1. Physics 152 Magnetism Walker, Chapter 22

2. 2 Magnetism Known since antiquity –Pieces of Magnetite, also called lodestone (Fe 3 O 4 ) known by Greeks to exert both forces of attraction and repulsion on each other Chinese invented compass for navigation –The earth exerts a force on magnetite.

3. 3 Basic model of Magnetic Materials All magnetic materials have two poles –Labeled: North and South poles Just as in electrostatics, Like repels like and opposites attract. N repels N, S repels S, N attracts S

4. 4 Magnetic Monopoles Unlike electrostatics: Magnetic monopoles have never been detected.

5. 5 Modern View of Magnetism (Oersted, Faraday, Maxwell, 19 th Century plus Quantum Mechanics – 20 th Century) charges in motionMagnetism is associated with charges in motion These currents can be microscopic currents in the atoms of magnetic materials.These currents can be microscopic currents in the atoms of magnetic materials. These currents can be macroscopic currents in the windings of an electromagnet.These currents can be macroscopic currents in the windings of an electromagnet.

6. 6 Magnetic Field Lines Magnetic field lines (outside the object) always go from the N pole to the S pole S N

7. 7 The Earth’s Magnetic Field The spinning iron core of the earth produces a magnetic field. The magnetic north pole corresponds to the geographic south pole...

8. 8 Magnetic Fields

9. 9 Magnetic Forces 1. Forces on moving charges 2. Forces on currents in wires or fluids particle is movingA charged particle in a static (not changing with time) magnetic field will experience a magnetic force only if the particle is moving. If a charge q with velocity v moves in a magnetic field B and v makes an angle  w.r.t. B, then the magnitude of the force on the charge is: |F| = | q v B sin  q v  B

10. 10 Magnetic Units We can determine the magnetic field by measuring the force on a moving charge: The mks unit of magnetic field is the Tesla (T). Dimensional analysis: 1 T = 1 N·s / (C·m) = 1 V ·s / m 2

11. 11 Direction of Magnetic Forces The magnetic force is in a direction perpendicular to the plane formed by B and v. To determine the direction, you must apply the right hand rule. F  v x = negative z direction (into the plane of paper) = positive z direction

12. 12 Direction of Magnetic Forces + v q x B palm: B fingers: v thumb: F F

13. 13 Right Hand Rule Draw vectors v and B with their tails at the location of the charge q. Point fingers of right hand along velocity vector v. Curl fingers towards Magnetic field vector B. Thumb points in direction of magnetic force F on q, perpendicular to both v and B.

14. 14 Walker, Problem 22-6 A particle with a charge of 14 µC experiences a force of 2.1x10 -4 N when it moves at right angles to a magnetic field with a speed of 28.1 m/s. What force does this particle experience when it moves with a speed of 7.0 m/s at an angle of 24.6° relative to the magnetic field? answer: 2.17x10 -5 N.

15. 15 Question A negative charge q is moving in the +y direction with velocity v. The magnetic field B is in the +x direction What is the direction of the force on q? a.-y direction b.Force is zero c.+z direction d.-z direction +z x y v B

16. 16 Force on a Current Carrying Wire Recall that a current in a wire is a collection of moving charges; therefore, a current carrying wire in a magnetic field also experiences a force. If a wire of length L, carrying a current I, makes an angle  with a magnetic field B, then the total force on the wire is: |F| = |I L B sin      F

17. 17 Right-Hand-Rule for Magnetic Force on a Wire Direction of magnetic force  L B sin  on wire is perpendicular to direction of I and to direction of B. Orientation of Force is determined by curling fingers from direction to I to direction of B.

18. 18 Motion of Charges in B Fields If a charged particle is moving in a direction perpendicular to a uniform magnetic field, then its trajectory will be a circle because the force F=qvB is always perpendicular to the motion, and therefore centripetal. Recall that so From which we find the radius of the circular trajectory is:

19. 19 Mass Spectrometer ++++  Ion source Position Sensitive Detector E  B Velocity Selector                                                                                                                                 sample Magnetic Field  to drawing qB mv r orbition of radius 

20. 20 Mass Spectrometer Measures m/q of ions