Mastering Magnetism: Key Concepts and Practice Problems

1. Magnetism – Part 3

2. Calendar • Monday – Recorded Lecture. • Today – Brief review of the material + a few problems. • Today, Friday we will continue with chapter 20 materials. • There will be a Quiz on Magnetism on Friday.

3. About those exams - • Grades look pretty bad. I will review on Friday after I have a look at the papers. • Each exam (both sections) had similar problems. • A Kirchoff Law Problem – simple • A combine either capacitors or resistors and calculate what was happening at one of them. • A problem involving polarization – a thinker. • A question on how much energy was required to bring three charges together.

4. These were the hints I gave you! • Anything in the three chapters is fair game. • Read the sections on charge and charge effects very carefully. We didn’t cover some of this in class. (Problem 2) • Know the difference between Potential and Potential energy. • Know how much work it takes to create a charge distribution. We did it in class. (Problem 1) • Know how to add capacitors and resistors and how to solve simple circuitproblems. (Problem 3) • There WILL be a Kirchhoff's Law problem. (Problem 4) • Coulomb’s Law and the addition of forces. Calculation of the potential (scalar) • Be sure to understand all of the HW problems that were assigned – or not assigned!

5. The force is perpendicular to the direction of motion. The force has a constant magnitude = Bqv This will produce circular motion as in PHY2053.

7. Look at the direction of the force and the velocity

10. Off Angle P

11. PROBLEM • An electron at point A in the figure has a speed v0 of 1.4 x 106 m/s. Find • the magnitude and direction of the magnetic field that will cause the electron to follow the semicircular path from A to B and • the time required for the electron to move from A to B. • What magnetic field would be needed if the particle were a proton instead of an electron? m=9.1E-31 Kg e=1.6E-19 C

12. Force on a Wire Carrying a Current in a B Field

13. A straight vertical wire carries a current of 1.20 A downward in a region between the poles of a large electromagnet where the field strength is 0.588 T and is horizontal. What are the magnitude and direction of the magnetic force on a 1.00 cm section of this wire if the magnetic-field direction is • toward the east, • (b) toward the south

14. Novel applications have been devised to make use of the force that a magnetic field exerts on a conductor carrying current.

15. Current Loop What is force on the ends?? Loop will tend to rotate due to the torque the field applies to the loop.

16. pivot The Loop (From the top) OBSERVATION Force on Side 2 is out of the paper and that on the opposite side is into the paper. No net force tending to rotate the loop due to either of these forces. The net force on the loop is also zero,

17. The other sides t1=F1 (b/2)Sin(q) =(B i a) x (b/2)Sin(q) total torque on the loop is: 2t1 Total torque: t=(iaB) bSin(q) =iABSin(q) (A=Area)

18. Application: The motor • If the conductor is a loop, the torque can create an electric motor.

22. A circular coil of wire 8.6 cm in diameter has 15 turns and carries a current of 2.7 A. The coil is in a region where the magnetic field is 0.56 T. What orientation of the coil gives the maximum torque on the coil. What is this maximum torque?

23. Another ApplicationThe Galvanometer

24. Currents Cause Magnetic Fields

25. Magnetic field of long straight conductor – • Placed over a compass, the wire would cause the compass needle to deflect. This was the classic demonstration done by Oersted as he demonstrated the effect.

26. Result r

27. Force Between Two Current Carrying Conductors First wire produces a magnetic field at the second wire position. The second wire therefore feels a force = Bil

28. Two Wires

29. Currents in a loop –

30. Field of a Current Loop N turns of wire

32. Solonoid B=~0 outside

33. The solenoid – B=0 outside

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