Electricity Flashcards
coulomb
SI unit of charge
current
The rate of flow of charge
anode
positive electrode
cathode
negative electrode
power
rate of work done
elementary charge
the magnitude of the charge of an electron
describe what is meant by mean drift velocity
the average displacement of electrons along a wire per second, electrons collide with lattice, so go in diff dirrections
Ohm’s Law
voltage and current are directly proportional when the temperature is constant. V=IR
semiconductor examples
silicon and germanium
semiconductors get hot because
electrons need to travel faster to get the same current
I = Anev
current = area x number density x elementary charge x drift velocity
cell symbol short side
negative terminal
thermionic emission
the thermally induced flow of charge carriers from a surface or over a potential-energy barrier
eV = 1/2mv^2
elementary charge x p.d = kinetic energy = work done on charge
ohm
unit of resistance
resistivity
A material’s opposition to the flow of charge.
p = RA/L
resistivity = resistance x area / length
Negative temperature coefficient
resistance of semiconductor decreases when temperature is increased
Kirchoff’s Second Law
sum of e.m.f. is equal to sum of p.d.s in a closed loop
potential difference
measure of energy transferred per unit charge between two points.
conventional current
positive to negative. opposite to electron flow
electron flow
negative to positive. opposite to conventional current.
Kirchoff’s First Law
sum of currents entering junction is equal to sum of current out of junction
quantised
charge on an object always multiple of e
number density
number of free electrons per cubic metre
cell symbol long side
positive terminal
potential difference in relation to charges
work done by the charges, they lose energy through component
electromotive force
work done on the charges, they gain energy through component
Threshold voltage
The minimum potential difference at which a diode begins to conduct
large current needed (car battery)
low internal resistance
small current needed (school power supply)
high internal resistance
what causes internal resistance?
wires and components in power supplies, chemicals in cells
resistance
A measure of how difficult it is for current to flow.
lost volts
The difference between a source’s emf and the terminal voltage.
terminal potential difference
The potential difference across the terminals of a power source. It is
equal to the source’s emf minus any voltage drop over the source’s internal
resistance.
number density of copper?
10^29
What is the difference between rms speed and drift velocity of electrons?
r.m.s. speed is random motion
Drift velocity is the mean velocity (of the free electrons)
What is conductivity?
1/resistivity
What is band theory?
In semiconductors, increasing thermal energy gives electrons enough energy to move from valence band to conduction band
Why would you connect a resistor in series with an LED?
Minimises current flow, ensuring it doesn’t overheat.
In series, if bulb fails, what happens to pd?
Components’ pds can be ignored.
If two resistors are connected in parallel, and one’s resistance is 0, what is the total resistance?
0
What is special about an open circuit?
Resistance = 0
What is special about a shorted circuit?
Resistance = 0
