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

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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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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

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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)