Phy2049: Magnetism

1. Phy2049: Magnetism • Last lecture: Biot-Savart’s and Ampere’s law: • Magnetic Field due to a straight wire • Current loops (whole or bits)and solenoids • Today: reminder and Faraday’s law.

2. The following clicker numbers are not registered. Please register them by June 20. • 417331 420441 461221 • 462681 497478 625833 • 405726

3. Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is: e A. into the page B. out of the page C. to the left D. to the right E. zero

4. Long parallel wires carry equal currents into or out of the page. Rank according to the magnitude of the net magnetic field at the center of the square, highest first. (Parenthesis means the same magnitude). 1. C,D, (A,B) 2. A, B, (C,D) 3. B, A, C, D 4. D, (A, B), C

5. Long, straight, parallel wires carry equal currents into or out of page. Rank according to the magnitude of the force on the central wire. 1. d, c, a, b 2. a, b, c, d 3. b, c, d, a 4. c, a, b, d 5. b, d, c, a

6. Magnetic Field Units • From the expression for force on a current-carrying wire: • B = Fmax / I L • Units: Newtons/Am  Tesla (SI unit) • Another unit: 1 gauss = 10-4 Tesla • Some sample magnetic field strengths: • Earth: B = 0.5 gauss = 0.5 x 10-4 T = 5 x 10-5 T • Galaxy: B  10-6 gauss = 10-10 T • Bar magnet: B  100 – 200 gauss • Strong electromagnet: B = 2 T • Superconducting magnet: B = 20 – 40 T • Pulse magnet: B  100 T • Neutron star: B  108 – 109 T • Magnetar: B  1011 T

7. (30 –11)

8. (30 –12)

9. (30 –13)

10. (30 –14)

11. (30 –15)

12. (30 –16)

13. loop 1 loop 2 (30 –17)

15. (30 –19)

16. (30 –20)

17. (30 –21)

18. N2 N1 (30 –22)

19. N2 N1 (30 –23)

20. N2 N1 (30 –24)