Conductance Flashcards
How conduction occurs
by promoting electrons into conducting states, that starts right above the Fermi level
Conducting states and valence band
separated from valence band by an infinitesimal amount
Electric current
-Amount of charge per unit time that passes through a plane that passes completely through the conductor
Current formula (definition of current)
i = dq/dt
SI units for current
Coulomb per second (C/s),
called an ampere (A)
direction of current
-current arrow drawn in direction in which positive charge carriers would move
actual charge carriers
- negative
- move in opposite direction of current
direction of conventional current
-always from a point of higher potential toward a point of lower potential, ie from positive toward negative terminal
When conductor has no current through it
- conduction electrons move randomly
- no net motion in any direction
Drift speed
- when conductor has current through it,
- electrons still move randomly,
- but tend to drift w/ a drift speed v𝒹
drift speed direction
-in direction opposite that of applied electric field that causes current
drift speed formula
i = Nₑ(v𝒹)(Ae)
Ne = number of conduction electrons per cubic meter
resistance
-ratio of voltage applied across a piece of material to the current through the material
resistance formula
R = V/I
SI unit of resistance
Volt/ampere (V/A)
also called
ohm (Ω)
resistivity
defined as ρ = E/J
J = current density = I/A (amps/m²)
unit of resistivity
ohm-metre (Ωm)
conductivity of a material
σ, the reciprocal of resistivity
conductivity formula
σ = 1/ρ
what is conductivity of a material proportional to?
proportional to number of free electrons and electron mobility
σ = Nₑ |e| μ
Ne = number of “free”/conduction electrons per m³
|e| = absolute charge on a free electron
μ = electron mobility
|e|
1.609 x 10⁻¹⁹ C
μ unit
m²V⁻¹s⁻¹
what resistivity of material depends on
temperature
temperature coefficient of resistivity
has the unit of reciprocal temperature
used in:
ρ = ρ₀[1 + α(T - T₀)]
conductance
the reciprocal of resistance
Conductance formula
G = I/V = 1/R
Conductance unit
the siemens (S)
= amps/volts = 1/Ω
Ohm’s law
an assertion that current through a device is always directly proportional to potential difference applied to device
Ohm’s law formula
I = V/R
ohmic material
type of material that obeys Ohm’s law
non-ohmic material
type of material which does not obey Ohm’s law
when a conducting device obeys Ohm’s law
-when resistance of the device is independent of the magnitude + polarity of the applied potential difference
when a conducting material obeys Ohm’s law
-when the resistivity of the material is independent of the magnitude + direction of applied electric field
resistance formula and resistivity
R = ρl/A
where l = length, A = area
Work done on electron by a force F in moving electron through a small distance 𝛿x (i think this is dx?)
𝑊 = 𝐹𝛿x
what is power equal to?
equal to rate of doing work
Power formula
Pₑ = F𝛿x/𝛿t
resistors
-devices used in circuits to provide resistance of a specified value
