Physics 121 - Electricity and Magnetism Lecture 09 - Charges & Currents in Magnetic Fields Y&F Chapter 27, Sec. 1 - 8 What Produces Magnetic Field? Properties of Magnetic versus Electric Fields Force on a Charge Moving through Magnetic Field Magnetic Field Lines A Charged Particle Circulating in a Magnetic Field – Cyclotron Frequency The Cyclotron, the Mass Spectrometer, the Earth’s Field Crossed Electric and Magnetic Fields The e/m Ratio for Electrons Magnetic Force on a Current-Carrying Wire Torque on a Current Loop: the Motor Effect The Magnetic Dipole Moment Summary

Magnetic Field Electrostatic & Gravitational forces act through vector fields Now: Magnetic force on moving charge acts through Force law: Effect of a given B field on charges & currents in wires Next week: How to create B field. Currents in loops of wire, moving charges Intrinsic spins of e-,p+  elementary currents  magnetic dipole moment Spins can align permanently to form natural magnets B & E fields  Maxwell’s equations, electromagnetic waves (not covered) “Permanent Magnets”: North- and South- seeking poles Natural magnets interact via field Compass – Earth has a magnetic field Ferromagnetic materials magnetize when cooled in B field (Fe, Ni, Co) - - Spins align into“domains” (magnets) Other materials (plastic, copper, wood,…) at most slightly affected (para- and dia- magnetism) REPELS ATTRACTS DIPOLES ALIGN IN B FIELD UNIFORM N S

Similarities: Electric Field versus Magnetic Field Electric force acts at a distance through electric field. Vector field, E. Source: electric charge. Positive (+) & negative (-) charge. Opposite charges attract Like charges repel. Field lines show the direction and magnitude of E. Magnetic force acts at a distance through magnetic field. Vector field, B Source: moving electric charge (current, even in permanent magnets). North pole (N) and south pole (S) Opposite poles attract Like poles repel. Field lines show the direction and magnitude of B.

Differences between magnetic & electrostatic field Test charge and electric field Single electric poles exist Magnetic “charge” p and magnetic field ? Single N or S magnetic poles have never been seen. Magnets always occur as dipole pairs.. Cut up a bar magnet small, complete magnets Electrostatic Gauss Law Magnetic Gauss Law There is no magnetic monopole... ...dipoles are the basic units Magnetic field exerts force on moving charges (current) only Magnetic flux through each and every Gaussian surface = 0 Magnetic field lines are closed curves - no beginning or end

Magnetic Force on a moving Charged Particle Particle moves with a velocity v through a magnetic field B Typical Magnitudes: Earth’s field: 10-4 T. Bar Magnet: 10-2 T. Electromagnet: 10-1 T. “LORENTZ FORCE” Units: 1 “GAUSS” = 10-4 Tesla See Lecture 01 for cross product definitions and examples FB is proportional to speed v, charge q, and field B. FB depends on complex geometry using cross product FB = 0 if v is parallel to B. FB is normal to plane of both v and B. FB reverses sign for opposite sign of charge Strength of B field created also depends on qv [current x length]. Electrostatic force can do work on a charged particle…BUT… … magnetic force cannot do work on moving particles since FB.v = 0.

Magnetic force geometry examples: A simple geometry x y z charge +q B along –z direction v along +x direction CONVENTION Head Tail More simple examples charge +q Fm is down |Fm| = qv0B0 x charge -q B into page Fm is still down Fm is out of the page charge +q or -q v parallel to B |Fm| = qv0B0sin(0)

A numerical example Electron beam moving in plane of sketch force for +e Electron beam moving in plane of sketch v = 107 m/s along +x B = 10-3 T. out of page along +y z axis is down a) Find the magnetic force on an electron : Negative sign means force is opposite to result of using the RH rule b) Find the electron’s acceleration : Direction is the same as that of the force: Fov = 0

When is magnetic force = zero? 9-1: A particle in a magnetic field is found to have zero magnetic force on it. Which of the situations could not be the case? The particle is neutral. The particle is stationary. The motion of the particle is parallel to the magnetic field. The motion of the particle is opposite to the magnetic field. All of them are possible.

Direction of Magnetic Force 9-2: The figures show five situations in which a positively charged particle with velocity v travels through a uniform magnetic field B. For which situation does the magnetic force point along the +x axis ? Hint: Use Right Hand Rule. x z y B v A C D E

Magnetic Units and Field Line Examples SI unit of magnetic field: Tesla (T) 1T = 1 N/[Cm/s] = 1 N/[Am] = 104 gauss At surface of neutron star 108 T Near big electromagnet 1.5 T Inside sunspot 10-1 T Near small bar magnet 10-2 T At Earth’s surface 10-4 T In interstellar space 10-10 T Magnetic field lines – Similarities to E The tangent to a magnetic field line at any point gives the direction of B at that point (not direction of force). The spacing of the lines represents the magnitude of B – the magnetic field is stronger where the lines are closer together, and conversely. Differences from E field lines B field direction is not force direction on charges B field lines have no beginning or end – closed curves But magnetic dipoles will align with B field

Charged Particles Circle at Constant Speed in a Uniform Magnetic Field: Centripetal Force Uniform B along z, v in x-y plane tangent to path FB is normal to both v & B ... so Power = F.v = 0. Magnetic force cannot change particle’s speed or KE. Force direction does change A charged particle moving in a plane perpendicular to a B field circles in the plane with constant speed v FB is a centripetal force (Uniform Circular Motion) x y z v Fm B CW rotation Set Radius of the path Period Cyclotron angular frequency tc and ωc do not depend on velocity. Fast particles move in large circles and slow ones in small circles All particles with the same charge-to-mass ratio have the same period. Positive and negative particles rotate in opposite directions. If v not normal to B particle spirals around B

Cyclotron particle accelerator Early nuclear physics research, Biomedical applications gap magnetic field + - Inject charged particles in center at S Charged plates (“Dees”) reverse polarity of E field in gap with frequency wc. This accelerates particles crossing gap. Particles spiral out in magnetic field as they gain KE and are detected. Frequency of Dee polarity reversal can be constant! It does not depend on speed or radius of path.

Earth’s field shields us from the Solar Wind and produces the Aurora Earth’s magnetic field deflects charged solar wind particles (via Lorentz force) . Protects Earth . Makes life possible (magnetosphere). Solar wind particles spiral around the Earth’s magnetic field lines producing the Aurora at high latitudes. They can also be trapped.

Circulating Charged Particle 9-3: The figures show circular paths of two particles having the same speed in a uniform magnetic field B, which is directed into the page. One particle is a proton; the other is an electron (much less massive). Which figure is physically reasonable? A B C D E

Charged particle in both E and B fields Add the electrostatic and magnetic forces Does F = ma change if you observe this while moving at constant velocity v’? Velocity Selector: Crossed E and B fields B out of paper E up and normal to B q is + charge v normal to both E & B CONVENTION OUT IN Free Body Diagram for q E B v Fe Fm OPPOSED FORCES Equilibrium when... Independent of charge q Charges with speed v are un-deflected Can select particles with a particular velocity

Measuring e/m ratio for the electron (J. J. Thompson, 1897) + - e- CRT screen y electron gun L For E = 0, B = 0 beam hits center of screen Add E field in –y direction…  y = deflection of beam from center Add a crossed B field (into page, E remains) FM points along –y, opposite to FE (negative charge) Adjust B until beam deflection y = 0 (FE cancels FM) to find vx and time t flight time t = L / vx ( vx is constant)

Mass spectrometer - Another cyclotron effect device: Separates molecules with different charge/mass ratios Ionized particle paths have separate radii, forming “mass spectrum” Molecular mass Ionized molecules or isotopes are accelerated varying masses & q/m same potential V & KE some types use velocity selector here Mass spectrum of a peptide shows the isotopic distribution. Relative abundances are plotted as functions of the ratio of mass m to charge z.

Force due to crossed E and B fields 9-4: The figure shows four possible directions for the velocity vector v of a positively charged particle moving through a uniform electric field E (into the page) and a uniform magnetic field B (pointing to the right). The speed is E/B. Which direction of the velocity produces zero net force? E A v v D B v B v C

Force on a straight wire carrying current in a B field Current is flowing opposite to electron’s drift velocity vd Lorentz force on electron = - evd x B (normal to wire) Motor effect: wire is pushed right by the charges dq = idt is the charge moving in wire segment whose length dx = -vddt. dx is along current direction. iL uniform B Fm Integrate along the length L of the wire (assume B is uniform )

Motor effect on a wire: which direction is the pull? B F = 0 F is into slide F is out of slide A current-carrying loop experiences a torque (but zero net force)

Torque on a current loop in a magnetic field z x Upper sketch: y-axis toward viewer Lower sketch: y-axis toward right Loop can rotate about y-axis B field is along z axis Apply to each side of the loop |F1|= |F3| = iaB: Forces cancel but net torque is not zero! |F2|= |F4| = i.b.Bcos(q): Forces cancel, Same line of action  zero torque z into paper x y Moment arms for F1 & F3 equal b.sin(q)/2 Force F1 produces CW torque equal to t1 = i.a.B.b.sin(q)/2 Same for F3 Torque vector is along –y (rotation axis)

Torque on a current loop in B field z x Restoring Torque: Maximum torque: Oscillates about Current loop in B field is like electric dipole maximum torque zero

Current loops are basic magnetic dipoles q Represents a current loop as a vector y z (B) -x q Magnetic dipole moment m measures strength of response to external B field strength as source of a dipole field

Comparison: Magnetic & Electric Dipole Moments TORQUE POTENTIAL ENERGY MAGNETIC DIPOLE ELECTRIC MOMENT SAMPLE DIPOLE VALUES [J / T] m ~ 1.4×10-26 J/T Proton (intrinsic) m ~ 9.3×10-24 J/T Electron (intrinsic) m ~ 8.0×1022 J/T Earth m ~ 5 J/T Small bar magnet

MOVING COIL GALVANOMETER Basis of most 19th & 20th century analog instruments: voltmeter, ohmmeter, ammeter, speedometer, gas gauge, …. Coil spring calibrated to balance torque at proper mark on scale ELECTRIC MOTOR: Reverse current I when torque t changes sign (q = 0 ) Use mechanical “commutator” for AC or DC Multiple turns of wire increase torque N turns assumed in flat, planar coil Real motors use multiple coils (smooth torque)

The DC Motor Maximum Torque CCW Zero Torque Cross Commutator Gap (Reversed)

Torque and potential energy of a magnetic dipole 9-5: In which configuration (see below) does the torque on the dipole have it’s maximum value? 9-6: In which configuration (see below) does the potential energy of the dipole have it’s smallest value? B A C D E