Current, Ohm’s Law, Etc.

where R is resistance Resistance does not vary with the applied voltage resistor

Experimentally it is found that R depends on the material the wire is made of and its dimensions. Does not depend on the shape. In a wire of uniform resistivity and cross sectional area, the electric field is a constant for constant currents.

+ - V

Exercise 1 Given the resistivity of copper, about Ohm-m, what length of 0.5 cm diameter wire will yield a resistance of 10 Ohms?

Current Density Consider current flowing in a homogeneous wire with cross sectional area A.

The Continuity Equation for Steady State Currents Currents and current densities are constant in time – steady state. The flux of out of any closed surface must be zero.

Another form of Ohm’s Law

For steady state situation

Metal: ρ increases with increasing T Resistivity and temperature

Semiconductors: ρ decreases with increasing T

Superconductor Once a current has been established in a superconducting ring, it continues indefinitely without the presence of any driving field Dutch physicist Heike Kamerlingh Onnes - Hg 2003

Water NitrogenOxygen Boils Freezes 100 C (212 F)-196 C (-322 F)-183 C (-297 F) 0 C (32 F) -210 C (-346 F) -223 C (-369 F) Our air is ¾ Nitrogen and ¼ Oxygen

1908- liquefied helium produced First discovered in mercury by Kamerlingh-Onnes in Critical temperature 4.21K. Nobel Prize in Superconductivity

High-T c Superconductivity Liquid nitrogen temperature 77 K Complex ceramic materials were discovered in They exhibit superconductivity at much higher temperatures – above LN temperature! Muller and Bednortz, Nobel Prize 1987

Meissner effect and magnetic levitation

Current, Ohm’s Law, Etc.

The Continuity Equation for Steady State Currents Currents and current densities are constant in time – steady state. The flux of out of any closed surface must be zero.

For steady state situation

Problem 4 Two wires having different resistivities ρ 1 and ρ 2 and equal cross sections, a, are connected end to end. Their lengths are l 1 and l 2. If a battery is connected to this system such that a potential difference of V is maintained between the ends, a)What will be the current densities in the wires? b) What will be the potential difference across each wire? c)Will there be any charge on the surface where the wires are connected?

Exercise 5 Consider a cylindrical shell, inner radius a and outer radius b. It is made of material with resistivity ρ. Suppose a current can be made to flow out from the inner surface to the outer. What would the resistance be for this current?

For steady state situation 1.Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero. 2.Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop must be zero.