University:
Kentucky State University
Course:
PHY 212 | General Physics II
Academic year:

2021
Views:

135

Pages:

4
Author:

teespagmaxrtf

Ohm’s law Empirically, in many materials: current density is proportional to electric field 1 J  E  E  Ohm’s law •  is electrical conductivity •  is electrical resistivity •  and  are material (substance) properties V/m V • unit of :  m  m 2 A/m A Ohm Caution!  is not volume density!  is not surface density! Ohmic vs non-Ohmic materials • materials that follow Ohm’s Law are called “ohmic” materials • resistivity  is constant • linear J vs. E graph ------------------------------------------------------• materials that do not follow Ohm’s Law are called “non-Ohmic” materials • nonlinear J vs. E graph -----------------------------------------------------• resistivities vary enormously among substances • roughly 10-8 ·m for copper • roughly 10+15 ·m for hard rubber • incredible range of 23 orders of magnitude J slope=1/ E J E Example 3 : the 12-gauge copper wire in a home has a crosssectional area of 3.31x10-6 m2 and carries a current of 10 A. Calculate the magnitude of the electric field in the wire.  of copper I E  J   A (1.72 108   m) 10 A  E (3.31106 m2 ) E  5.20 102 V/m Homework hint: you can look up the resistivity of a material in a table in your text. Resistance Given a wire (i.e. object):  A • length L, cross section A • material (substance) of resistivity  L start from E  J I V  EL  JL   L  IR A L R A resistance of the wire, Ohm’ law (derived formula) V  IR unit V   (Ohm) A

Ohm’s Law

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