• Examples
• gas - lightning
• liquid - electrolyte tank
• solid - metal wire

• electrical appliances, etc...

• charge will flow to equalize charge on each object
• motion of charge will define current

average current

Instantaneous Current

• sign of moving charges or charge carriers
• positive - ?
• negative - ?
• Cannot distinguish from electrical arguments
• Convention:
• Direction of Current = Direction + charges would move

• Battery provides Electric field
• Field exerts force on charges
• force causes acceleration
• charges move faster and faster
• implies increasing current
• THIS IS NOT THE CASE
• current is the same everywhere along a wire

• charges do not have a clear path
• will collide with stationary ions and each other
• motion is random
• overall drift parallel to E
• look at average velocity

• amount of charge in volume (V = x A)
• DQ = nqDxA

• n = carriers/volume
• q = charge/carrier
• DQ/D t= nqADx /Dt
• I = nqvA

experiment

• hook source of potential difference to conducting wire
• make graph

• R = resistance - opposes motion of charges
• Ohmic materials
• obey Ohm's Law - resistance stays constant
• Non-Ohmic materials
• do not obey Ohm's Law

• getting from A to B
• wider cross-section
• easier to get through
• longer
• more time
• type of material
• resistivity
• Resistance

• resistance increases with temperature
• increasing amplitude of vibration - more difficult for charges to get through

• change in size can be neglected

• effective resistance decreases with temp
• increased # of charge carriers

• limited range

• 1) Heavy ions move less?
• 2) Electrons might become sluggish?
• R = infinity if electrons "freeze"

• trying to liquefy helium
• Hg at 4.2 K, resistance is zero
• experimental limits : R < 10^-24 W
• Table 17.2 - Critical Temperatures

• Barium, Lanthanum and Copper compound

• BCS theory - Cooper pairs
• freezing in "velocity space"