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Electricity and Magnetism
Physics 208 Dr. Tatiana Erukhimova Lecture 24
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Why does a light bulb burn out when you switch the light on and never when you turn it off?
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The amount of current drawn by a given device is determined by its power input
The current in a 100-W light bulb is
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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?
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To prevent the fuse from blowing, you replace the fuse with one rated at 40 A.
Is this a reasonable thing to do?
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Wheatstone bridge
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British scientist and inventor
Charles Wheatstone British scientist and inventor Samuel Hunter Christie, British scientist and mathematician
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Wheatstone English concertina
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Electromotive force
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105-119 Heldenfels Second exam Tuesday, March 18, 7 pm
Sections 521, 522 room 105 Sections 523, 524 room 107 Section 525 room 109
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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
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Conductors and insulators
Charges reside at the surface of the conductor + + + + + + + + Conductor + + E=0 + + + + +
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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.
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Insulating sphere with charge Q uniformly spread throughout the volume
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Conducting sphere with charge Q
V r A r
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A Charged, Thin Sheet of Insulating Material
+ + + + + + + + + + +
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A field in a cavity of a conductor Faraday’s cage
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Electric field near a surface of a conductor
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Spherical capacitor; Cylindrical capacitor
The capacitance is: Spherical capacitor; Cylindrical capacitor
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Current Density for j =Const only!
Consider current flowing in a homogeneous wire with cross sectional area A. for j =Const only!
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Current, Ohm’s Law, Etc.
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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.
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Joule’s Law
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