Lesson 20 Magnetism Eleanor Roosevelt High School Chin-Sung Lin

Lesson 20

Magnetism

Eleanor Roosevelt High SchoolChin-Sung Lin

History of Magnetism

History

The lodestone, which contains iron ore, was found more than 2000 years ago in the region of Magnesia in Greece

History

The earliest Chinese literature reference to magnetism lies in the 4th century BC writings Guiguzi (鬼谷子 ): "The lodestone attracts iron”

History

Zheng He used the Chinese compass as a navigational aid in his voyage between 1405 and 1433

History

In the 18th century, the French physicist Charles Coulomb studied the force between lodestones

History

In 1820 Danish physicist and chemist who discovered that electric currents create magnetic fields

Magnetic Poles

Magnetic Poles

Magnets attract and repel without touching

The interaction depends on the distance

Magnetic poles produce magnetic forces

Magnetic Poles

Magnet can act as a compass

The end that points northward is called north pole, and the end that points south is call the south pole

Magnetic Poles

All magnets have north and south poles

They can never be separated from each other

If you break the magnet in half, what will happen?

Magnetic Poles

Each half will become a complete magnet

Unlike electric charge, you cannot have north or south pole alone

Magnetic Poles

Like poles repels; opposite poles attract

Magnetic Fields

Magnetic Fields

The space around the magnet is filled with a magnetic field

Magnetic Fields

The magnetic field lines spread from the north pole to the south pole

Where the lines are closer (at the poles), the field strength is stronger

Magnetic Fields

The magnetic field unit:

Units: tesla (T) or gauss (G)

1 tesla = 10,000 gauss

Magnetic Fields

What will happen If we place a compass in the field?

Magnetic Fields

A magnet or small compass in the field will line up with the field

Magnetic Fields

Electric charge is surrounded by an electric filed

The same charge is surrounded by a magnetic field if it is moving

Which types of electron motion exist in magnetic materials?

Magnetic Fields

Electrons are in constant motion about atomic nuclei

This moving charge constitutes a tiny current and produces a magnetic field

Magnetic Fields

Electrons spinning about their own axes constitute a charge in motion and thus creates another magnetic field

Every spinning electron is a tiny magnet

Magnetic Fields

Electrons spinning in the same direction makes up a stronger magnet

Spinning in opposite directions cancels out

The field due to spinning is larger than the one due to orbital motion

Magnetic Fields

For ferromagnetic elements: iron, nickel, and cobalt, the fields do not cancel one another entirely

Each iron atom is a tiny magnet

Magnetic Domain

Magnetic Domain

Interactions among iron atoms cause large clusters of them to line up with one another

These cluster of aligned atoms are called magnetic domains

Magnetic Domain

There are many magnetic domains in a crystal iron

The difference between a piece of ordinary iron and an iron magnet is the alignment of domains

Magnetic Domain

Iron in a magnetic field:

A growth in the size of the domains that is oriented in the direction of the magnetic field

A rotation of domains as they are brought into alignment

Magnetic Domain

Permanent magnets:

Place pieces of iron or certain iron alloys in strong magnetic fields

Stroke a piece of iron with a magnet

Electric Currents &Magnetic Fields

Electric Currents & Magnetic Fields

Current-Carrying Wire:

A moving electron produces a magnetic field

Electric current also produces magnetic field

A current-carrying conductor is surrounded by a magnetic field


Right-hand rule:

Grasp a current-carrying wire with your right hand

Your thumb pointing to the direction of the current

Your fingers would curl around the wire in the direction of the magnetic field (from N to S)


What will happen to the compasses if the current is upward?

?


The current-carrying wire deflects a magnetic compass


Current-Carrying Loop:

A wire loop with current produces a magnetic field


Current-Carrying Loop:

A wire loop with current produces a magnetic field


Coiled wire— Solenoid:A solenoid can be made of many wire loops


Coiled wire— Solenoid:A current-carrying coil of wire with many loopsThe magnetic field lines bunch inside the loop


Coiled wire— Solenoid:A coil wound into a tightly packed helix which produces a magnetic field when an electric current is passed through it

Solenoids can create controlled magnetic fields and can be used as electromagnets


Intensity of Magnetic Field of Electromagnet (B):Increased as the number of loops increased (B ~ N)Increased as the Current increased (B ~ I)Intensity is enhanced by the iron core (B ~ μ)

BN

I


Permeability:The measure of the ability of a material to support the formation of a magnetic field within itself. Magnetic permeability is typically represented by the Greek letter μ

B μ


Permeability:

Medium Permeability μ [H/m]

Relative Permeability μ/μ0

Mu-metal (nickel-iron alloy) 2.5×10−2 20,000

Ferrite (nickel zinc) 2.0×10−5 – 8.0×10−4 16 – 640

Steel 8.75×10−4 100

Vacuum 1.2566371×10−6 (μ0) 1

Water 1.2566270×10−6 0.999992

Superconductors 0 0


Direction of magnetic field of electromagnet follows the Right-hand Rule:

Your fingers indicate the direction of the current (I)your thumb points the direction of the field (B)

B

I

Magnetic Forces on Moving Charged Particles


When a charged particle moves in a magnetic field, it will experience a deflecting force (FB)

I

+


When a charged particle moves in a magnetic field, it will experience a deflecting force (FB)

FB = qvB

FB magnetic force [N]

q electric charge [C]v velocity perpendicular to the field

[m/s]B magnetic field strength [T, Teslas]

I


The magnetic field unit:

Units: tesla (T) or gauss (G)

1 tesla = 10,000 gauss

tesla = (newton × second)/(coulomb × meter)

T = Ns / (Cm)


Direction of the magnetic force (FB) follows the

Fleming’s Left Hand Motor Rule

I


What will happen to the positively charged particle?

+


The positively charged particle will experience a force always perpendicular to the motion

The particle will have a circular motion


The magnetic field has been used to detect particles in the cloud chamber

What will happen to the different radiation?



α He2+ helium nucleus (+)

β e– electron (–)

γ uncharged EM ray



α He2+ helium nucleus (+)

β e– electron (–)

γ uncharged EM ray


The magnetic field has been used to deflect the electron beam. Where will the electron beam hit the screen?

electron beam S

N screen

magnet

A

BC

D


Mass spectrometry:To determine masses of particles, for determining the elemental composition of a molecule


Mass spectrometry:

magnetic force = centripetal force

FB = FC

qvB = mv2/r

r = (mv)/(qB)


Mass spectrometry:

r = (mv)/(qB)• the faster it is travelling the bigger the circles

• the bigger its mass is the bigger the circles

• the bigger its momentum the larger the circles

• the stronger the magnetic field the smaller the circles

• the larger the charge the smaller the circles


A positively charged particle moving along a spiral path inside a uniform magnetic field

Magnetic Force on Current-Carrying Wires


What will happen to the current carrying wires?

I I


The current-carrying wire also follows Fleming’s left hand motor rule


The current-carrying wire deflects a magnetic compass and a magnet deflects a current-carrying wire are different effect of the same phenomena


Magnetic Force Between Wires:

What will happen to the parallel wires if both current are in the same direction?

I1I2



Parallel wires carrying currents will exert forces on each other

When the current goes the same way in the two wires, the force is attractive

When the currents go opposite ways, the force is repulsive



What will happen to the parallel wires if the current are in the opposite direction?

I1I2

Galvanometers & Motors

Galvanometer

A sensitive current-indicating instrument The coil turns against a spring, so the greater the current, the greater its deflection

Galvanometer

A galvanometer may be calibrated to measure current— an ammeter

A galvanometer may be calibrated to measure voltage— a voltmeter

Motor

Converts electrical energy into mechanical energy

Motors operate through interacting magnetic fields and current-carrying conductors to generate force

DC Motor

The current-carrying wire of the motor coil follows Fleming’s left hand motor rule

DC Motor

AC Motor

AC Motor

Earth’s Magnetic Field


Earth itself is a huge magnet

The magnetic poles of Earth do not coincide with the geographic North pole – magnetic declination


Magnetic Pole Shift:

The magnetic poles of Earth keep changing

The pole kept going north at an average speed of 10 km per year, lately accelerating to 40 km per year


Magnetic Pole Weakening:The strength of the magnetic field of Earth keep decreasingThe magnetic field has weakened 10% since the 19th century Earth's Magnetic Field Trends

52,000.00

53,000.00

54,000.00

55,000.00

56,000.00

57,000.00

58,000.00

59,000.00

60,000.00

Year

Tota

l Int

ensi

ty (

nT)


A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged


A positively charged particle moving along a spiral path inside a uniform magnetic field


Earth’s magnetic field will deflect the charged particles from outer space to reduce the cosmic rays striking Earth’s surface


Van Allen radiation belt: is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field


Van Allen radiation belt: energetic electrons forming the outer belt and a combination of protons and electrons creating the inner belt


Aurora: a natural light display in the sky, particularly in the polar regions, caused by the collision of charged particles directed by the Earth's magnetic field

The End

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Lesson 20 Magnetism Eleanor Roosevelt High School Chin-Sung Lin