PHYSICS UNIT 8: MAGNETISM

1. PHYSICS UNIT 8: MAGNETISM

2. MAGNETISM • Magnetic Fields are always dipolar: N and S poles • law of magnetism: like poles repel, unlike poles attract • magnetic force is directly proportional to the magnets’ strength, inversely proportional to the square of the distance between the magnets • only certain materials can be magnetized – ferromagnetic materials(Fe,Co,Ni ores/alloys)

3. MAGNETISM • Magnets are always dipoles: north-seeking N pole, south-seeking S pole

4. MAGNETISM • magnetic field: region of magnetic force • detect using a compass (needle points in field direction) • field runs N to S outsidemagnet, S to N inside magnet

5. • MAGNETISM • magnetic field strength, B: strength of the magnetic field,in Tesla, T • earth’s magnetic field at the surface = 0.5 x 10-4 T • field directions: •  left right  • up  down  • out toward you in away from you ×

6. MAGNETISM • Earth is a magnet • north geographic pole is an S pole • field like a bar magnet

7. MAGNETISM • Source of Magnetism • electrons are magnetic due to “spin” • paired electrons: opposite “spin”, cancel out • occurs in most materials • unpaired electrons  magnetic atoms • occurs in ferromagnetic materials

8. MAGNETISM • magnetic domains: regions of aligned magnetic atoms • magnetic object: alignedmagnetic domains

9. ELECTROMAGNETISM • Field Around A Wire: a current (moving charge) produces a magnetic field encircling the wire • right hand rule: grasp conductor in right hand, with thumb pointing in I direction; fingers will circle wire in B direction

10. ELECTROMAGNETISM • magnetic field strength around a wire depends on amount of current in wire, & distance from wire • B=m0I/2pr B: magnetic field strength (T) I: current in wire (A) r: distance from wire (m) m0=4p×10-7 Tm/A

11. ELECTROMAGNETISM • Field Around A Solenoid(coil) • like bar magnet when carrying current • add iron core: electromagnet(much stronger field) • solenoid field strength, B=m0nIn = #loops/length of coil

12. ELECTROMAGNETISM • right hand rule for solenoids: grasp coil in right hand, with fingers circling coil in I direction; thumb will point to N pole • core is pulled into solenoid when turned on (striker, switch)

13. ELECTROMAGNETISM • parallel wires exert forces on each other due to their magnetic fields • currents in same direction – wires attract • currents in opposite direction – wires repel

14. PHYSICS UNIT 8: MAGNETISM

15. MAGNETIC FORCES • cathode ray tube (TV & monitor picture tube)

16. MAGNETIC FORCES • cathode ray tube (TV & monitor picture tube)

17. MAGNETIC FORCES • Magnetic fields exert force on moving charges • magnetic force on a moving charge, • F = qv x B (vector cross product) • F: force (N) • B: field strength (T) • q: particle charge (C) • v: speed (m/s)

18. MAGNETIC FORCES • F is perpendicular to v and B • can only make the particle turn • F is a centripetal force • Bqv = mv2/r • mass spectrometer: shoot particles into magnetic field to find their massm = Bqr/v

19. proton: electron: v N S S N v MAGNETIC FORCES • right hand rule for + particles, left hand rule for – particles: point thumb in v direction, index finger in B direction (N to S); other fingers will bend in F direction

20. MAGNETIC FORCES • Magnetic fields exert force on current-bearing wires • magnetic force on a wire, F = BIL • B: magnetic field strength (T) • I: current (A) • L: length of wire in field (m) • for a coil, L = length of wire in field × # of loops

21. × × × × × × × × × × × × •• •• •• •• •• •• I I MAGNETIC FORCES • right hand rule for wires: point thumb in I direction, index finger in B direction; other fingers will bend in F direction

22. PHYSICS UNIT 8: MAGNETISM

23. ELECTROMAGNETIC INDUCTION • Electromagnetic Induction: a voltage (and a current) is induced in a wire when it moves in a magnetic field • induced voltage V = BLv • V: potential difference between ends of wire (V) • L: wire length inside field (m) • v: wire speed (m/s) • induced current I = V/R

24. ELECTROMAGNETIC INDUCTION • either the wire or the field can move • wire must cut across field lines to induce a voltage

25. × × × × × × × × × × × × •• •• •• •• •• •• v v ELECTROMAGNETIC INDUCTION • right hand rule for electromagnetic induction: point thumb in v direction, index finger in B direction; other fingers will bend to point I direction

26. ELECTROMAGNETIC INDUCTION • Generators: spinning a coil in a B field induces current in the coil Green wire voltage

27. ELECTROMAGNETIC INDUCTION • generator current is alternating current (AC) • frequency: current cycles/sec (Hz) = coil rotation rate • DC generator: coil connects to commutator, which switches external connection when current switches

28. ELECTROMAGNETIC INDUCTION • Lenz’s Law: an induced current generates a magnetic field which opposes the change that induced it • mechanical energy must go in to get electrical energy out • ex: The Drop Zone - magnets on cars move past metal fins; current is induced in fins which takes energy from the cars, slowing themdown

29. PHYSICS UNIT 8: MAGNETISM

30. m0 = 4p×10-7 Tm/A B = m0I/2pr B = m0nI n = #loops/length of coil F = Bqv m = Bqr/v F = BIL V = BLv V = IR e = +/- 1.60x10-19 C right hand rule for field around a wire right hand rule for field around a solenoid right (or left) hand rule for force on particles right hand rule for force on wires right hand rule for induced voltage UNIT 8 REVIEW