Electricity Flashcards
Potential difference
The pd across a device is equal to the work done per unit charge.
V = W/Q
Electromotive force (EMF)
The emf of a power supply is equal to the energy supplied per unit charge by the power supply.
E = W/Q
Work down by charge carriers
W= IVt
Electrical power equations
P = IV
P = I^2 R
P = V^2/ R
Ohm’s Law
Under constant physical conditions for an ohmic conductor current is proportional to voltage.
Ideal ammeter
0 resistance
Ideal voltmeter
Infinite resistance
Resistance
A measure of the difficulty of making a current pass through a substance.
It is caused by repeated collisions between the charge carriers and the positive ions of a substance.
Resistivity
Rho = RA/l
Units= ohm m
Material property
Superconductor
Has 0 resistance below a critical temperature. Below Tc when a current flows there is no pd because the resistance is 0 therefore the current has no heating effect.
Uses of superconductors
- MRI scanners
- particle accelerators
Uses of diodes
Polarity protection
Rectification circuits
Pd in potential dividers
Vo R1 / (R1 + R2) = V1
V1/V2 = R1/R2
Series rules
I same everywhere
pds add up
resistances add up
Parallel rules
Is add up
Pd same everywhere
1/R = 1/R1 + 1/ R2 + …
Cells in series
Combined emf = nE
Combined internal resistance= nr
Cells in parallel
Combined emf = E
Combined internal resistance = r/n
Internal resistance and terminal pd equation
V = E - Ir
Short circuit
Max current from supply
No external resistance ( only resistance in the circuit is the internal resistance)
Imax = E/r
Open circuit
I = 0A
V = emf
Max terminal pd
Power with internal resistance
Power is dissipated in internal resistor and in load resistor
Total power = I^2 R + I^2 r
Peak power when R=r
