Electric Currents and Electric Currents and Magnetic Fields Magnetic Fields

HistoryHistory Lodestones were discovered 2000 Lodestones were discovered 2000

years ago and were magnetic. They years ago and were magnetic. They were named after Magnesia which is were named after Magnesia which is a region in Greece where they were a region in Greece where they were found. found.

The Chinese used them for The Chinese used them for navigating ships in the 12th century. navigating ships in the 12th century. We know they contained iron ore We know they contained iron ore which is called magnetite. which is called magnetite.

Moving Charges Create Moving Charges Create Magnetic FieldsMagnetic Fields

In 1820 a Danish scientist Hans In 1820 a Danish scientist Hans Christian Oersted noticed that a wire Christian Oersted noticed that a wire containing current would deflect a containing current would deflect a compass needle.compass needle.

This indicated the relationship between This indicated the relationship between electricity and magnetism.electricity and magnetism.

Moving charges create a magnetic field.Moving charges create a magnetic field. Current running through a wire will create a Current running through a wire will create a

magnetic field.magnetic field. The direction of magnetic field can be The direction of magnetic field can be

determined by the right hand rule.determined by the right hand rule.

Magnetic field linesMagnetic field lines Bar MagnetBar Magnet Wire with Wire with

currentcurrent

Earth’s Magnetic FieldEarth’s Magnetic Field

The earth is basically one huge The earth is basically one huge magnet. However, the north and magnet. However, the north and south poles are not located exactly south poles are not located exactly at the locations we call the north and at the locations we call the north and south poles. south poles.

Earth’s Magnetic Field Earth’s Magnetic Field Changes?Changes?

The angle between the geographic The angle between the geographic (true) North Pole according to the (true) North Pole according to the earth’s magnetic field and the earth’s magnetic field and the magnetic north that compasses point magnetic north that compasses point to is called magnetic declination. to is called magnetic declination.

Magnetic north has wandered Magnetic north has wandered throughout geologic time. More than 20 throughout geologic time. More than 20 reversalsreversals have been recorded in the have been recorded in the orientation of the dipoles in rocks on orientation of the dipoles in rocks on earth (over 5 billion years!)earth (over 5 billion years!)

Compasses Align in the Compasses Align in the direction of the fielddirection of the field

Magnetic fields created by Magnetic fields created by wireswires

Charged Particles in Magnetic fieldsCharged Particles in Magnetic fields

Moving charges create a magnetic Moving charges create a magnetic field.field.– When charged particles move through When charged particles move through

an external magnetic field their an external magnetic field their magnetic fields interact and the charge magnetic fields interact and the charge experiences a force.experiences a force.

– The force the charge experiences is The force the charge experiences is always perpendicular to the charge’s always perpendicular to the charge’s velocity and the external magnetic field.velocity and the external magnetic field.

Right Hand RuleRight Hand Rule

Use your right hand:Use your right hand:– The palm of your hand pushes in the The palm of your hand pushes in the

direction of the force the charge direction of the force the charge experiences.experiences.

– Your outstretched fingers point in the Your outstretched fingers point in the direction of the external magnetic field.direction of the external magnetic field.

– Your thumb points in the direction of the Your thumb points in the direction of the current or the velocity of a negative current or the velocity of a negative charge charge moving through the charge charge moving through the external field.external field.

Right Hand Rule for charges Right Hand Rule for charges entering a magnetic fieldentering a magnetic field

Right Hand rule is used to determine Right Hand rule is used to determine the direction in which the metal rod is the direction in which the metal rod is

deflecteddeflected

To calculate the force a charge To calculate the force a charge experiences when traveling in an experiences when traveling in an

external magnetic fieldexternal magnetic field FFmagneticmagnetic = B= Bvvqqsinsinθθ

– FFmagnetic magnetic magnetic force exerted on the charge by the magnetic force exerted on the charge by the external magnetic field. external magnetic field.

Unit - NewtonsUnit - Newtons

– q q the magnitude of the charge traveling in the magnetic the magnitude of the charge traveling in the magnetic field.field.

Unit - CoulombsUnit - Coulombs

– v v the velocity of the charge traveling in the magnetic fieldthe velocity of the charge traveling in the magnetic field Unit - m/sUnit - m/s

– B B The magnitude the the external magnetic field.The magnitude the the external magnetic field. Unit - TeslaUnit - Tesla

One Tesla is equal to 1 C of charge is moving 1 m/s One Tesla is equal to 1 C of charge is moving 1 m/s perpendicular to the external magnetic field and experiencing 1 perpendicular to the external magnetic field and experiencing 1 N of force.N of force.

sinsinθθ angle between v and Bangle between v and B

Degree of force the charge experiences Degree of force the charge experiences depends on the direction of motion depends on the direction of motion relative to the external magnetic field. relative to the external magnetic field. – The force has its maximum value when it is The force has its maximum value when it is

traveling perpendicular to the magnetic field.traveling perpendicular to the magnetic field.– The force decreases when the charge moves The force decreases when the charge moves

at angles to the magnetic field.at angles to the magnetic field.– The charge will experience zero force when it The charge will experience zero force when it

is moving parallel to the magnetic field lines. is moving parallel to the magnetic field lines. – When a charge is traveling perpendicular to When a charge is traveling perpendicular to

a uniform magnetic field the particle will a uniform magnetic field the particle will make a circular path.make a circular path.

Right Hand Rule for current Right Hand Rule for current Flowing through a wire.Flowing through a wire.

Force exerted on a wire carrying charge Force exerted on a wire carrying charge perpendicular to an external magnetic fieldperpendicular to an external magnetic field

FFmagneticmagnetic = B = B l l II sinsinθθ

– FFmagnetic magnetic Force wire experiences due to the Force wire experiences due to the external external magnetic field. magnetic field.

Unit – NewtonsUnit – Newtons

– B B Magnetic FieldMagnetic Field Unit – TeslaUnit – Tesla

– l (L) l (L) Length of the wireLength of the wire Unit – metersUnit – meters

– II Current in the wireCurrent in the wire Unit – amperesUnit – amperes

– sinsinθθ angle between I and Bangle between I and B

Devices Utilize Devices Utilize ElectromagnetismElectromagnetism

ElectromagnetsElectromagnets GalvanometerGalvanometer DC MotorsDC Motors GeneratorsGenerators TransformersTransformers

ElectromagnetsElectromagnets

A coil of wire attached to a battery creates A coil of wire attached to a battery creates a magnetic field inside the coil.a magnetic field inside the coil.

The overlapping magnetic field lines The overlapping magnetic field lines intensify the field inside the coil.intensify the field inside the coil.

If a piece of metal is placed inside the coil, If a piece of metal is placed inside the coil, the domains of the metal become aligned the domains of the metal become aligned with the magnetic field inside the coil and with the magnetic field inside the coil and strengthens the field.strengthens the field.

Change the direction of current and you Change the direction of current and you change the direction of the magnetic field.change the direction of the magnetic field.

GalvanometersGalvanometers

Device that measures current using Device that measures current using magnetic force.magnetic force.– A coil of wire is wrapped around a soft A coil of wire is wrapped around a soft

iron core that is mounted on a pivot iron core that is mounted on a pivot attached to a needle.attached to a needle.

– When current is run through the wire the When current is run through the wire the magnetic field created by the current magnetic field created by the current interacts with an external magnetic field.interacts with an external magnetic field.

– The degree of deflection is related to the The degree of deflection is related to the amount of current in the coil.amount of current in the coil.

Electric MotorsElectric Motors Motors use electricity to create mechanical Motors use electricity to create mechanical

motionmotion..– Motors use direct current.Motors use direct current.– A coil attached to a battery is rotated in the A coil attached to a battery is rotated in the

presence of a permanent magnetpresence of a permanent magnet..– Current flows in opposite directions in the Current flows in opposite directions in the

upper and lower part of the loopupper and lower part of the loop..– As the current moves through the upper part of As the current moves through the upper part of

the loop it is deflected by the permanent the loop it is deflected by the permanent magnetic field. And rotates half a turn.magnetic field. And rotates half a turn.

– The direction of current is switched every half The direction of current is switched every half rotation by contacts at the end of the shaft rotation by contacts at the end of the shaft which keeps the loop rotating in the same which keeps the loop rotating in the same direction.direction.

DC MotorDC Motor

DC Motor

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Electromagnetic Electromagnetic InductionInduction Electric current can create a magnetic field Electric current can create a magnetic field

but can a magnetic field create current?but can a magnetic field create current?– British scientist Michael Faraday and American British scientist Michael Faraday and American

scientist John Henry in 1831 independently scientist John Henry in 1831 independently discovered the answer is yes.discovered the answer is yes.

– They found by moving a magnet in and out of a They found by moving a magnet in and out of a coil of wire electric current is induced.coil of wire electric current is induced.

– A changing magnetic field can create current.A changing magnetic field can create current.– The amount of current induced is dependent on:The amount of current induced is dependent on:

The rate in which the magnet s moved in and out of the The rate in which the magnet s moved in and out of the coli of wire.coli of wire.

The number of coils in the wire.The number of coils in the wire.

Generator Simulation

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• http://www.physclips.unsw.edu.au/jw/electricmotors.html#mandg