Module 4 Resistivity Flashcards
What factors effect the resistance of a wire?
temperature
material the wire is made from
length of the wire
cross sectional area of the wire
How are resistance and length of a wire related?
R directly proportional L
What is the unit for resistivity?
ohms-metre
What must be kept constant when using the formula for resistivity?
Temperature
If diameter doubles and length doubles in a wire, explain the effect on resistivity
no change, resistivity is independent of length and area, will not be effected
Definition of resistivity
resistivity of a material at a given temperature
is the product of the resistance of the component made the material and its cross sectional area divided by its length
How does temperature effect resistivity
increase in temperature increases resistance, resistance is directly proportional to resistivity
Describe the method of finding the resistivity of a wire using a power supply
1.Set up a series circuit with a power supply, an ammeter, and a voltmeter across the length of the wire being tested
2. Measure voltage and current for the length of the wire L
3. Calculate resistance using R=V/I
4. Measure cross sectional surface area by using πr^2 and using a micrometer to find d than r
5. Adjust length of the wire from 10-80cm in intervals of 10cm
6. Calculate resistivity for each length using using ρ = RA/L and take an average (or plot graph of R against L)
Describe the method of finding the resistivity of a wire without using a power supply
- Attach an ohmmeter to a suspended wire using crocodile clips
- Vary (from 10 to 80cm in intervals of 10 cm)
and measure the length/distance between points of contact along the wire and record the resistance - Calculate resistivity for each length using using ρ = RA/L and take an average
How do you find the cross sectional area of a wire?
Use a micrometer to measure diameter
Repeat at different points along the wire and calculate a mean
Use this value to find the cross sectional area by using π(d/2)^2
Why, when finding out the resistivity of a wire, is it advantages to not use a power source?
No current is conducted
Wire does not heat up - increase in temp causes in increase in resistance, and resistivity is proportional to resistance, so more than one variable will be changes
No current means it’s safer to handle the exposed piece of wire as a shock may be delivered if a current was introduced
Explain how using a graph of resistance on the y, and length on the x axis the resistivity may be found
ρ = RA/L
The gradient will be R/L giving us grad = ρ/A
ρ = grad x A
Read this micrometer
9mm from top + 0.5mm from bottom + 0.18mm from turning barrel so 9.68mm
