*W07D1Magnetic Dipoles, Force and Torque on a Dipole, Experiment 2W07D1 Magnetic Dipoles, Torque and Force on a Dipole, Experiment 2: Magnetic Dipole in a Helmholtz Coil http://web.mit.edu/8.02t/www/materials/Experiments/expMagForcesDipoleHelmholtz.pdf

Reading Course Notes: Sections 8.4, 8.6.4, 8.10.4, 8.13, 9.5, 9.9

Announcements

Exam Two Reviews:Monday from 7-9 pm in 26-152Tuesday from 9-11 pm in 26-152Wednesday from 9-11 pm in 26-152

PS 6 due W07 Tuesday at 9 pm in boxes outside 32-082 or 26-152

Exam 2 Thursday March 21 7:30 - 9:30 pm: See announcement pages for section room assignments

Conflict Friday March 22 9-11 am in 32-082; 10-12 noon in 6-120

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*Magnetic Field of Bar Magnet(1) A magnet has two poles, North (N) and South (S)(2) Magnetic field lines leave from N, end at S

*Magnetism Bar MagnetLike poles repel, opposite poles attract

*Bar Magnets Are Dipoles!NO! Magnetic monopoles do not exist in isolation Create Dipole Field Rotate to orient with FieldIs there magnetic mass or magnetic charge?

*Magnetic Monopoles?Magnetic monopoles do not exist in isolationq-qElectric DipoleWhen cut: 2 monopoles (charges)Magnetic DipoleWhen cut: 2 dipolesAnother Maxwells Equation! (2 of 4)Gausss LawMagnetic Gausss Law

*Conservation of Magnetic Flux:

*Torque on a Current Loop in a Uniform Magnetic Field

If the wire is a uniform magnetic field then

If the wire is also straight then*Review: Magnetic Force on Current-Carrying Wire

*Group Problem: Current LoopPlace rectangular current loop in uniform B field 1)What is the net force on this loop?2)What is the net torque on this loop?3)Describe the motion the loop makes

*Torque on Rectangular LoopNo net force but there is a torqueArea vector

Magnetic Dipole Momenthttp://web.mit.edu/viz/EM/visualizations/magnetostatics/calculatingMagneticFields/RingMagField/RingMagField.htm

*Torque on Current LoopPlace rectangular current loop in uniform B field Magnetic moment points out of the page

torque tries to align the magnetic moment vector in the direction of the magnetic field

*Concept Question: Magnetic Field LinesThe picture shows the field lines outside a permanent magnet The field lines inside the magnet point:UpDownLeft to rightRight to leftThe field inside is zeroI dont know

*Concept Q. Answer: Magnetic Field LinesMagnetic field lines are continuous.E field lines begin and end on charges.There are no magnetic charges (monopoles) so B field lines never begin or endAnswer: 1. They point up inside the magnet

*Demonstration:Deflection of a Compass Needle by a Magnet G1http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%201&show=0

*Demonstration:Galvanometer principle G10 http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%2010&show=0

*Concept Question: Dipole in FieldmFrom rest, the coil above will:

rotate clockwise, not moverotate counterclockwise, not movemove to the right, not rotatemove to the left, not rotatemove in another direction, without rotatingboth move and rotateneither rotate nor moveI dont know

*Concept Q. Answer: Dipole in FieldAnswer: 1. Coil will rotate clockwise (not move) No net force so no center of mass motion. BUT Magnetic dipoles rotate to align with external field (think compass)

*Force on a Dipole in a Non-Uniform Field

*Dipoles dont move???This dipole rotates but doesnt feel a net forcein a uniform magnetic fieldBut dipoles can feel magnetic force.

*Dipoles in Non-Uniform Fields:Magnetic Force

*Concept Question: Dipole in FieldThe current carrying coil above will feel a net forceupwardsdownwardsof zeroI dont know

*Concept Q. Answer: Dipole in FieldAnswer: 2. Feels downward force. The forces shown produce a net downward force

*Force on Magnetic DipoleWhat makes the field pictured? Bar magnet below dipole, with N pole on top. It is aligned with the dipole pictured, they attract! NSm

Work Done by Interaction to Anti-align Magnetic Dipole*

Potential Energy: Dipole in Magnetic Field*Set zero reference point Lowest energy state (aligned) : Highest energy state (anti-aligned):

*Force on Magnetic DipoleNSNSalong z-axism

Magnetic Field Profiles Experiment 2

Concept Question: Dipole in HelmholtzA dipole pointing along the positive x-direction and located at the center of a Helmholtz coil will feel:a force but not a torque.a torque but not a force.both a torque and a force.neither force nor torque.

Concept Q. Answer: Dipole in HelmholtzAnswer: 2. a torque but not a force. The Helmholtz coil makes a UNIFORM FIELD.Dipole feels only torque (need gradient for force).

Concept Question: Dipole in Anti-Helmholtz CoilA dipole pointing along the positive z-direction and located at the center of an anti- Helmholtz coil will feel:a force but not a torque.a torque but not a force.both a torque and a force.neither force nor torque.

Concept Q. Answer: Dipole in Anti-Helmholtz CoilAnswer: 1. A force because there is a gradient in the magnetic field but no torque because the magnetic field at the center is zero.

Experiment 2:Magnetic Forces on Dipolein Fields of Helmholtz Coil

Getting Started: Set up current supplyOpen circuit (disconnect a lead)

Turn current knob full CCW (off)

Increase voltage to ~12 VThis will act as a protection: V

Appendix Experiment 2:Magnetic Forces on Dipolein Fields of Helmholtz Coil

Field Configurations and Concept Questions

Appendix 2:Gausss Law for Magnetism

*Animation: Magnetic Field Generated by a Current Loophttp://web.mit.edu/viz/EM/visualizations/magnetostatics/calculatingMagneticFields/RingMagInt/RingMagIntegration.htm

*Demonstration:Magnetic Field Linesfrom Bar Magnet G2http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%202&show=0

Class 18*Class 18Class 15*Class 18Class 12*Class 18Class 12*Class 18Class 12*Class 18Class 12*Class 18Class 12*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 15*Class 18Class 18*Class 18Class 15*Class 18Class 15*Class 18Class 12*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18Class 18*Class 18\begin{vmatrix}\vec{\tau}\end{vmatrix} = \mu B \sin \thetaW = -\int_0^\pi \tau d\theta = -\int_0^\pi \mu B \sin \theta \,d\theta

Class 15*Class 18Class 18*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Week 08, Day 2Week 08, Day 2Class 19Class 19*Week 08, Day 2Class 19*Class 18Class 18*Class 18Class 12*Class 18