Magnetism

1. Magnetism AP-Physics

2. Magnetism • In physics, magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials. • Some well-known materials that exhibit easily detectable magnetic properties are nickel, iron, cobalt, gadolinium and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.

3. Magnets • By convention, in physics we say that magnetic fields leave the magnetic north pole of a magnet and enter the magnetic south pole.

4. Magnets • Iron filings can be placed on clear film with a bar magnet placed below the film to reveal the magnetic field lines of the magnet. • As you can see, the field lines look very similar to the electric field lines between two oppositely charged particles

5. The Earth as a Magnet • The Earth has a magnetic field associated with it due to the flow of its molten iron core. • Notice that the North Pole is really the magnetic south pole. • Compass needles point toward the direction of the Earth’s magnetic field, thus pointing north.

6. Source of Magnetism • Every electron, on account of its spin, is a small magnet. • In most materials, the countless electrons have randomly oriented spins, leaving a net magnetic effect of zero on average. • However, in magnetic substances, a majority of the electron spins are aligned in the same direction, so they act cooperatively, creating a net magnetic field.

7. Sources of Magnetism • Electron spin is not the only source of magnetism. • Hans Christian Oersted is credited with being the first person to deduce that moving currents also create magnetic fields. • While preparing for an evening lecture on April 21, 1819, Oersted developed an experiment which provided evidence that surprised him.

8. Sources of Magnetism • As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. • This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.

9. Charged Particles in a Magnetic Field • When moving through a magnetic field, a charged particle experiences a magnetic force. • This force is greatest when the particle enters the field perpendicular to the direction of the field (B). (θ = 90) • The force is zero when it moves along the field lines. • The force on the particle can be found with the formula: • F = qvBsinθ • F = force on particle • q = charge on particle • v = velocity of particle • B = magnetic field strength (measured in Teslas or N/A*m) • Small electric fields are sometimes measured in Gauss’s • 1 T = 1 x 104 G

10. A Few Typical B Values • Conventional laboratory magnets • 25000 G or 2.5 T • Superconducting magnets • 300000 G or 30 T • Earth’s magnetic field • 0.5 G or 5 x 10-5 T

11. Finding the Direction of Magnetic Force • The direction of the magnetic force is always perpendicular to both B and V • Fmax occurs when B is perpendicular to V. • F = 0 when B is parallel to V.

12. Open Hand Right Hand Rule • If we point our fingers in the direction of the magnetic field and let our thumb point in the direction of the moving charge, the magnetic force will leave our palm. • (you must use right hand and will only work for positive charges – negative charges will go out the back of your hand)

13. RHR Practice • Determine the direction of the force on a positive test charge on each of the 6 diagrams below. Use up page, down page, left, right, out of page, into page and no force as your possible answers.

14. Answers • A. out of page • B. Right • C. up the page • D. Right • E. Down • F. No force

15. Example Problem #1 • A proton moving to the right at 6 x 105 m/s enters a magnetic field of 8 Teslas directed into the page at an angle of 90°. What is the magnitude and direction of the force on the proton? • V = 6 x 105 m/s • B = 8 T • Θ = 90° • q = 1.6 x 10-19 C • F = (1.6 x 10-19)(6 x 105)(8)(sin90) • F = 7.68 x 10-13 N up the page

16. Example Problem #2 • A proton is moving vertically upward with a velocity of 4.2 x 105 m/s. The magnetic field is north with a strength of 2 T. What is the magnitude and direction of the force? • (change in terminology common in different texts) • Vertically upward – out of page • Vertically downward – into page • North – up page • South – down page • West – left • East - right • F =1.34 x 10-13 N to the left

17. Example #3 • An electron is moving west. It enters a magnetic field directed into the page. In what direction is the force on the charge? • North or up the page.

18. Particle in Magnetic Field • A charged particle moving in a plane perpendicular to a magnetic field will move in a circular orbit with the magnetic force playing the role of centripetal force. The direction of the force is given by the right-hand rule.

19. Example Problem #4 • Derive an equation for the radius of a charged particle moving in an magnetic field. • F = qvBsinθ • Fc = mac • ac = v2/r • mv2/r = qvBsinθ • r = mv/qB

20. Magnetism in a Wire • Because current is moving charges, we can conclude that there must be a magnetic force acting on a wire carrying an electric current. • The magnetic field due to a current carrying wire circulates around the wire in a direction given by what is usually called the closed right hand rule.

21. Closed Right Hand Rule • To determine the direction of the magnetic field around a wire, you just need to visualize yourself wrapping your right hand around the wire and pointing your thumb in the direction of the current flow.

22. Force on Wire in Magnetic Field • F = BILsinθ • F = magnetic force acting on wire • B = magnetic field strength • I = current in wire • L = length of wire • Θ = angle between I and B

23. Magnitude of the Field of a Long Straight Wire • The magnitude of the field at a distance r from a wire carrying a current of I is: • µo = 4  x 10-7 T.m / A • µo is called the permeability of free space • Direction still found with RHR.