PHY 2054 Magnetism - I An Attractive New Topic

This Magnetic Week Today we begin chapter 20 – Magnetism Today we begin chapter 20 – Magnetism There will be NO CLASS on MONDAY There will be NO CLASS on MONDAY There will be a PowerPoint presentation posted. Run it as a presentation (click the screen icon bottom right) and it will play as a recording. Print it as usual.There will be a PowerPoint presentation posted. Run it as a presentation (click the screen icon bottom right) and it will play as a recording. Print it as usual. Wednesday – We will continue with magnetism – assuming that you have viewed the Monday Lecture. Wednesday – We will continue with magnetism – assuming that you have viewed the Monday Lecture. Quiz next Friday Quiz next Friday Magnetism2

WELCOME BACK EXAMS WILL EVENTUALLY BE RETURNED Maybe Magnetism3

How Did You Do??  A  B  C  D  E  F. Less than 0 Magnetism4

Did the Card Help? A. A Lot B. A Little C. Not really D. No Magnetism5

Magnetism was known long ago. Refrigerator Magnetics

Magnetism7 Lodestone (Mineral) Lodestones attracted iron filings. Lodestones seemed to attract each other. Lodestone is a natural magnet.

Magnetism8 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.

Magnetism9 Refrigerators are attracted to magnets!

Where is Magnetism Used?? Motors Navigation – Compass Magnetic Tapes – Music, Data Older Television Tubes & Oscilloscopes – Beam deflection Coil Magnetic Resonance Imaging Magnetism10

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 Magnetism12 NSNS 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. Magnetism14 Shaded End is NORTH Pole Shaded 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.) Magnetism15

Magnets Magnetism16 Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles. Magnetic Field N S

Consider a Permanent Magnet Magnetism17 NS The magnetic Field B goes from North to South.

Introduce Another Permanent Magnet Magnetism18 NS N S The bar magnet (a magnetic dipole) wants to align with the B-field. pivot

Field of a Permanent Magnet Magnetism19 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. NS N S

Field of a Permanent Magnet Magnetism20 NS NS The bar magnet (a magnetic dipole) aligns with the B-field. It is now happy!

Electric field of an electric dipole Magnetism21 Electric Field Magnetic Field The magnet behaves just like the Electric dipole and aligns itself with A 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 Magnetism24 X Field INTO Paper Field OUT of Paper B

Typical Representation Magnetism25 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. Magnetism26

Current Carrying Wire Magnetism27 Current into the page. B Right hand Rule- Thumb in direction of the current Fingers curl in the direction of B

Current 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. Magnetism28

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! Magnetism29

Planet Earth Magnetism30

Inside it all. Magnetism Miles

On the surface it looks like this.. Magnetism32

Inside: Warmer than Floriduh Magnetism33

Much Warmer than Floriduh Magnetism34

Finally Magnetism35

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. Magnetism36

Magnetism37 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 Magnetism39 Compass Direction Navigation DIRECTION N S If N direction is pointed to by the NORTH pole of the Compass Needle, then the pole at the NORTH of our planet must be a SOUTH MAGNETIC POLE! Magnetic DIRECTION S N And it REVERSES from time to time.

A Look at the Physics Magnetism40 q There is NO force on a charge placed into a magnetic field if the charge is NOT moving. q If the charge is moving in a different direction, there is a force on the charge, perpendicular to both v and B. F = q v B There is no force if the charge moves parallel to the field.

Nicer Picture Magnetism41

Another Picture The Vector Cross Product Magnetism42

Practice Magnetism43 Which way is the Force???

Units Magnetism44

t e s l a s are Magnetism45

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 !! (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 Magnetism50 Smaller Mass

Magnetism51

An Example Magnetism52 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

Problem Continued Magnetism53 r

Magnetism54 crossed Let’s Look at the effect of crossed E and B Fields: x x x x x x q, m B v E

Magnetism55 What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?. FEFE FBFB F E = q E and F B = q v B If F E = F B the particle will move following a straight line trajectory q E = q v B v = E / B x x x x x x q m B v E B E