is known as known as electromagnetelectromagnet. .

Applications of Electromagnets:Applications of Electromagnets: Electric Meters:Electric Meters:

The strength of the magnetic field produced by The strength of the magnetic field produced by an electromagnet is proportional to the an electromagnet is proportional to the electric current in the electromagnetelectric current in the electromagnet..

A galvanometer measures electrical current by A galvanometer measures electrical current by measuring the magnetic fieldmeasuring the magnetic field..

A galvanometer can measure current, A galvanometer can measure current, potential difference, and resistancepotential difference, and resistance..

A A galvanometergalvanometer measures the direction measures the direction and relative strength of an electric and relative strength of an electric current from the magnetic field it current from the magnetic field it producesproduces. A coil of wire wrapped around an . A coil of wire wrapped around an iron core becomes an electromagnet that iron core becomes an electromagnet that rotates in the field of a permanent magnet. rotates in the field of a permanent magnet. The rotation moves pointer on a scale.The rotation moves pointer on a scale.

Electric Motors:Electric Motors:

An electrical motor is an electromagnetic An electrical motor is an electromagnetic device that device that converts electrical energy into converts electrical energy into mechanical energymechanical energy..

A motor has two working parts - A motor has two working parts - a a stationary magnet called a field magnetstationary magnet called a field magnet and and a cylindrical, movable electromagnet a cylindrical, movable electromagnet called an armaturecalled an armature..

The armature is on an axle and rotates in the The armature is on an axle and rotates in the magnetic field of the field magnet.magnetic field of the field magnet.

The The axle is used to do workaxle is used to do work..

Electromagnetic InductionElectromagnetic Induction

Induced Current:Induced Current: If a loop of wire is moved in a magnetic field a voltage is If a loop of wire is moved in a magnetic field a voltage is

induced in the wire.induced in the wire. The The voltagevoltage is called an is called an induced voltageinduced voltage and the resulting current is and the resulting current is

called an called an induced currentinduced current.. The induction is called The induction is called electromagnetic inductionelectromagnetic induction..

A current is induced in aA current is induced in a

coil of wire moved coil of wire moved

through a magnetic field.through a magnetic field.

The direction of the The direction of the

current depends on the current depends on the

direction of motion.direction of motion.

The The magnitude of the induced voltagemagnitude of the induced voltage is proportional to: is proportional to:

The number of wire loops cutting The number of wire loops cutting across the magnetic field lines.across the magnetic field lines.

The strength of the magnetic field.The strength of the magnetic field.

The rate at which magnetic field lines The rate at which magnetic field lines are cut by the wire.are cut by the wire.

Applications:Applications: DC and AC Generators,DC and AC Generators, Transformers (step-up and step-down).Transformers (step-up and step-down).

ElectromagnetElectromagnet – – a a magnet made from a magnet made from a current bearing coil current bearing coil of wire wrapped of wire wrapped around an around an iron or iron or steel core.steel core.

What is an What is an electromagnetelectromagnet??

GeneratorGenerator – – a machine a machine that changes that changes mechanicalmechanical energy to energy to electricalelectrical energyenergy

Usually use Usually use movingmoving magnets to create magnets to create currentscurrents in coils of wire. in coils of wire.

What is a generator?What is a generator?

MotorMotor – a device that – a device that changes changes electrical electrical energy to energy to mechanical mechanical energy that can do energy that can do workwork..

What is a motor?What is a motor?