PHY 2054 Magnetism - IPHY 2054 Magnetism - I

An Attractive New TopicAn Attractive New Topic

This Magnetic WeekThis Magnetic Week

Today we begin chapter 20 – MagnetismToday we begin chapter 20 – Magnetism There will be NO CLASS on MONDAYThere will be NO CLASS on MONDAY

• There will be a PowerPoint presentation posted. There will be a PowerPoint presentation posted. Run it as a presentation (click the screen icon Run it as a presentation (click the screen icon bottom right) and it will play as a recording. bottom right) and it will play as a recording. Print it as usual.Print it as usual.

Wednesday – We will continue with Wednesday – We will continue with magnetism – assuming that you have magnetism – assuming that you have viewed the Monday Lecture.viewed the Monday Lecture.

Quiz next FridayQuiz next FridayMagnetismMagnetism 22

WELCOME BACKWELCOME BACKEXAMS WILL EVENTUALLY BE RETURNED MaybeEXAMS WILL EVENTUALLY BE RETURNED Maybe

MagnetismMagnetism 33

How Did You Do??How Did You Do?? A. 80-100A. 80-100 B. 60-79B. 60-79 C. 40-59C. 40-59 D. 20-39D. 20-39 E. 0-19E. 0-19 F. Less than 0F. Less than 0

MagnetismMagnetism 44

Did the Card Help?Did the Card Help?

A.A. A LotA Lot

B.B. A LittleA Little

C.C. Not reallyNot really

D.D. NoNo

MagnetismMagnetism 55

Magnetism was known Magnetism was known long ago.long ago.

RefrigeratorMagnetics

Magnetism 7

Lodestone (Mineral)

• Lodestones attracted iron filings.

• Lodestones seemed to attract each other.

• Lodestone is a natural magnet.

Magnetism 8

New Concept

The Magnetic Field– We give it the symbol B.– A compass will line up

with it.– It has Magnitude and

direction so it is a VECTOR.

• There are some similarities with the Electric Field but also some significant differences.

Magnetism 9

Magnetism

• Refrigerators are attracted to magnets!

Where is Magnetism Used??Where is Magnetism Used??

• Motors• Navigation – Compass• Magnetic Tapes

– Music, Data

• Older Television Tubes & Oscilloscopes– Beam deflection Coil

• Magnetic Resonance Imaging

Magnetism 10

And in magnets!!Although the magnet on the left is an electromagnet/huge and the one on the right is a permanent magnet/small, the idea is the same.

Compare to Electrostatics

Magnetism 12

N

S

Pivot

Magnet

What Happens??

• Notice the general behavior trends of attraction and repulsion, dipole or monopole.

Magnets

• Like Poles Repel• Opposite Poles

Attract• Magnetic Poles

are only found in pairs.– No magnetic

monopoles have ever been observed.

Magnetism 14

Shaded End is NORTH PoleShaded End of a compass points

to the NORTH.

S N

Observations• Bring a magnet to an electrically charged object

and the observed attraction will be a result of charge induction or polarization.

• Magnetic poles do not interact with stationary electric charges.

• Bring a magnet near some metals (Co, Fe, Ni …) and it will be attracted to the magnet.– The metal will be attracted to both the N and S poles

independently.– Some metals are not attracted at all. (Al, Cu, Ag, Au)– Wood is NOT attracted to a magnet.– Neither is water.

• A magnet will force a compass needle to align with it. (No big Surprise.)

Magnetism 15

Magnets

Magnetism 16

Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles.

Magnetic Field

N S N S

Consider a Permanent Magnet

Magnetism 17

N S

B

The magnetic Field B goes from North to South.

Introduce Another Permanent Magnet

Magnetism 18

N S

B

N

S

The bar magnet (a magnetic dipole) wants to align with the B-field.

pivot

Field of a Permanent Magnet

Magnetism 19

The south pole of the small bar magnet is attracted towards the north pole of the big magnet.

The North pole of the small magnet is repelled by the north pole of the large magnet. The South pole of the large magnet creates a smaller force on the small magnet than does the North pole. DISTANCE effect.

The field attracts and exerts a torque on the small magnet.

N S

B

N

S

Field of a Permanent Magnet

Magnetism 20

N S

B

N S

The bar magnet (a magnetic dipole) aligns with the B-field.It is now happy!

Electric field of an electric dipole

Magnetism 21

Electric Field Magnetic Field

The magnet behaves just like theElectric dipole and aligns itself withA MAGNETIC field.

Similarities will continue.

Iron filings will align as a compass does – • Each small filing lines up tangent to the field lines allowing a visual demonstration

Examples of Creating Magnetic fields• Fields are created by electric currents in a variety of ways and observed in a variety of places.

Convention For Magnetic Fields

Magnetism 24

X

Field INTO Paper Field OUT of Paper

B

Typical Representation

Magnetism 25

B

B is a vector!

Experiments with Magnets Show

• Current carrying wire produces a circular magnetic field around it.

• Force (actually torque) on a Compass Needle (or magnet) increases with current.

Magnetism 26

Current Carrying Wire

Magnetism 27

Current intothe page.

B

Right hand Rule-Thumb in direction of the currentFingers curl in the direction of B

Current Carrying WireCurrent Carrying Wire

• B field is created at ALL POINTS in space surrounding the wire.

• The B field has magnitude and direction.• Force on a magnet increases with the current.• Force is found to vary as ~(1/d) from the wire.

Magnetism 28

Compass and B Field• Observations

– North Pole of magnets tend to move toward the direction of B while S pole goes the other way.

– Field exerts a TORQUE on a compass needle.

– Compass needle is a magnetic dipole.

– North Pole of compass points toward the NORTH.

– The NORTH geographic pole of the planet is therefore a magnetic South pole!

Magnetism 29

Planet Earth

Magnetism 30

Inside it all.

Magnetism 31

8000Miles

On the surface it looks like this..

Magnetism 32

Inside: Warmer than Floriduh

Magnetism 33

Much Warmer than Floriduh

Magnetism 34

Finally

Magnetism 35

In Between

• The molten iron core exists in a magnetic field that had been created from other sources (sun…).

• The fluid is rotating in this field.• This motion causes a current in the molten metal.• The current causes a magnetic field.• The process is self-sustaining.• The driving force is the heat (energy) that is

generated in the core of the planet.

Magnetism 36

Magnetism 37

After molten lava emerges from a volcano, it solidifies to a rock. In most cases it is a black rock known as basalt, which is faintly magnetic, like iron emerging from a melt. Its magnetization is in the direction of the local magnetic force at the time when it cools down.

Instruments can measure the magnetization of basalt. Therefore, if a volcano has produced many lava flows over a past period, scientists can analyze the magnetizations of the various flows and from them get an idea on how the direction of the local Earth's field varied in the past. Surprisingly, this procedure suggested that times existed when the magnetization had the opposite direction from today's. All sorts of explanation were proposed, but in the end the only one which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was sometimes reversed.

Our Earth has a magnetic field.

Repeat

Magnetism 39

CompassDirection

NavigationDIRECTION

N

S

If N directionis pointed to bythe NORTH poleof the Compass Needle, then the

pole at the NORTHof our planet must

be a SOUTH MAGNETICPOLE!

MagneticDIRECTION

S

N

And it REVERSES from time to time.

A Look at the Physics

Magnetism 40

B

q

There is NO force ona charge placed into amagnetic field if thecharge is NOT moving.

Bq

v

• If the charge is moving in a different direction, thereis a force on the charge,perpendicular to both v and B. F = q v B

There is no force if the chargemoves parallel to the field.

Nicer Picture

Magnetism 41

Another Picture The Vector Cross Product

Magnetism 42

)sin(qvBF

q

BvF

Practice

Magnetism 43

Which way is the Force???

Units

Magnetism 44

m)-N/(A 1 T 1 tesla1

mAmp

N

/

:

)

sCm

N

qv

FB

Units

Bqv Sin(θF

teslas are

Magnetism 45

The Magnetic Force is Different From the Electric Force.

Whereas the electric force acts in the same direction as the field:

The magnetic force acts in a direction orthogonal to the field:

And --- the charge must be moving !!

)sin(qvBF

F qE

(Use “Right-Hand” Rule to determine direction of F)

• The effect of an existing magnetic field on a charge depends on the charges direction of motion relative to the field.

The Right Hand Rule

The effect of the sign of a moving

Positive and negative charges will feel opposite effects from a magnetic field.

Mass Spectrometer

Magnetism 50

Smaller Mass

Magnetism 51

An Example

Magnetism 52

A beam of electrons whose kinetic energy is K emerges from a thin-foil “window” at the end of an accelerator tube. There is a metal plate a distance d from this window and perpendicular to the direction of the emerging beam. Show that we can prevent the beam from hitting the plate if we apply a uniform magnetic field B  such that

22

2

de

mKB

Problem Continued

Magnetism 53

r

22

22

2

e

2mKB

:Bfor Solvee

22

eB

m

2 so

2

1

qB

mvr

Before From

d

dB

mK

m

Kr

m

KvmvK

Magnetism 54

Let’s Look at the effect of crossedcrossed E and B Fields:

x

x xx

xx

•q , m

B

v

E

Magnetism 55

What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?.

•FEFB

FE = q E and FB = q v B

If FE = FB the particle will movefollowing a straight line trajectory

q E = q v B

v = E / B

x

x xx

xx

•q m

B

v

EB

E