Electrons, waves and photons Flashcards
Charge carriers
in liquids(electrolytes), generally ions
in metals, free delocalised electrons
if one end of a wire is positive and one end is negative, electrons will flow
factors affecting current
temperature
cross sectional area
speed of electrons
conventional current
positive to negative
number density
Definition and values
number of free charge carriers per unit volume.
for conductors n ≈ 10^28
for semi conductors n≈10^17
for insulators, n is much lower
derrivation of equation for current
I =Q/t
no. electrons = nV(density x volume)
charge = neV (no. electrons x e)
I = (neV)/t
V/t = (Axl)/t = Av where v is mean drift velocity
I =Anev
How does an electron gun work
Thermionic emission - metal filament is heated by electrical current; some electrons gain enough KE to escape surface of metal
What does an electron gun do and what is it used for
Fires narrow beam of electrons
Can be used to ionise particles
used in electron microscopes, mass spectrometers or oscilloscopes
why does higher temperature increase resistance
if temperature increases, positive ions have more internal energy and vibrate with greater frequency about their mean positions
frequency of collisions with charge carriers increases, so resistance increases as more work is done by charge carriers
factors affecting resistance
temperature
material
length
cross sectional area
relationship between length and resistance
R is proportional to L
relationship between cross sectional area and resistance
R is proportional 1/A
Resistivity
How hard it is for charge to flow through a material
ρ(rho), measured in ohm meters
Resistivity of conductors and insulators
Good conductors : ρ is of the order of10^-8
Insulators: ρ is of the order of 10^16
Semiconductors are in between
Negative temperature coefficient
As temperature increases, resistance decreases
In some semiconductors, as temp increases increases, number density increases
Uses of thermistors
Used in thermometers, thermostats and inside electrical devices
How do LDRs work
Made from semiconductors in which number density changes depending on light intensity
As light intensity increases, number density increases so resistance decreases
Derivation of power equation
P =W/t
W=QV
P=QV/t
I=Q/t
P=IV
What is one unit of energy
1 kWh (kilowatt-hour)
Laws in series circuits
Current is the same in every position
e.m.f is shared between components- components with greater resistance take greater share
Laws in parallel circuit
Current is split between branches - branch with higher resistance have smaller current
Each loop has equal p.d and this must be equal to the emf
Derivation of resistance equation (series)
Vt = V1+V2…
It = I1=I2
V=IR
IRt =IR1+IR2…
Rt = R1+R2…
Derivation of resistance equation (parallel)
I=I1+I2…
Vt = V1=V2…
R=V/I
It/V = I1/V + I2/V…
1/Rt = 1/R1 +1/R2 …
lost volts
difference between emf and terminal pd(measured at the terminals of the power source)
Calculating lost volts
r= internal resistance
lost volts = Ir
draw a graph of V against I
emf = Ir +V
y-intercept = emf
gradient = -r