Current flow through a metal. 

Current flow through a vacuum. (Note: The electrons are colored differently to make tracking easier.) 
Q:
" A 9.0 volt battery is connected across a light
bulb (R = 3.0 ) A: R = V / I . Current, I, is the time rate of flow of charge, Q, through an area: I = Q / t. The amount of charge, Q, is simply a number of particles, z, multiplied by the charge per particle, q: (Q = z q) Using these facts we can derive and equation for the number of particles, z, passing through a conductor with resistance, R, in an amount of time, t: R = V / I > I = V / R = I = V / R = Q / t = ( z q ) / ( t ) = V / R > z = ( V * t ) / (R * q) Now that we have a formula we can
solve the problem. Solve for z: z = ( 9.0 V * 60
sec) / (3.0 * 1.6021917 x 10^19 C / electron) 1.123 x 10^{22} electrons pass through the resistor in one minute. b. The power, P, loss in a conductor is the square of the current, I^{ 2}, multiplied by the resistance, R: P = I^{ 2} R. Using the fact that I = V / R we can rewrite this as: P = V^{ 2} / R . Power, P, is energy, E, divided by time, t: (P = E / t ). Rewrite the equations: P = V^{ 2} / R as E / t = V^{ 2} / R. This yields E = V^{ 2} t / R. Now we can find the energy lost through the light bulb. E = V^{
2} t / R The energy dissipated by the light bulb in 1 minute is 1620 Joules. 

Q: Do all conductors obey Ohm's Law? A: Yes, conductors are also called "Ohmic materials." 

Q:
Two light bulbs use the standard 110 V light socket. One is rated
at 60 Watts and the other at 100 Watts. A:
The formula for power is P = I * V = (V/R)
* V = V^2/R.

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Semiconductors: materials with few conduction electrons but with available conductionlevel states that are close, in energy to their valance bands. These materials become conductors when they are doped with other atoms that contribute electrons to the conduction band.
Superconductors: materials that lose all electrical resistance at low temperatures.
The SI unit of electric current is the ampere
(A): 1 A = 1 C/s.
The SI unit of resistivity is the ohmmeter ( m).
Ohm's Law: for many materials (including most metals),
the ratio of the current density to the electric field is a constant, , that is
independent of the electric field producing the current.
Electric current (i)  
Current density (J)  
Drift speed of the charge carriers  
Resistance of a conductor  R = V / i 
Resistivity () and conductivity ()  
Ohm's Law  J = E 
Resistance of a conductive wire of length L and uniform cross section  
Change of resistivity () with temperature   _{0} = _{0} (T  T_{0}) 
Resistivity of metals  
Power (rate of energy transfer)  P = iV 
Resistive dissipation 
Color Code For Resistors 

Number  Multiplier  Tolerance (%)  
Black  0  1  
Brown  1  10^{1}  
Red  2  10^{2}  
Orange  3  10^{3}  
Yellow  4  10^{4}  
Green  5  10^{5}  
Blue  6  10^{6}  
Violet  7  10^{7}  
Gray  8  10^{8}  
White  9  10^{9}  
Gold  10^{1}  5%  
Silver  10^{2}  10%  
Colorless  20% 
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