Chapter 9.7 - Resistace & Resitivity Flashcards
1
Q
What is resistivity
A
• Product of resistance of a component made of a given material and the material’s cross sectional area divided by its length
-at given temperature
- Measured in Ωm
- resistivity = (resistance X area) / length
2
Q
What are the four factors that affect resistance
A
- Temperature
- Material of wire
- Length of wire
- Cross-sectional area of wire
3
Q
Why is the reisrance of a wire directly proportional to its length
A
• For any given current increasing wire length increases pd across it
• Doubling length doubles pd
-resistance also doubles
• V = IR
4
Q
Why is the resistance of a wire inversely proportional to its cross-sectional area
A
- For any given pd doubling the cross-sectional area will double the current in the wire
- So resistance must have halved
- V = IR
5
Q
What is the new equation for resistance
A
• Resistance = (Resistivity x Length) / Area
• Resistance against Length graph
-gradient x Area = resistivity
6
Q
What are the different resistivities of materials
A
- Conductors - 10^-8 Ωm
- Insulators - 10^16 Ωm
- Semi-conductors in between
7
Q
What is superconductivity
A
- Resistivity of a material falls to zero when material is cooled to a Very low temperature
- LHC at CERN uses superconducting wires carrying up to 20,000 A to produce strong magnetic fields
- Wires become superconducting at around 4K
