Chapter 26 CURRENT ELECTRICITY

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Chapter 26 CURRENT ELECTRICITY

• Electric Current (I)
• rate of positive charge passing through a surface
  IdQ/dt
• units Coulomb/Second Ampere (C/s A)

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Current Flow of Charge
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Atomic Model for current

• Number of charges moving with a velocity through
  the area
• Metallic conductors
• Negative charges move
• drift velocity is slow
• electrons collide with atomic cores frequently
• collisions transfer energy (cause heating)

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Atomic Model for current
n number of charges/vol e magnitude of charge
take limit as Dt goes to 0
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Current Density

• Current per area

vector quantity
relation to current via dot product
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Problem 12
Find current in section bounded by r
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Ohms Law

• Vector field equation

Conductivity and resistivity
Scalar Equation
Ohms Law vector form
Resistance and resistivity
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Resistance
Units Ohm Volt / Ampere (W V / A)
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Ohmic devices vs. no Ohmic
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Sample problem 26-4
Find resistance of rectangular block of iron
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Temperature Dependence of Resistance
Define temp coefficient of resistivity
If a is small and constant
Units of a is (oC)-1
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Microscopic view of Ohms Law
Resistivity is a constant
Free electron Theory
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Low temperature behavior of resistance

• Superconductivity

Semiconductor
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Electric Power
Substituting Ohms Law yields
Units watt amp volt