Resistance of wire investigation

Discussion in 'Supporting Physics' started by JHRoss, May 15, 2019.

  1. Hi there,

    I am doing a lot of electrical practicals this week, one of them is the investigation of resistance of a wire as a function of it's diameter or length. I have experienced a lot of burned wires using voltages up to 12V on a constantan wire 28SWG 0,375 mm. Any tips / suggestions?

    Thanks in advance
     
  2. Nick Mitchener

    Nick Mitchener COMMITTEE

    don't use 12V?

    We use low voltage on a metre of nichrome. we always have the metre of wire in the circuit and use a flying lead on the voltmeter to measure pd for different lengths.

    That way the wire stays at a constant temperature, a requirement for ohm's law to hold, the current stays constant.
     
    Andrew Goloskof likes this.
  3. Limit the voltage and limit the amount of time it's running for during readings

    We instruct them to start at the lowest and step up until they can get a reasonable reading on the multimeter
    Then they only have the pack on long enough to take a reading at that distance then turn it off while they record it

    If nothing else as it heats it'll affect their results
     
  4. We do exactly the same Nick, 32 SWG Nichrome which is about 18.99Ω per metre. Wire is on 4cm trunking with 4mm binding posts and a one metre ruler screwed onto the trunking underneath. No mishaps with over ten years worth of use.
     
    Anna17 likes this.
  5. The teachers always tell the kids to start from low voltages, however they often try high voltages to see what happens. So I test the nichrome as you have told and I witnessed that it is more difficult to burn.
     
  6. When a wire get burned what are we supposed to do? Disposal? Is it totally useless?
     
  7. Nick Mitchener

    Nick Mitchener COMMITTEE

    the problem with the heated wire is the oxidised layer is an insulator. You can use abrasive paper to remove it.

    If you use batteries for lower school and teachers with limited class skills you can protect the equipment. If you use the system we described you should not have a problem anyway.
     
  8. 26 or 28 SWG has always been the wire I have used, unless the practical is to looks at a wider range of wire SWG though no more than 34 and no less than 20 SWG. 1.5V usually is enough but no more than 6V and supply being batteries. It is also good practical to have a tapping switch in the circuit so the current isnt always flowing.
     
  9. Nick Mitchener

    Nick Mitchener COMMITTEE

    we use 28 swg nichrome too, resistance is about 11 ohms per metre. We give them the Satz meters that read to 2 decimal places, you can certainly do it with 1.5V, we either give them low current variable power supplies or 4 cell strips of D cells. We have the wire on wooden battens, so that it doesn't tie up metre rules which are supplied separately. Normally open push to close switches would help if you can't get therm to behave but you are relying on the teacher getting them to include them in the circuit.

    We rarely give anybody switches for anything as they are an unnecessary expense.
     
  10. Alternatively, measure with a multimeter instead of power packs...
     
  11. Sharon Elsdon

    Sharon Elsdon COMMITTEE

    We use 26swg or 28swg constantan or nichrome.
    Our wire is attached to wooden boards also and it works well.
    We limit the power supplies to 3v max, otherwise we have burnt wires that need changing each lesson!
     
  12. PhysicsSimon

    PhysicsSimon COMMITTEE

    As previously posted (including pictures and circuit diagrams) it is possible to set this up so the wire never gets hot....we (including my old HOD when 'teaching the teachers' in evening sessions sponsored by the IOP) always use this even though it is slightly different to that shown in the book. Basically you put 6V across the whole wire (usually 32swg nichrome - but we also have 28,30 & 34 set up if they want to look at area effects as well as 32swg iron, constantan & copper (only 1V if copper!)) but have the voltmeter (with its approx infinite resistance) on a flying lead that you touch to the wire - the current stays the same, a nice non-heating low value no matter how short the sample you're measuring. Produces lovely straight line graphs & is intrinsically safe (as long as nobody forgets the voltmeter on the flying lead - if they do you'll still get a v hot wire at 10cm)
     
  13. To be fair the edexcel version doesn't even have a power pack involved

    You just have the wire,ruler, clips leads and the multimeter set to resistance
    It uses a 34swg constantan wire
     
  14. PhysicsSimon

    PhysicsSimon COMMITTEE

    But, but....that's cheating! And more to the point, where's the fun in that?
     
  15. And yet we still have to explain it to half of them.....
     
  16. Nick Mitchener

    Nick Mitchener COMMITTEE


    It also doesn't really do much towards the required practical.

    I had a physics specialist who wanted to replace the flying lead on a voltmeter with a flying lead on a multimeter set to resistance. Perhaps I should have let him. I'm still not sure what would have happened.
     
  17. As I said, this IS the Edexcel required practical
     
  18. We use 28swg Nichrome wire on 'modified' 1M rulers and a cell.

    Resistance Wire & Cell.jpg
     
  19. Nick Mitchener

    Nick Mitchener COMMITTEE

    I thought he was replying to Claire. However edexcel sounds like it doesn't stretch the students much.
     
  20. Can you please describe me what does a flying lead look like? Or attach a picture of it?