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Magnetism A Whole New Topic March 8, 2006 Magnetism
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Introductory Material
Magnetism
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Dis Here Week Today we begin magnetism – a topic that will occupy us for most of the remainder of the semester. I do not yet have the exam papers (Tuesday PM) but I will return them if I get them. Friday – Quiz on DC Circuits New material on the web New WebAssign on board – READ THE CHAPTER!! Magnetism
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DEMO ON Magnetism Magnetism
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Lodestone (Mineral) Lodestones attracted iron filings.
Lodestones seemed to attract each other. Used as a compass. One end always pointed north. Lodestone is a natural magnet. Magnetism
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Magnetism Refrigerators are attracted to magnets! Magnetism
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Applications Motors Navigation – Compass Magnetic Tapes Television
Music, Data Television Beam deflection Coil Magnetic Resonance Imaging High Energy Physics Research Magnetism
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Magnets Like Poles Repel Opposite Poles Attract
Magnetic Poles are only found in pairs. No magnetic monopoles have ever been observed. S N Shaded End is NORTH Pole Shaded End of a compass points to the NORTH. Magnetism
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Observations Bring a magnet to an electrically charged object and nothing happens. No forces. 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. 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
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Magnets Magnetic Field Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles. Magnetism
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Consider a Permanent Magnet
Magnetism
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Introduce Another Permanent Magnet
S pivot The bar magnet (a magnetic dipole) wants to align with the B-field. Magnetism
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Field of a Permanent Magnet
S The south pole of the small bar magnet is attracted towards the north pole of the big magnet. Also, the small bar magnet (a magnetic dipole) wants to align with the B-field. The field attracts and exerts a torque on the small magnet. Magnetism
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Field of a Permanent Magnet
S Show disk 19, demo 6 (minimum energy configuration of magnets) The bar magnet (a magnetic dipole) wants to align with the B-field. Magnetism
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The Magnetic Field Similar to Electric Field … exists in space.
Has Magnitude AND Direction. The “stronger” this field, the greater is the ability of the field to interact with a magnet. Magnetism
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Convention For Magnetic Fields
B X Field INTO Paper Field OUT of Paper Magnetism
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Experiments with Magnets Show
Current carrying wire produces a circular magnetic field around it. Force on Compass Needle (or magnet) increases with current. Magnetism
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Current Carrying Wire B Right hand Rule-
Current into the page. Right hand Rule- Thumb in direction of the current Fingers curl in the direction of B Magnetism
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Current Carrying Wire B field is created at ALL POINTS in space surrounding the wire. The B field had magnitude and direction. Force on a magnet increases with the current. Force is found to vary as ~(1/d) from the wire. Magnetism
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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. Magnetism
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Planet Earth Magnetism
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Inside it all. 8000 Miles Magnetism
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On the surface it looks like this..
Magnetism
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Inside: Warmer than Floriduh
Magnetism
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Much Warmer than Floriduh
Magnetism
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Finally Hot Hot Hot Magnetism
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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
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After molten lava emerges from a volcano, it solidifies to a rock
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. Magnetism
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Ancient Navigation Magnetism
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This planet is really screwed up!
NORTH POLE SOUTH POLE Magnetism
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Repeat And it REVERSES from time to time. 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! Compass Direction And it REVERSES from time to time. Magnetism
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Rowland’s Experiment xxx xxx B INSULATING Field is created by
any moving charge. Increases with charge on the disk. angular velocity of the disk. Electrical curent is a moving charge. Rotating INSULATING Disk which is CHARGED + or – on exterior. xxx xxx B ++ Magnetism
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So much for the observations.
Let’s do the physics! Magnetism
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A Look at the Physics q If the charge is moving, there
There is NO force on a charge placed into a magnetic field if the charge is NOT moving. q There is no force if the charge moves parallel to the field. q If the charge is moving, there is a force on the charge, perpendicular to both v and B. F = q v x B Magnetism
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WHAT THE HECK IS THAT??? A WHAT PRODUCT?
A CROSS PRODUCT – Like an angry one?? Alas, yes …. F=qv X B Magnetism
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The Lorentz Force This can be summarized as: F or: v q m B
q is the angle between B and V Magnetism
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Nicer Picture Magnetism
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VECTOR CALCULATIONS Magnetism
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See proof of previous approach on the website.
Note See proof of previous approach on the website. Magnetism
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Practice B and v are parallel. Crossproduct is zero. So is the force.
Which way is the Force??? Magnetism
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Units Magnetism
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teslas are HUGE! Magnetism
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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: (Use “Right-Hand” Rule to determine direction of F) And --- the charge must be moving !! Magnetism
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So… A moving charge can create a magnetic field.
A moving charge is acted upon by a magnetic field. In Magnetism, things move. In the Electric Field, forces and the field can be created by stationary charges. Magnetism
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Wires A wire with a current contains moving charges.
A magnetic field will apply a force to those moving charges. This results in a force on the wire itself. The electron’s sort of PUSH on the side of the wire. F Remember: Electrons go the “other way”. Magnetism
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The Wire in More Detail Assume all electrons are moving
with the same velocity vd. B out of plane of the paper Magnetism
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Magnetic Levitation Magnetic Force Current = i mg
Where does B point???? Into the paper. Magnetism
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MagLev Magnetism
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There was a crooked man who lived in a crooked house that was wired with crooked wires
Magnetism
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Crooked Wire (in a plane) in a constant B field
Magnetism
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Case 1 The magnetic force on a curved current carrying conductor in a uniform magnetic field is the same as that of a straight conductor carrying the same current between the two points a and b. Magnetism
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The net magnetic force on a closed current carrying loop is ZERO!
Case 2 The net magnetic force on a closed current carrying loop is ZERO! Magnetism
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Current Loop What is force on the ends?? Loop will tend to rotate due to the torque the field applies to the loop. Magnetism
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The Loop OBSERVATION Force on Side 2 is out of the paper and that on
the opposite side is into the paper. No net force tending to rotate the loop due to either of these forces. The net force on the loop is also zero, pivot Magnetism
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An Application The Galvanometer
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The other sides t1=F1 (b/2)Sin(q) =(B i a) x (b/2)Sin(q)
total torque on the loop is: 2t1 Total torque: t=(iaB) bSin(q) =iABSin(q) (A=Area) Magnetism
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Don't hurt yourself doing this!
A Coil Normal to the coil RIGHT HAND RULE TO FIND NORMAL TO THE COIL: “Point or curl you’re the fingers of your right hand in the direction of the current and your thumb will point in the direction of the normal to the coil. Don't hurt yourself doing this! Magnetism
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Dipole Moment Definition
Define the magnetic dipole moment of the coil m as: =NiA We can convert this to a vector with A as defined as being normal to the area as in the previous slide. Magnetism
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