BoardWork

1. BoardWork Indicate which pole of the magnet is north and which is south. (The blue lines represent magnetic field lines.)

2. Magnets and Currents interacting and inseparable

3. Objectives • Describe the magnetic field caused by an electrical current. • Determine the force on an electric charge in a magnetic field.

4. What’s the Point? • What is the force that underlies electric motors?

5. Current Creates Magnetism An electric current creates a magnetic field. Look, Ma! No poles!

6. Left Right Up Down In Out Vector Direction Conventions

7. I A. D. B. E. C. F. Poll Question If a wire in front of you carries a current from left to right, what is the direction of the resulting magnetic field where you are?

8. Board Work An electric current creates a magnetic field whose lines circle right-handed around it. Draw lines for the magnetic field created by a ring of current. I

9. N dipole field S Magnetic Field of Current Ring Source: Griffith, The Physics of Everyday Phenomena

10. N S Solenoid Magnetic Field Source: Griffith, The Physics of Everyday Phenomena

11. Electrons are Magnets! spin

12. Electrons are Magnets! current

13. Electrons are Magnets! magnetic field

14. Electrons are Magnets! N magnetic dipole S

15. Types of Magnets • Electromagnets • currents travel through conducting coils • Permanent Magnets • materials whose electrons have aligned spins or orbits Moving charges create the fields!

16. The Lorentz Force making electrons work for us

17. Magnetic Force on a Charge • Currents create magnetic fields. • Currents are made of moving charges. • Moving charges are magnets. • Magnets apply forces to each other. • Magnets apply forces to moving charges. How do those forces behave?

18. F = qv  B Lorentz Force qv Source: Griffith, The Physics of Everyday Phenomena Right-Hand Rule

19. A. D. B. E. C. F. Poll Question What is the direction of the cross product A B? A B

20. Cross Product Review a A q B b AB = area of parallelogram

21. a A q B b AB=–(BA) Cross Product Review • Curl right-hand fingers in direction of q • Right-hand thumb points in direction of cross-product • Not commutative

22. If q, v or B reverse, direction of F reverses Lorentz Force Properties • F = 0 unless charge is moving • F = 0 if velocity is  to field • F = maximum if velocity is  to field • F  0 only if charge crosses B field lines

23. paper square Lorentz Force creases:in out qv magnetic current field vectors Make an Origami Right Hand fold over

24. B v Think Question What is the direction of the force on the object moving with velocity v through magnetic field B? A. D. + B. E. C. F.

25. B q 2q v v Group Poll Question What is the magnitude of the force on object A compared to the magnitude of the force on object B? • FB = 4FA • FB = 2FA • FB = FA • FB = FA/2 • FB = FA/4 A B

26. B q 2q v v Group Poll Question How does the work done on object A compare to the work done on object B? A B • wB > wA. • wB = wA. • wB < wA.

27. F = qvB does not explicitly include a reaction force. • Magnetic fields (B) are always created by moving charges. • Moving charges (qv) always create magnetic fields. • The moving charge creating B “feels” the field of qv. So F = qvB goes both ways. Lorentz and Newton’s third law Here’s where it is:

28. Board Work From F = qv× B, find the SI unit of magnetic field B.

29. Challenge Question A current runs through one wire of a pair of parallel wires. What is the direction of the resulting magnetic field at the location of the other wire? I ?

30. Force between parallel currents What is the force on this current?I B

31. Definition of Ampere • If two parallel wires are held 1 m apart, • with currents of 1 A through each wire, • the attractive force between the wires is 2  10–7 N for each meter of length of the wires.

32. Reading for Next Time • Faraday’s law • Big Ideas • A changing magnetic field can create an electric potential • AC Transformers can be understood using Faraday’s law and conservation of energy • Very abstract • You are ready • It is very cool