Electricity and Magnetism Physics 208 Dr. Tatiana Erukhimova Lectures 24-25

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

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

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?

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

Wheatstone bridge I

British scientist and inventor Charles Wheatstone British scientist and inventor 1802-1875 Samuel Hunter Christie, British scientist and mathematician

Wheatstone English concertina

Electromotive force

105-107 Heldenfels Second exam Tuesday, March 24, 7 pm Sections 521, 522 room 105 Sections 525, 526 room 107

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 .

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

Electric field of a ball of charge Q 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.

Insulating sphere with charge Q uniformly spread throughout the volume

Conducting sphere with charge Q V r A r

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

Electric field near a surface of a conductor

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

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

Current, Ohm’s Law, Etc.

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.

Joule’s Law

Highest 100 Average = 71 Lowest 12 Median = 73.5 22 19 18 16 13 13 >90 >80 >70 >60 >50 <50

The Magnetic Field

The force on a charge q moving with a velocity The magnitude of the force

left-hand rule right-hand rule

Motion in magnetic field 1) Uniform , 2) Uniform , 3) Nonuniform