PHY 2054 Magnetism - IAn Attractive New Topic
This Magnetic WeekToday we begin chapter 20 MagnetismThere will be NO CLASS on MONDAYThere 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.Quiz next FridayMagnetism*
WELCOME BACKEXAMS WILL EVENTUALLY BE RETURNED MaybeMagnetism*
How Did You Do??A. 80-100B. 60-79C. 40-59D. 20-39E. 0-19F. Less than 0Magnetism*
Did the Card Help?A LotA LittleNot reallyNoMagnetism*
Magnetism was known long ago.RefrigeratorMagnetics
Magnetism*Lodestone (Mineral)Lodestones attracted iron filings.Lodestones seemed to attract each other.Lodestone is a natural magnet.
Magnetism*New ConceptThe Magnetic FieldWe 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*MagnetismRefrigerators are attracted to magnets!
Where is Magnetism Used??MotorsNavigation CompassMagnetic TapesMusic, DataOlder Television Tubes & OscilloscopesBeam deflection CoilMagnetic Resonance ImagingMagnetism*
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 ElectrostaticsMagnetism*N
Notice the general behavior trends of attraction and repulsion, dipole or monopole.
MagnetsLike Poles RepelOpposite Poles AttractMagnetic Poles are only found in pairs.No magnetic monopoles have ever been observed.Magnetism*Shaded End is NORTH PoleShaded End of a compass pointsto the NORTH.S N
ObservationsBring 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*
MagnetsMagnetism*Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles.Magnetic FieldN SN S
Consider a Permanent MagnetMagnetism*The magnetic Field B goes from North to South.
Introduce Another Permanent MagnetMagnetism*The bar magnet (a magnetic dipole) wants to align with the B-field.pivot
Field of a Permanent MagnetMagnetism*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.
Field of a Permanent MagnetMagnetism*The bar magnet (a magnetic dipole) aligns with the B-field.It is now happy!
Electric field of an electric dipoleMagnetism*Electric FieldMagnetic FieldThe 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 FieldsMagnetism* X
Field INTO Paper Field OUT of Paper B
Typical RepresentationMagnetism*BB is a vector!
Experiments with Magnets ShowCurrent carrying wire produces a circular magnetic field around it.
Force (actually torque) on a Compass Needle (or magnet) increases with current.Magnetism*
Current Carrying WireMagnetism*Current intothe page.Right hand Rule-Thumb in direction of the currentFingers curl in the direction of B
Current Carrying WireB 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*
Compass and B FieldObservationsNorth 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*
Inside it all.Magnetism*8000Miles
On the surface it looks like this..Magnetism*
Inside: Warmer than FloriduhMagnetism*
Much Warmer than FloriduhMagnetism*
In BetweenThe 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*
Magnetism*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.
SIf N directionis pointed to bythe NORTH poleof the Compass Needle, then thepole at the NORTHof our planet mustbe a SOUTH MAGNETICPOLE!And it REVERSES from time to time.
A Look at the PhysicsMagnetism*There is NO force ona charge placed into amagnetic field if thecharge is NOT moving.There is no force if the chargemoves parallel to the field.
Another Picture The Vector Cross ProductMagnetism*
PracticeMagnetism*Which way is the Force???
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 movingPositive and negative charges will feel opposite effects from a magnetic field.
An ExampleMagnetism*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
Magnetism*Lets Look at the effect of crossed E and B Fields:xxxxxxq , mBvE
Magnetism*What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?.FEFBFE = q E and FB = q v BIf FE = FB the particle will movefollowing a straight line trajectoryq E = q v B
****Show disk 19, demo 6 (minimum energy configuration of magnets)