Chapter 9 - Energy, Power and Resistance (pt2) Flashcards
Resistivity
An electrical property of a material at a given temperature, the resistance of a 1m wire with a 1m^2 cross-section area
Resistance relationship with length and cross-section area
R ∝ L
R ∝ 1/A
Resistance equation in terms of resistivity
R = ρL/A
How to find resisitivity?
RA/L
Multiply the gradient of a resistance length graph by the area
Resistivity of each classification
In the order of 10^-8 Ωm for conductors
In the order of 10^16 Ωm for insulators
Semi-conductors between
Superconductivity
Cooling a material enough to the point where its resistivity is zero and thus 0 resistance. Needs cooling to -190 to -270 degrees
How do thermistors work?
Some semi-conductors have a negative temperature coefficient so increasing the temperature increases the number density, decreasing the resistance.
Thermistor uses
Simple thermometers, thermostats, heat alarms
Resistance against temperature graph thermistor
Almost vertical decrease at low temperatures before flattening
Finite resistance at 0 degrees
IV graph thermistor
Similar to the filament lamp but at high/low p.d. the current changes more rather than less
How do LDRs work
Increasing the light intensity increases the number density, decreasing the resistance
Resistance against light intensity graph LDRs
Almost vertical decrease at low light intensities before flattening
Finite resistance at 0 degrees
Electrical Power
The rate of energy transfer by each electrical component
Power equations
P = VI
Assuming Ohm’s Law:
P = I^2 R
P = V^2/R
1 kWh in J
3.6 MJ
