8/4/08 Lecture 1 Part 1 1
Electricity, Magnetism,and Fields
Prof. Daniel Stump
Lecture Notes:www.pa.msu.edu/people/stump/VIPP2008/
8/4/08 Lecture 1 Part 1 2
Field Theory
Electricit
y
전기
221
r
qKqF
Michael Faraday James Clerk Maxwell
The force on an electric charge is caused by a field, E(x).
q
q
FE(x)
E(x)F
Electric Charge and Electric Field
—electricity—
장의 이론
8/4/08 Lecture 1 Part 1 3
Example: The electric field outside a charged sphere
rE(x)
rFF
E(x)
ˆ implies
ˆ and
2
2
r
KQr
KQqq
What is electric charge? Charge is a property of subatomic particles (electrons and protons) which creates the interaction with an electric field.
Interactions of particles and fields
E(x) and { q1 q2 q3 … }
E exerts a force on each qi
Each qi adds to E(x)
… that is “Field Theory”
r(x)E
)E(xF
ˆ
2r
Kq
q
ii
iii
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The Interaction between Particles (+,-) and the Electric Field
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Potential energy — Electric potential — Voltage
2r
KQqF
units:… newton
… joule = N m
… volt = J/C
Force
Potential energy
Electric potential
rKQq
U
rKQ
V
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Field Theory
Electric field lines and equipotential surfaces
HIGHpotential (+)
LOWpotential (-)
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Electric field lines and equipotential surfaces.The equipotential surfaces are similar to contour lines on a topographic map.
지형 지도 —topographic map—
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Topographic Map --- --- contour lines are equipotentials of gravitational potential energy
지형 지도 —topographic map—
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Field Theory
Electric field lines and equipotential surfaces
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8/4/08 Lecture 1 Part 1 11
NEC 1 farad capacitor
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Capacitors
Cap. EquationsQ = C V
U = ½ Q V
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Electric Current
The human race has created a remarkable technology based on electric current. (Thomas Edison patent for light bulb = 1880)
nevAI
A 1.9C/s 9.1mm 6.12mm)2(14.3rA
mm/s 0.1 m/s10coulomb 10602.1
mmelectrons/ 10melectrons/ 10
222
4
19
319328
nevA
ven
-
I
Example
전류 + E
E
8/4/08 Lecture 1 Part 1 14
Ohm’s Law
Demonstration:The “marbles and nails model” of electrical resistance to current.
Why is the mean electron velocity constant as the electrons move to lower electric potential energy? I.e., the electrons do not accelerate, on average.
Georg Ohm (1789-1854)
I = V/R (Ohm)Recall I = n e v AV v(voltage) (average electron velocity)
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