Resistivity

Question 1

Resistivity - At a constant temperature

Answer 1

Property of a material: Fixed value that is different for every material

Question 2

Factors affecting resistance

Answer 2

Temperature
Material the wire is made from
Length of the wire
Cross sectional area of the wire

Question 3

What is Resistivity

Answer 3

Resistance ∝ Length (For a constant temperature)
As cross-sectional area increases, the resistance decreases: R ∝ 1/A
Combining these gives: R ∝ L/A
The constant of proportionality is the resistivity ρ (rho)

Question 4

Equation for resistivity

Answer 4

R = ρL/A
R: resistance in Ohms (Ω)
ρ: resistivity in Ohm-metres (Ωm)
L: length in metres (m)
A: cross-sectional area in metres squared (m^2)
Watch for changing diameters -> A = πd^2/4

Question 5

Defining Resistivity

Answer 5

Use the equation as the basis for the definition
ρ = RA/L
Definition: The resistivity of a material at a given temperature (must say) (constant) is the product of the resistance of a component made of the material and it’s cross-sectional area, divided by it’s length

Question 6

Changing Values

Answer 6

If A and L are constant: ρ ∝ R
An increase in temperature causes an increase in resistance - Meaning the resistivity increases
Resistivity of a material varies with temperature in the same way as the resistance of most components varies with temperature

Question 7

Experimental Methods to find resistivity - 1

Answer 7

With a power supply
Set up a series circuit with a power supply an ammeter and a voltmeter across the length of wire being tested
Measure the voltage and current for each length of wire L
Calculate resistance at each using R = V/I
Calculate resistivity at each using ρ = RA/L then take an average
To find Area: Measure the diameter and calculate area
Check for constant temperature

Question 8

Experimental Methods to find resistivity - 2

Answer 8

Using an Ohmmeter which measures the resistance of a wire directly
Vary and measure the length of wire and record the resistance of the wire in Ohms
Calculate the resistivity at each length using ρ = RA/L then calculate an average (ignore anomalies)
When describing include lengths of wire and the intervals used

Question 9

Advantages without a power supply

Answer 9

There is less danger with this experiment due to heating effects: No heating as there is no current
This is because there is no power supply
Reduces danger even with small values of p.d.

Question 10

Using a graph to find resistivity

Answer 10

Once we have recorded values of resistance and length, we plot a graph of R against L
Passes through the origin
y = mx so ρ/A = m
resistivity = gradient x cross-sectional area