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Chapter 21: Magnetic Forces and Magnetic Fields

Chapter 21: Magnetic Forces and Magnetic Fields. Where did the term “magnetism” originate?. 1 st person to study magnets: Thales (2600 yrs ago) Rocks (lodestones) from a town called Magnesia could attract bits of iron He called them “ho mangetes lithos” which means “ The Magnesian Rock ”

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Chapter 21: Magnetic Forces and Magnetic Fields

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  1. Chapter 21: Magnetic Forces and Magnetic Fields

  2. Where did the term “magnetism” originate? • 1st person to study magnets: Thales (2600 yrs ago) • Rocks (lodestones) from a town called Magnesia could attract bits of iron • He called them “ho mangetes lithos” which means “The Magnesian Rock” • Don’t get it confused with Milk of Magnesia…. That’s a laxative

  3. Similarities

  4. Gravitational/Electric/Magnetic FIELDS • Any mass will make a gravitational field, which will attract all other masses. • Any charge will make an electric field, which will attract or repel all other charges. • Any movingcharge will make a magnetic field, which will attract or repel any other moving charges.

  5. Magnets • Two poles (N & S) • Cut in two  each new magnet have 2 poles • Does a monopole exist? (one pole) • Types • Ferromagnetic: STRONGLYattracted to magnets (iron, cobalt, nickel, gadolinium, dysprosium) • Diamagnetic: weakly repelled by magnets (water, glass, copper, graphite, salt, lead, rubber, diamond, wood, some plastics) • Paramagnetic: weakly attracted to magnets (aluminum, oxygen, sodium, platinum and uranium)

  6. Permanent Magnets • “Fridge Magnets” • Stay magnetized • Ways to remove their magnetism: • Heat past Curie point • Hit them hard! • In AP Physics we only study magnetized iron and steel (numerous other strange materials can be used to make magnets…even ceramics)

  7. They are magnetic because e- are moving around the nuclei. • So, why isn’t everything a magnet?? • Ans: Magnetic field around each atom points in a random direction and when lots of them are together, they cancel each other out. • Hmmm… they are vectors (size, direction, and can cancel each other out)

  8. Magnetism is induced by aligning areas called domains within a magnetic field.

  9. Steel • Takes work to magnetize • Stays magnetized for days, weeks, or years • Takes work to demagnetize

  10. Iron • Easy to magnetize • Loses magnetism easily (seconds or less when removed from a magnetizer)

  11. Poles of Earth and Magnet

  12. Electric Fields vs. Magnetic Fields

  13. N S Magnetic Fields • Lines around the magnet show the direction and strength of the field. • Also called magnetic lines of flux • Travel through the magnet • Leave North pole • Travel through the air in a curve • Enter South pole • Line tangent to any point on a line of flux shows the direction of field • Lines closer together = stronger field • Direction of the field is N to S

  14. Magnetic field induces magnetism in iron filings. MAGNETIC FIELD The concept of a field is applied to magnetism as well as gravity and electricity. A magnetic fieldB surrounds every magnet and is also produced by a charged particle in motion relative to some reference point. The presence of the magnetic field about a bar magnet can be seen by placing a piece of paper over the bar magnet and sprinkling the paper with iron filings.

  15. N N N S Field Lines Between Magnets Unlike poles Attraction Leave N and enter S Repulsion Like poles

  16. A current loop is the same as a barmagnet; it has north and south faces.B field lines leave north "faces", enter at south "faces".  (In thiscase, the "faces" are left and rightside of the circular plane.)

  17. Nicola Tesla (1856-1943) Friedrich Gauss (1777-1855) Magnetic Fields • Symbol: B • Units: • Tesla (T) • Gauss (G) • Weber/meter square (Wb/m2)

  18. One Tesla (T) = 10,000 Gauss------------------------------------------ Source Magnetic Field        (Gauss) Earth          0.5 Appliance          10 Bar magnet         100 Human limit       2000 Large electro-magnet     50,000

  19. Forces and Fields: Place a charged up balloon in a magnetic field – nothing happens. Magnetic fields don’t affect stationary charges. Moving charge traveling through a magnetic field will experience a force. The force exerted will be perpendicular to the motion of the charge and perpendicular to the direction of the field. The result of the force is to cause a deflection of the charged particle. It gets pushed to the side.

  20. If charges move at an angle with respect to the directionof the magnetic field, they experience a sideways force. Magnetic Fields Exert Forces on Moving Charges

  21. Magnetic Force If charge q moves with velocity v through magnetic field B, it will experience a magnetic force FB with magnitude: is the angle between vand B

  22. A charge +q=6 X 10-6C moves with speed v=4X105 m/s through a magnetic field of strength B=0.4T, as shown in the figure below. What is the magnitude of the magnetic force experienced by q?

  23. The force F is always perpendicular to both v and B.

  24. Right-Hand Rule for Moving Charges F = q v B sin q

  25. Fields into and out of page “Heads”of vectors “Tails”of vectors

  26. Right Hand Rule for Force Fingers point in direction of magnetic field B. Thumb points in direction ofthe velocity vector v. Palm shows the direction of the force F. x into • out

  27. Positive • b) Negative

  28. Determine the direction of the force acting on the following charges: Out of page To left of page It’s a NEGATIVE CHARGE Top of page

  29. Differences between FE & FB

  30. A particle of mass m and charge +q is projected with velocity v into a uniform magnetic field B as drawn below. How will the particle move? Since v is perpendicular to B, the particle will feel magnetic force of strength qvB Since v is perpendicular to B, the particle will undergo uniform circular motion

  31. The magnetic force is always perpendicular to the velocity. Moving Charges in an Electric Field and a B-Field            Magnetic Field Electric Field

  32. Circular Paths in Magnetic Field Fmagnetic = Fcentripetal SIMULATION

  33. A particle of charge –q is shot into a region that contains an electric field, E, crossed with a perpendicular magnetic field, B. If E=2X104N/C and B=.5T, what must be the speed of the particle if it is to cross this region without being deflected? Electric force must cancel magnetic force if undeflected FE & FB point in OPPOSITE directions Electric force ↑ because negative Magnetic force ↓ because right hand rule

  34. A particle with charge +q, traveling with velocity v, enters a uniform magnetic field B, as shown below. Describe the particle’s subsequent motion. If particle’s velocity were parallel to B, then it would be unaffected by B. If v were perpendicular to B, then it would undergo uniform circular motion It’s got a little of both! The particle’s trajectory will be a helix

  35. The magnetic force is always perpendicular to the velocity. Moving Charges in an Electric Field and a B-Field            Magnetic Field Electric Field

  36. Magnetic Force on a Current-Carrying Wire • Magnetic fields affect moving charges • Guess what’s in a current-carrying wire?? • ANSWER: Moving charges • THEREFORE: Current-carrying wires feel a magnetic force when in a magnetic field The “Free Bill” Equation θ is angle between l and B

  37. F = BIlsin θThe direction of the force on the wire may be determined by a second right-hand rule.

  38. What is the direction of the force on the wire below?

  39. A U-shaped wire of mass m is lowered into a magnetic field B. How much current I must pass through the wire in order to cause the net force on the wire to be zero? FBT= FB1+FB2+FB3 FB1 = LEFT (right hand rule) FB3 = RIGHT (right hand rule) SAME LENGTH  THEY CANCEL FBT= FB2= UPWARD (right hand rule)

  40. A rectangular loop of wire that carries a current I is placed in a uniform magnetic field B, as shown in the diagram below. What torque does it experience? Into page Out of page Ignoring tiny gap in wire, There’s two l1 lengths two l2 lengths l2 are parallel to B Fl1right = out of page Fl1 left = into page Loop is FREE to move creating TORQUE turning the loop BOTH torques are in same direction:

  41. Magnetic Fields Created By Current-Carrying Wires • Remember: source of magnetic fields are electric charges that MOVE. • Spin, circulate, move through space, flow down a wire • Current generates a magnetic field in the surrounding space of magnitude: r is distance from wire μo is permeability of free space

  42. 3rd Right Hand Rule For Magnetic Field Created by Wire Fingers curl in direction of magnetic field lines

  43. The diagram below shows a proton moving with a speed of 200,000 m/s, initially parallel to, and 4cm from, a long, straight wire. If the current in the wire is 20A, what’s the magnetic force on the proton?

  44. The diagram below shows a pair of long, straight, parallel wires, separated by a small distance, r. If currents I1 and I2 are established in the wires, what is the magnetic force per unit length they exert on each other? Magnitude of magnetic force per unit length felt by Wire 2, due to magnetic field generated by Wire 1, is: It’s the same magnitude of force Wire 1 feels due to magnetic field of Wire 2. ATTRACTIVE FORCE If currents were reversed  wires would repel each other

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