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PHYSICS 7D (30)

William Molzon (8)

Lecture 12

Department

PhysicsCourse Code

PHYSICS 7DProfessor

William MolzonLecture

12This

**preview**shows half of the first page. to view the full**2 pages of the document.**Lecture 12: Currents and Resistance

I. Drift Velocity

- In a conductor of length L and area A, the number of

conduction electrons = nV= nLA, where n is the

number of conductor electrons per unit volume.

- An electron takes a time t= L/vd to pass through this

length of conductor: in time L/vd, all free charge in

length L exits through the downstream end, so

II. Resistance and Resistivity

- Ohm’s Law: For some materials, the current through a piece of material is directly

proportional to the voltage difference between the two points. For these materials, the

constant R that relates the voltage and the current is called the resistance.

- R ≡ Units: ≡ Ohm ( )

I

V volt

ampere Ω

- Resistance depends on geometry. If we don’t want to deal with a particular object but with

materials- define resistivity

- ≡ Units: = m = * m⍴j

E V /m

A/m2A

VΩ

- E = pj

- The inverse of the resistivity is the conductivity:

- ≡ j= Eσ1

⍴σ

III. Ohmic and non-ohmic materials

- A conducting device obeys Ohm’s law

when the resistance is independent of

magnitude and polarity of V.

- For these materials, Ohm's law means

that resistivity is independent of

magnitude and direction of E.

- For other materials, the relationship

between V and I can be complicated.

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