Physics - Resistivity
OCR A-Level Physics 2022
What resistivity depends on
What three things does the resistivity depend on?
- The material of the wire.
- The length of the wire.
- The cross-sectional area of the wire.
If you double the length of a wire, what happens to the resistance?
It also doubles.
What’s the difference between resistivity and resistance?
Resistivity is the property of a material whereas resistance is just a measurement of a component.
Deriving the resistivity equation
How would you write that the resistance $R$ is directly proportional to its length $L$?
How would you write that the resistance $R$ of a wire is inversely proportional to its cross-sectional area $A$?
If $R \propto L$ and $R \propto \frac{1}{A}$, how could you combine these equations?
\[R \propto \frac{L}{A}\]
How would you write this using a constant of proportionality?
\[R = \frac{\rho L}{A}\]
What does $\rho$ represent in this equation?
The resistivity.
Resistivity from a graph
How would you work out the resistivity from the gradient?
How would you work out the resistivity from the gradient?Measuring resistivity experimentally
What factor is not accounted for in the resistivity equation?
Temperature.
What experiment could you perform to determine the resistivity of a material?
Measure how the resistance of a wire varies with its length.
Roughly, what’s the order of magnitude for the resistivity of conductors?
How can you keep the current constant as you change the length of a wire?
Use a variable resistor in the circuit.
Why is it important to keep current constant in an experiment measuring resistance?
Because changing the current will affect the temperature of the wire, which will also affect the resistance of the wire.
Superconductors
What does it mean for a component to be a super conductor?
The material has no resistance.
What is the current issue with superconductors?
They have to be kept very cold.
What does $T _ c$ represent here?
What does $T _ c$ represent here?The critical temperature of the superconductor.