Electricity Flashcards
Find current (using charge)
Charge passing a point/time
Q/t
Find voltage (using work done)
Work done/charge
W/Q
Charge of an electron in coulombs (or e)
1.6 x10^-19
Kirchhoff current law
Sum of current into a point = sum of current out
(ΣIin = ΣIout)
Because conservation of charge
Number density
Number of free electrons per cubic metre of material
Find current (using mean drift velocity)
Current = cross sectional area*number density*e*mean drift velocity (I = Anev)
Find voltage (using current)
Voltage = current*resistance (V = IR)
Find resistance (using resistivity)
Resistance = resistivity*length/cross sectional area (R = ρL/A)
Find resistivity
Resistivity = resistance*cross sectional area/length (ρ = RA/L)
Resistivity
How much a material opposes electron flow
Resistance
How much a component opposes electron flow
Current
Rate of flow of charge
Potential difference (p.d.)
Energy lost (work done) by each unit of charge flowing through a component
Electromotive force (e.m.f.)
Energy gained by each unit of charge passing through a cell
How to measure p.d. of a component?
Connect a voltmeter in parallel
How to measure current through a component?
Connect an ammeter in series
Power
Rate of work done by a component
Find power (of a component)
Power = voltage*current (P = VI)
Find power (using only current and resistance)
Power = resistance*current^2 (P = I²R)
Find power (using only voltage and resistance)
Power = voltage^2/resistance (P = V²/R)
Find kinetic energy of an electron from an electron gun
Kinetic energy = e*accelerating p.d.
0.5mv² = eV
Diode
- only allows current in one direction
- very high resistance until threshold p.d. is reached
- non ohmic component (V not proportional to I)
Thermistor
- can have positive or negative temperature coefficient
- NTC decreases resistance with temp rise
- PTC increases resistance with temp rise
- non ohmic
Photoresistor (or LDR)
- decreases resistance as light intensity increases
- non ohmic
Find total resistance in series
Rt = R1 + R2 + …
Find total resistance in parallel
1/Rt = 1/R1 + 1/R2 + …
Kirchhoff voltage law
In any loop,
Sum of e.m.f. = sum of p.d.
(Σε = ΣV)
Because conservation of energy
Find p.d. across R1 of a potential divider
V1 = Vt*R1/Rt
Find e.m.f. of a cell (using internal resistance r)
e.m.f. = terminal p.d. + p.d. of internal resistance
(ε = V + Ir
or
ε = I(R + r))
On a terminal p.d. vs current graph, how do you find:
- e.m.f.
- internal resistance
ε = V + Ir (because of voltage law) V = -rI + ε so: - e.m.f. = y intercept - internal resistance = negative of gradient
Terminal p.d.
p.d. across a cell (same as p.d. across rest of circuit)
Direction of current flow
Positive to negative
Direction of electron flow
Negative to positive
n values for different materials
Conductors - high
Semiconductors - lower
Insulators - very low
I-V graph of a resistor
- resistance stays the same
- straight line through the origin
- ohmic conductor
I-V graph of a filament lamp
- resistance increases with V (because of heating)
- curve with gradient that reduces to 0
I-V graph of a thermistor
- resistance decreases with V (because of heating)
- current curves upwards like a tan graph
I-V graph of a diode
- resistance goes from high to low at threshold voltage
- curves sharply upwards at V = 0.7 (different for LEDs)
Explain the effect of temperature on resistance / resistivity
High temperature causes ions in the metal to vibrate, making them more likely to block electron flow
Find work done by a component
Work done = voltage*charge
(W = VQ
or
W = VIt)
Kilowatt hour
Unit of energy used to calculate energy costs
Ratio of voltages in a potential divider
Ratio of voltages = ratio of resistances
V1/V2 = R1/R2
Potentiometer
Resistor with three terminals and a sliding contact - can be used as a variable resistor or a potential divider
Ohm’s law
Voltage is proportional to current at constant temperature
