Chapter 9.7 - Resistace & Resitivity

Question 1

What is resistivity

Answer 1

• 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

Question 2

What are the four factors that affect resistance

Answer 2

• Temperature
• Material of wire
• Length of wire
• Cross-sectional area of wire

Question 3

Why is the reisrance of a wire directly proportional to its length

Answer 3

• For any given current increasing wire length increases pd across it

• Doubling length doubles pd
-resistance also doubles

• V = IR

Question 4

Why is the resistance of a wire inversely proportional to its cross-sectional area

Answer 4

• For any given pd doubling the cross-sectional area will double the current in the wire
• So resistance must have halved
• V = IR

Question 5

What is the new equation for resistance

Answer 5

• Resistance = (Resistivity x Length) / Area

• Resistance against Length graph
-gradient x Area = resistivity

Question 6

What are the different resistivities of materials

Answer 6

• Conductors - 10^-8 Ωm
• Insulators - 10^16 Ωm
• Semi-conductors in between

Question 7

What is superconductivity

Answer 7

• 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