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Field Around Magnet

Use a compass to map the direction of the magnetic field surrounding a magnet. Use iron filings to visualize the field lines of different magnets. Explore the relationship between field strength, distance, and the magnet's poles.

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Field Around Magnet

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  1. Field Around Magnet • Use a compass to map the direction of the magnetic field surrounding a magnet. • White board your results. In particular: • how does the strength of the field vary with distance from the wire? • how does the field direction relate to the poles of the magnet? Magnetism

  2. Activity: Map Field of Magnets • Use iron filings to map the field of a • bar magnet • horseshoe magnet • White board results • draw field lines. • how might magnets generate magnetic fields? Magnetism

  3. Magnetic Field Lines • direction of magnetic field, B, is parallel to field line • number of lines per area is proportional to strength of field • field lines point • from N to S • field lines formclosed loops Magnetism

  4. Magnetism No magnetic monopoles! Magnetism

  5. Magnets are similar to Electric Dipoles Magnetism

  6. Ferromagnetism • Ferromagnetic material • iron or other materials that can be made into magnets • You can make a magnet from iron by placing it in a strong B field • individual domains become aligned with external B field • Loss of magnetism from: • dropping • heating • Curie temperature • 1043 K for iron Preferentially downwards Random Magnetism

  7. Cross Product – Right Hand Rule Magnetism

  8. Specifying 3 Dimensions • out of page • tip of arrow • into page • tail of arrow Magnetism

  9. Force on a moving charge • Right Hand Rule (#2) • qv = fingers • B = bend fingers • F = thumb • Find the direction of the force on a negative charge for each diagram shown. Magnetism

  10. Magnetism

  11. Think-Pair-Share • Derive an expression for the radius of an e-’s orbit in a uniform B field. Express your answer in terms of me, v, qe, and B. Turn in your solution! Magnetism

  12. Earth’s Magnetic Field • magnetic declination • angular difference between geographic north and magnetic north • varies with latitude Magnetism

  13. Tactics: Right-hand rule for fields

  14. The Source of the Magnetic Field: Moving Charges The magnetic field of a charged particle q moving with velocity v is given by the Biot-Savart law: where r is the distance from the charge and θ is the angle between v and r. The Biot-Savart law can be written in terms of the cross product as

  15. EXAMPLE 33.1 The magnetic field of a proton QUESTION:

  16. EXAMPLE 33.1 The magnetic field of a proton

  17. EXAMPLE 33.1 The magnetic field of a proton

  18. EXAMPLE 33.1 The magnetic field of a proton

  19. The Magnetic Field of a Current The magnetic field of a long, straight wire carrying current I, at a distance d from the wire is The magnetic field at the center of a coil of N turns and radius R, carrying a current I is

  20. EXAMPLE 33.4 The magnetic field strength near a heater wire QUESTION:

  21. EXAMPLE 33.4 The magnetic field strength near a heater wire

  22. Practice Problems • Magnetism: Worksheets 1 and 2 • Finish before next class Magnetism

  23. Tactics: Finding the magnetic field direction of a current loop

  24. Magnetic Dipoles The magnetic dipole moment of a current loop enclosing an area A is defined as The SI units of the magnetic dipole moment are A m2. The on-axis field of a magnetic dipole is

  25. EXAMPLE 33.7 The field of a magnetic dipole QUESTIONS:

  26. EXAMPLE 33.7 The field of a magnetic dipole

  27. Tactics: Evaluating line integrals

  28. Ampère’s law Whenever total current Ithrough passes through an area bounded by a closed curve, the line integral of the magnetic field around the curve is given by Ampère’s law:

  29. The strength of the uniform magnetic field inside a solenoid is where n = N/l is the number of turns per unit length.

  30. The Magnetic Force on a Moving Charge The magnetic force on a charge q as it moves through a magnetic field B with velocity v is where α is the angle between v and B.

  31. Magnetic Forces on Current-Carrying Wires Consider a segment of wire of length l carrying current I in the direction of the vector l. The wire exists in a constant magnetic field B. The magnetic force on the wire is where α is the angle between the direction of the current and the magnetic field.

  32. EXAMPLE 33.13 Magnetic Levitation QUESTION:

  33. EXAMPLE 33.13 Magnetic Levitation

  34. General Principles

  35. General Principles

  36. General Principles

  37. Applications

  38. Applications

  39. Applications

  40. Does the compass needle rotate clockwise (cw), counterclockwise (ccw) or not at all? • Clockwise • Counterclockwise • Not at all

  41. Does the compass needle rotate clockwise (cw), counterclockwise (ccw) or not at all? • Clockwise • Counterclockwise • Not at all

  42. The magnetic field at the position P points • Into the page. • Up. • Down. • Out of the page.

  43. The magnetic field at the position P points • Into the page. • Up. • Down. • Out of the page.

  44. The positive charge is moving straight out of the page. What is the direction of the magnetic field at the position of the dot? • Left • Right • Down • Up

  45. The positive charge is moving straight out of the page. What is the direction of the magnetic field at the position of the dot? • Left • Right • Down • Up

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