1. Electricity and Magnetism
Physics 208
Dr. Tatiana Erukhimova
Lecture 24

2. Why does a light bulb burn out when you switch the light on and never when you turn it off?

3. The amount of current drawn by a given device is determined by its power input
The current in a 100-W light bulb is

4. A kitchen circuit
An 1800-W toaster, a 1.3-kW electric frying pan, and a 100-W lamp are plugged into the same 20-A, 120-V circuit.
What current is drawn by each device, and what is the resistance of each device?
Will this combination blow the fuse?

5. To prevent the fuse from blowing, you replace the fuse with one rated at 40 A.
Is this a reasonable thing to do?

6. Wheatstone bridge

7. British scientist and inventor
Charles Wheatstone
British scientist and inventor
Samuel Hunter Christie, British scientist and mathematician

8. Wheatstone English concertina

9. Electromotive force

10. 105-119 Heldenfels Second exam Tuesday, March 18, 7 pm
Sections 521, 522 room 105
Sections 523, 524 room 107
Section 525 room 109

11. Gauss’s Law
The total flux of electric field out of any closed surface is equal to the charge contained inside the surface divided by

12. Conductors and insulators
Charges reside at the surface of the conductor + + + + + + + +
Conductor + +
E=0 + + + + +

13. Electric field of a ball of chargeQ
Electric field outside of a charged sphere is exactly the same as the electric field produced by a point charge, located at the center of the sphere, with charge equal to the total charge on the sphere.

14. Insulating sphere with charge Q uniformly spread throughout the volume

15. Conducting sphere with charge QV r A r

16. A Charged, Thin Sheet of Insulating Material+ + + + + + + + + + +

17. A field in a cavity of a conductor Faraday’s cage

18. Electric field near a surface of a conductor

19. Spherical capacitor; Cylindrical capacitor
The capacitance is:
Spherical capacitor; Cylindrical capacitor

20. Current Density for j =Const only!
Consider current flowing in a homogeneous wire with cross sectional area A.
for j =Const only!

21. Current, Ohm’s Law, Etc.

22. For steady state situation
Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero.
Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop must be zero.

23. Joule’s Law