Experiment 2 Magnetic Fields Force and Torque on

Experiment 2: Magnetic Fields, Force and Torque on a Magnetic Dipole W 07 D 2

Announcements Quiz 1 is next week. It is instead of the learning sequence. You can start it anytime Tuesday 6 pm through Thursday 6 pm but you have 1. 5 hr once you start it.

Magnetic Dipole: Current Loop Current loops generate a dipole field.

Current Loops are Magnetic Dipoles Magnetic Dipole Moment μ Magnitude: product of current an area of loop Direction: Perpendicular to the plane of the loop, using right hand rule with respect to direction of current. Points from South to North.

Magnetic Field of Bar Magnet (1) A magnet has two poles: North (N) and South (S) (2) Magnetic field lines leave from N and end at S Remember: electric dipole Field lines from positive (N) charge and go to negative (S). Bar magnets are dipoles, too.

Magnetic Monopoles? Electric Dipole When cut: 2 charges monopoles Magnetic Dipole When cut: 2 magn. dipoles Magnetic monopoles have not been found, so far ; -) Gauss’s Law Magnetic Gauss’s Law

Conservation of Magnetic Flux

Torque on a Current Loop in a Uniform Magnetic Field

Magnetic Force on Current-Carrying Wire If the wire is in a uniform magnetic field then where the direction of the vector is the direction of the current If the wire is also straight then where the direction of the vector is the direction of the current REVIEW

WE: Current Loop Place rectangular current loop in uniform B field I current a b What is the force on the current loop? What is the torque on the current loop?

WE: Current Loop Place rectangular current loop in uniform B field. Net force is zero. The torque on the loop is

Torque on Current Loop Magnetic moment points out of the page Definition of magnetic moment Torque tries to align the magnetic moment vector in the direction of the magnetic field

Magnetic Force on Current Loop In Uniform Magnetic Field If a current loop is placed in a uniform magnetic field then Because the vector integral of the line element around a closed path is zero And what about two magnets…. Do they exert a force on each other?

Magnetic Force on a Dipole in a Non-Uniform Magnetic Field

Force on Dipole in Non-uniform Magnetic Field Dipoles can feel magnetic force in a non-uniform magnetic field. Simple example: Magnetism – Bar Magnet: Like poles repel, opposite poles attract

Force on Dipole in Non-uniform Magnetic Field The forces shown produce a net downward force on dipole into the region of greater field strength.

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

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

Force on Magnetic Dipole N S Special case: Magnetic field points along z-axis Dipole is attracted to region of greater field strength

Helmholtz Whoever, in the pursuit of science, seeks after immediate practical utility may rest assured that he seeks in vain. – Quote from Helmholtz. Hermann Ludwid Ferdinand von Helmholtz 1821 -1894 Professor of Anatomy, Physiology and Physics