1220S12 Magnetostatics Stud

8/12/2019 1220S12 Magnetostatics Stud

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Physics 1220/1320

Electromagnetism

&Thermodynamics

Lecture Magnetostatics, chapter 27-29


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Magnetism- Contrary to common opinion, magnetism is just as common

as electricity.

-Magnetic fields are due to the motion of electric charges

-All moving charges create magnetic fields

-Electric and magnetic fields will turn out to be coupled and theexpression of the more general phenomenon of electromagnetism

-This phenomenon will explain the whole range of radiation and its

ways of spreading.

- Unlike electric charges, magnetism always comes in the form of twoopposing poles (usually called North and South pole)

-The magnetic force, magnetic field lines behave differently than the

electric counterparts


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Unlike polesattract,

Like polesrepel


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Many metals can bemagnetized when broughtin contact with a magnet.


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Magnetic field B and magnetic force FB

Unit Tesla [T] = [N/(Am)] , 10 k[G] = 1[T]


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Magnetic Field Lines

Magnetic flux FB,Gausss Law

!


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Mass Spectrometers:Magnetic fields can act as velocity selectors for charged particles:

v = E/B

ie only particles with the right speed can pass through

(condition: SFy=0)

In the famous Thompsonexperiment, this effect was used

to determine the ratio e/m for

electrons.

In the mass spectrometer, the effect

is used to determine the mass of unknownparticles with high precision.

http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=53

http://www.pk-applets.de/phy/thomson/thomson.html
http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=53http://www.pk-applets.de/phy/thomson/thomson.htmlhttp://www.pk-applets.de/phy/thomson/thomson.htmlhttp://www.pk-applets.de/phy/thomson/thomson.htmlhttp://www.pk-applets.de/phy/thomson/thomson.htmlhttp://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=53


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Hall Effect

Force on chargecarrier in B

Transverse E builds through charge accumulation

Due to FBuntil FEequal+opposite to FBHall voltage


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Force on Current-Carrying Conductor


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Force and Torque on a Loop

Net force is zero

Torque is zero if dA parallel B and max

if perpendicular to B

Magnetic dipole moment m=IA


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Loops are important because electrons often perform loops,so material properties can be understood if one understands

B for conductor loops.

A potential energy is associated with the dipole moment in B.


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A case of practical importance is the energy of a coil in B:Consider a coil which rotates from an initial position into one

where its mis parallel to B.

Note:


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How magnets work:

Forces on current loops in non-uniform B

dF = I dl x B

Magnets in non-uniform fields

If free to move, all magnets will orientsuch that their axis // B


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Permanent magnets:

http://ist-socrates.berkeley.edu/~cywon/Curie.html http://ist-socrates.berkeley.edu/~cywon/Stripe.html

Random order

Aligned atomic

ms

ttends to align

ms with B

Presence of B makes net m

Non-uniform Battractive force
http://ist-socrates.berkeley.edu/~cywon/Curie.htmlhttp://ist-socrates.berkeley.edu/~cywon/Stripe.htmlhttp://ist-socrates.berkeley.edu/~cywon/Stripe.htmlhttp://ist-socrates.berkeley.edu/~cywon/Stripe.htmlhttp://ist-socrates.berkeley.edu/~cywon/Stripe.htmlhttp://ist-socrates.berkeley.edu/~cywon/Curie.htmlhttp://ist-socrates.berkeley.edu/~cywon/Curie.htmlhttp://ist-socrates.berkeley.edu/~cywon/Curie.html


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Magnetic Field of moving charge

Unit Tesla [T] = [(Ns)/(Cm)] = [N/(Am)]

[m0] = [N/A2] = [Tm/A] permeability of free space

and c2= 1/(e0m0)


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Forces between two moving electrons


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Magnetic Field of a Current Element:

Biot-Savart


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B of Current Carrying Straight Conductor


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Magnetic field of two wires


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28.24

Find I4to make B at center of square zero:


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Magnetic Field of a Circular Loop(atoms & electrons!)


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Amperes Law


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A more general integration path gives thesame result, as long as the wire is includedand the surface of integration is closed:


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Group Task:

Find for the 4 surfaces a-d the values

for integral B dL

a:

b:

c:

d:


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Field Inside a Long Cylindrical Conductor


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Magnetic Field of a Solenoid

http://webphysics.davidson.edu/applets/BField/Solenoid.html

http://www.falstad.com/vector3dm/index.html
http://webphysics.davidson.edu/applets/BField/Solenoid.htmlhttp://www.falstad.com/vector3dm/index.htmlhttp://www.falstad.com/vector3dm/index.htmlhttp://webphysics.davidson.edu/applets/BField/Solenoid.html

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1220S12 Magnetostatics Stud