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Find out how resistivity changes as the length of wire is changed, and to find out the resistivity of a certain unknown wire.

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Section A- Plan


To find out how resistivity changes as the length of wire is changed, and to find out the resistivity of a certain unknown wire.


  • Power Pack
  • Ammeter
  • Voltmeter
  • Crocodile clips
  • Wire
  • Metre rule
  • Thermometer


  1. Measure the diameter of wire using a micrometer, taking measurements in 3 different places along wire and take an average.
  2. Take a piece of wire which is 100cm long, and attach t a metre rule.
  3. Connect up wire to a power Pac, ammeter and voltmeter using crocodile clips and wires, to produce a circuit.
  4. Vary the length of wire used in circuit by moving croc clips along the wire at 10cm intervals.
  5. Record in a table the voltage and current for each of the 10cm intervals.
  6. Find the resistance of wire for each recorded length, using the formula R= V/I
  7. Repeat the whole method 3 times and find the average resistance of each length of wire.
  8. Measure the temperature of the room with the thermometer and note this down.
  9. Use the average diameter to calculate the cross-sectional area using A= π(d/2)2  
  10. Plot resistance (y-axis) against length (x-axis). Draw in the line of best fit.
  11. y=mx+c    R= (ρ/A × length) + 0   Use gradient ρ/A to calculate resistivity (ρ).


Safety was one of my top priorities.

...read more.


The equation of the straight line is y= mx + c

As the line will go through the origin c =o

Therefore y= mx

The Resistance will be the y-axis and Length will be the x-axis. Therefore:-

        R= m×L

As R= ρL                The gradient of this is ρ/A (excluding R and L)




ρ= mA

The area will be found by measuring the wires diameter and using the formula A=π (d/2)2

...read more.


I believe my experiment was good because

Section D- Conclusion

Analysis of errors

Error in the area of the wire











D                = 0.19 ± 0.01mm

                = 0.19 ± 5%  

r= D/2                = 0.095 × 10-3 m ± 5%

A= πr2

As r is squared the % error is doubled

A=                2.8×10-8 ±10%

Error in the gradient

See the error graph

M                = 14.1 ± 1.4

                = 14.1 ± 10%

Error in the value of Resistivity of wire

ρ= Am

ρ        = (2.8×10-8) ×14.1

        = 39.4×10-8± 20%

        = 39.4×10-8 ± 7.9

Possible sources of error

Systematic errors

  • There may be inaccuracies in the measuring equipment. It’s possible that the micrometer may have been damaged and may not have read the thickness of the wire accurately.
  • There may have been an error in measuring the location of the crocodile clips, and this may have lead to inaccurate results.
  • The voltmeter takes a small amount of current from the wire

Random errors

  • The temperature of the wire and the room may have lead to increased or decreased resistances.
  • Thickness of the wire would not be constant, this would lead to different resistance values in different parts of the wire.
  • Unknown material of wire
  • The power supply may not have been constant.


  • Errors in meters- use 3 different meters to check current, and voltage values
  • Location of crocodile clips- use pointers instead of clips to get accurate measurements
  • Some current flows through the voltmeter- I could have used an oscilloscope as it has a very high resistance so current will not go through voltmeter.

...read more.

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