Sensing Flashcards
Current
Rate of flow of charge
Kirchhoffs first law
Conservation of charge
IT = I1 + I2
Potential difference
The potential energy difference per unit charge between two points
V = E / Q = W / Q
Energy dissipated in a circuit
name and formula
W = VQ = VIt
Power
P = E / t = IV = I^2R
Ohms Law
V = IR
If this relationship holds true a conductor is ohmic. Metals are ohmic conductors, however when heated resistance increases and they become non-ohmic.
Conductance
G = 1 / R = I / V
Parallel circuits
Current: splits at junctions and adds to the total
EMF: total p.d. across every parallel component
Resistance: 1/RT = 1/R1 + 1/R2 etc
Conductance: Add to the total
Series circuits
Current: same everywhere in the circuit
EMF: p.d. shared between components
Resistance: Adds to the total
Conductance: 1/GT = 1/G1 + 1/G2 etc
Conductivity
Measure of the ability of a material to conduct irrespective of dimensions.
G = σA / L
Resistivity
Measure of a materials resistance irrespective of dimensions.
R = ρL / A
Conduction model of metals
Regular lattice of positive metal ions, with free (delocalised) electrons moving in random directions with varying speeds until a potential difference is applied. As a metal is heated the positive ions vibrate more and the free electrons begin to be obstructed; this is what increases resistance
Current and drift velocity
I = nAve
Insulators
Materials without mobile charges
Semi-conductors
Materials that usually have very few mobile charges, but when heated this number increases greatly
Thermistors
Resistance drops as temperature increases
This is because the heat frees more electrons
LDR
Resistance drops as light intensity increases
This is because the photons free up more electrons
Potential dividers
P.d. is shared proportionately to the resistance of components.
Kirchhoffs second law
For any complete circuit:
EMF = the sum of all potential differences across all the resistances in the circuit
Internal resistance
ε = V + Ir
V = ( -r) I + ε (can plot this as y=mx+c)
