CP 2 - Measuring Resistivity Flashcards
what is the standard technique for measuring the resistivity of a material in the form of a wire
- calculate the cross-sectional area from measurements of the diameter
- then get a value for R / l using a graphical method
how do you measure and get a value for the diameter that will be used in the calculation from a wire
- you measure the diameter (aka thickness) with a micrometer
- at four places along its length at different orientations
why do you measure the diameter four times
to get a good average to be used in the calculation
what is done to the wire after its diameter is measured
it is taped to a half meter rule
how should you make sure the wire is taped to the ruler and why
- about 1cm should be protruding from the zero mark of the ruler
- so that it can easily be held on to with crocodile clips
what would you connect the wire to
a circuit
what components would the circuit contain
- a cell
- a voltmeter
- an ammeter
how would the circuit be contructed
- the voltmeter would be connected in parallel to the wire
- and the ammeter would be in a series to measure the total current
why is the voltmeter connected in parallel to the wire
- to measure the potential difference across the wire
- so that you can use V and I to calculate the resistance at different lengths
how would the length, l, be measured
- a crocodile clip is placed at the beginning of the wire
- while a second clip is placed (at a measured distance from the beginning) on another point on the wire
- the distance between the clips (or the reading on the ruler) is the length
what is actually done in this experiment
- the potential difference and current are recorded for a specific length of the wire connected to the circuit
- this is then used to calculate the resistance of the wire using R = V / I
- this is then repeated for varying lengths of the wire connected to the circuit by changing the distances between the clips
- a graph for R against l is then plotted
what are the units of the resistance and length
- resistance is in ohms
- length is in metres
what is the equation for resistance with resistivity and then what is the equation for resistivity
- R = pl / A
- so p = RA / l
what would A equal in this equation
- the diameter divided by 2 to get r (d/2)
- then use the equation for the area of a circle
- so pi r^2 would be pi x (d/2 x d/2)
- to give (pi d^2) / 4
how would a random error be shown on a graph when plotting R and l
- the points wouldnt perfectly align
- to give you a perfectly straight line
- instead you would have to draw a line of best fit
why could a random error occur in this practical despite the lengths being accurate
- there could be variations in pressure applied by the clips when being connected to the wire
- or there could simply be inconsistencies in the wire
- either possibility would change the resistance of the wire relative to the length
what does it mean if the line of best fit doesnt go through the origin
- that there is a systematic error caused by contact resistance
- due to poor contact between the clip and the wire
why is a graphical method good in terms of bypassing errors
- the impact of the random errors is reduced as the line of best fit gives us a good average to work with
- and the systematic error isnt even taken into consideration when calculating the gradient by change in R over change in l
