Electricity Flashcards
what is kirchoffs first law?
at any point in an electrical circuit the sum of currents into that point = sum of currents out of that point
what is the mean drift velocity
the average velocity of electrons moving through a wire
mean drift velocity equation
I=Anev I = current in conductor n = number density e = elementary charge v = mean drift velocity
what effect does a thinner wire have on the mean drift velocity
if current is constant then mean drift velocity has to increase as wire gets thinner
what does the ideal ammeter have
zero resistance
what is EMF
electro motive force
describes when work is done on charge carriers
where energy is transferred from other energy forms into electrical energy
what is PD
potential difference
describes when work is done charge carriers
when energy is transferred from electrical energy into other forms of energy
what is ohms law
for a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the potential difference across it
what is resistivity
an electrical property of a material.
resistance is proportional to length/area. the constant is resistivity
equation for resistivity
R= pl/A
as a material gets hotter what happens to its resistivity
as a material gets hotter, the resistivity increases
what are semiconductors
they have negative temperature coefficients so their resistance drops as thei temperature increases
what are thermistors made of
semiconductors
what is power
the rate of energy transfer
what is kirchoffs second law
the sum of electromotive forces is equal to the sum of pds around a closed loop
describe current in a series circuit
same in every position
describe voltage in a series circuit
emf is shared equally among resistors with the same resistance
if resistances are different then the resistor of higher resistance will take a higher proportion of the emf
total resistance in a series circuit
r1+ r2 +r3 e.t.c
describe current in a parallel circuit
current is shared between paths according to resistance. the higher the resistance the less current
describe voltage in a parallel circuit
pd across each branch is equal to emf in power supply
pd is shared between components in the branch
total resistance in a parallel circuit
1/R = 1/r1 + 1/r2 e.t.c
what is the difference between the pd in the circuit and the emf in the power supply
difference is lost volts
lost volts = Ir
I = current
r = internal resistance
how can you calculate emf
emf = terminal pd + lost volts
equation to find the force between two point charges
kQ1Q2/r^2 where k is a constant
other equation to find the force between two point charges
F=charge X electric field
IV graph for a diode
slopes up from around 0 one arm going up
IV graph for filament lamp
through origin, wavy arm going horizontal
equation for power
P=IV
equation for energy with voltage and current
E=VIt
what is EMF in terms of lost volts
emf=terminal pd + lost volts
equation for lost volts
lost volts = I X r where r= internal resistance
