Resistance and Resistivity Flashcards
Ohm’s Law
Current through a metallic conductor is directly proportional to the applied p.d provided the temperature is constant.
V = IR
Joule’s Law of Electrical Heating
IV Characteristics
A graph of I against V showing the relationship between these two quantities for a component.
Series circuit
Current is the same everywhere
Supply voltage is equal to the sum of the voltages across each of the components
Parallel Circuit
P.D is the same across each resistor
Total current is equal to the sum of the current through each resistor
Ohmic Conductor
Temperature must remain constant
Non-Ohmic Conductor
Temperature rises with current. As temperature rises, resistance rises.
Thermistor
Resistance of an NTC thermistor decreases as it heats up
Resistance of a metal and temperature
Resistance of a metal rises linearly with temperature. They are not directly proportional
Resistance of an NTC thermistor and temperature
The resistance of an NTC thermistor falls exponentially with temperature as shown.
Resistivity
Numerically equal to the resistance of a sample of the material 1m long with a cross-sectional area of 1m2. It is a property of a material whereas resistance is a property of a particular specimen
Internal Resistance
Sources of EMF have themselves some resistance to the electric current that passes through them.
- As more current is drawn, terminal p.d. falls
- The source of e.m.f is less that 100% efficient
Modelled as r.
Open-circuit voltage
The terminal p.d. when no current is being drawn, i.e. the e.m.f.
Internal Resistance and the Law of Conservation of Energy
Superconductivity
Superconductor – a material which loses all its electrical resistivity to become a perfect conductor when it is below its transition / critical / Curie temperature.
Uses:
- MRI scanners
- Maglev monorail systems
