1. Magnetism

2. Magnets You have likely played with magnets in the past. But probably not very formally.

3. Magnets are interesting Origin: Magnetite is a naturally occurring magnetic mineral First record of discovery of properties from 2600 years ago Named after region where it was discovered: the Greek island of Magnesia

4. Magnets are interesting The connection is closer than you think! Thanks, Einstein (and Derek Muller for explaining it so lucidly) explaining

5. Magnets are interesting Properties: All magnets have north and south pole: likes repel; opposites attract If you were to split a magnet in two, you would end with two magnets, each with a north and south pole. True even to the atomic level!

6. Magnetic field The force exerted on other objects can be described with a ‘magnetic field’. Like the electric field, imaginary but useful for describing interactions

7. Magnetic field Conventions Points away from north pole, towards south pole Lines never cross Tighter lines = more intense field “Flux”,  Greek letter phi Pronounced ‘fee’ Measures magnetic field strength per area

8. Magnets and currents A long wire carrying current creates a circular magnetic field Magnetism is connected to electricity In 1820, Danish scientist Hans Christian Oersted noticed deflection of compass needle in presence of electric current. Laid the foundation for Henry, Faraday, Maxwell, Tesla, and Einstein. A loop of wire carrying current creates a linear magnetic field See more: Magnetic Field Demonstrations Simple Wire CoilsMagnetic Field Demonstrations Simple Wire Coils

9. Right-hand rule #1 1.Point your thumb in the direction of the flow of current 1.by convention, from positive terminal to negative terminal 2.Imagine curling the fingers of your right-hand around the wire. 3.The circular magnetic field around a long current-carrying wire goes in the same direction as your fingers.

10. Magnetic forces on moving charged particles Positively charged particles move into page. Passing through a magnetic field goes from north to south (here: right to left) Moving particles experience upward force.

11. Magnetic forces on moving charged particles Positively charged particles move into page. Passing through a magnetic field goes from north to south (here: left to right) Moving particles experience downward force.

12. Magnetic forces on moving charged particles Positively charged particles come out of the page. Passing through a magnetic field goes from north to south (here: right to left) Moving particles experience downward force.

13. Right-hand rule #2 1.Point your right forefinger in the same direction as the movement of charge. 2.Point your middle finger in direction of magnetic field. 3.Your thumb points in the direction of the force POSITIVELY-charged moving particles will experience.

14. In real life… Walter Lewin Walter Lewin ~10:00 – 11:50

15. Try it! Positively charged particle come out of page Passing through a magnetic field goes from north to south (here: left to right) In what direction will the particles feel a force?

16. Try it Negatively charged particles move into page. Passing through a magnetic field goes from north to south (here: right to left) In what direction will the particles feel a force?

17. Think about it… Is this drawing accurate?

18. Applications Televisions and mass spectrometers use precisely this physical principal to work.

19. Mathematical model

20. Force is greatest when  = 90 . Current running perpendicular to magnetic field experiences force. Force is zero when  = 0  Current running parallel to magnetic field experiences no force.

21. Quantifying magnetic field strength

22. How big is a tesla? FactorExample 10 -12 Primates’ brains 10 -9 Magnetic strength of heliosphere 10 -6 Strength of magnetic tape near tape head 10 -5 Strength of Earth’s magnetic field 31  T near equator; 58  T @ 50  10 -3 Strength of typical refrigerator magnet 10 0 Typical coil gap of typical loudspeaker Strength of coin-sized neodymium magnet 10 1 Strength used to levitate froglevitate frog 10 2 Strongest pulsed magnet produced in lab 10 6 Strength of neutron star

23. Example A loop of wire (30 cm long and 10 cm wide) is partially suspended in a magnetic field and hangs from a scale that reads zero when the current is zero. If the scale reads 3.48 x 10 -2 N when current of 0.245 A passes through the wires, calculate the strength of the magnetic field. The magnetic field points into the page.

24. Solution Use the right-hand rule to compare the direction of the force on the downward portion of the loop with the direction of the force on upward portion of the loop. Use the right-hand rule to determine the direction of the force experienced by the bottom portion of the loop.

25. Example Try it first. Then delete this box. This process is used to precisely calculate the strength of electric fields.

26. Solution What would the force on the coil be if it were entirely inside the magnetic field?

27. Mathematical model v2

28. Example A negative charge –Q is placed at rest near a magnet. In what direction will it move? In what direction would a positively charged particle +Q move? @ v = 0 m/s, F = 0 N It won’t move at all! Try it first. Then delete this box.

29. Example A proton moving vertically upward at a speed of 5.0 x 10 6 m/s passes through a magnetic field. When it passes through the field, it experiences a 8.0 x 10 -14 N push to the west. If a northward-moving proton experiences 0 N, a)In what direction is the magnetic field in this area? b)How strong is the field in this area?

30. Solution - a A proton moving vertically upward passes through a magnetic field. When it passes through the field, it experiences a push to the west. A magnetic field exerts a force towards the west on a proton moving vertically upward If a northward-moving proton experiences 0 N, in what direction is the magnetic field in this area? Experiences force to the west By right-hand rule, upward traveling positive charge experiencing westward force must be traveling through field pointing north. Try it first. Then delete this box.

31. Solution - b A proton moving at a speed of 5.0 x 10 6 m/s experiences a 8.0 x 10 -14 N push to the west. How strong is the field in this area? Try it first. Then delete this box. This process is used to precisely calculate the charge and mass of particles.

32. There’s more! Walter Lewin