AQA Req prac - resistance

Discussion in 'Supporting Physics' started by Christine Broadbent, Jun 14, 2018.

  1. Christine Broadbent

    Christine Broadbent magic maiden

    I'm getting very confused which resistors, diodes and fixed resistors I need to buy for this prac.

    If you do AQA which size do you buy ? Which sort of diode is best to use? Is a metal film resistor better than a wirewound one or vica versa?

    AQA's worksheets are not very clear (unless I'm not looking properly!)

    Not really a physicsy person and we only have chem and bio specialists in the dept so any ideas and suggestions would be appreciated. Thanks.
     
  2. We use TVS diodes, 10r wirewound fixed resistors, linear rheostats and 12V filament bulbs. I'm not sure if any of it is correct, but its all stuff I had in my cupboards and I wasn't allowed to buy anything else!
     
    Christine Broadbent likes this.
  3. Nick Mitchener

    Nick Mitchener COMMITTEE

    AQA is very vague on values. We use 10 ohm or 15 ohm 3 or 5 watt resistors but that's just us. You want something the little darlings can't burn out.

    We use a mix of batteries or power supplies, but I favour batteries where possible again to prevent abuse. We have battery packs with 4 D cells.

    Our rheostats are 16 ohm so we use 3.5V bulbs to get the best results with those, and we will be going to single D cell with the diode, 250 ohm variable resistor and 5 ohm protective resistor, using equipment we have basically to get the best results.
     
    Christine Broadbent likes this.
  4. Do you mean resistors for a series and parallel circuit for the resistance required practical
    Or
    A fixed resistor with protective diode, a diode and bulb for IV characteristics required practical?

    If the former, at 1.5V a 10ohm 1/4W carbon film resistor was fine. Make sure they only have one battery, if you can't trust them go for 100ohm resistors, will be much cheaper than going for higher wattage 10ohm resistors.
    (V=IR, P=IV, 1.5V = I * 10ohm = 0.15A, P = 0.15A * 1.5V = 0.225W, 0.225W +5% = 0.236W)

    If the latter see above.
     
  5. Nick Mitchener

    Nick Mitchener COMMITTEE

    The required practical is the IV graph for bulb, resistor, and diode.

    The reason we use a lower resistance value is to keep the current up so they can get enough values without a multimeter, though we do give them a multimeter for the diode.
     
  6. Christine Broadbent

    Christine Broadbent magic maiden

    Thanks, guys, that's given me plenty to work on. One extra question tho... is a protective resistor something different to just a resistor?
     
  7. No. A plain old resistor (eg 10ohm) protects the diode by restricting the current.
     
  8. Nick Mitchener

    Nick Mitchener COMMITTEE

    the protective resistor is just another fixed resistor there to limit the current through the diode so as not to damage it. We are going to be using a single D cell in future and as a result don't really need one but we put a low value one in so that the students are familiar with its purpose as they will more than likely ask a question on it in the exam.
     
  9. Also a little sidenote, resistors are often described as 'R' instead of 'ohm' or the greek letter omega on websites, eg 10R for a 10ohm resistor.
     
  10. Christine Broadbent

    Christine Broadbent magic maiden

    Thank You. Feel I know a little bit more now!
     
  11. PhysicsSimon

    PhysicsSimon COMMITTEE

    Can I ask what are you using to measure the current? We've recently moved from multimeters to dedicated ammeters (and voltmeters) which the students understand much more easily and for the resistor/bulb/diode practical, wanted to keep the current reasonably high even at low voltages. For this reason we went to 22 ohm power resistors (7W max power) which are fine up to 12V (P=V2/R = 6.5W), the 10 ohm equivalents top out at 8V. H&S: they can get pretty warm which is why I've mounted them inside conduit - don't want the little darlings burning their fingers now do we?
     
  12. Christine Broadbent

    Christine Broadbent magic maiden

    We have the same ammeters as you, and I was going to use 15R resistors(3W). Will they be ok? I have made component holders to mount them in but the resistor will be 'on show'. Will they get too hot to handle?
     
  13. PhysicsSimon

    PhysicsSimon COMMITTEE

    Using the P=V2/R & solving for V, your planned resistors are fine (within spec) up to 6.7V....and as they only have to dissipate 3W compared to 7 for ours, may be ok temp wise but I'd try it - can always give a warning to the teacher that it's possible for them to burn if they touch after an amount of time on the top voltage - you'll be covered.

    On the subject of resistors & voltage, about 2 days after starting here there was a complaint about 'smoke coming out of the resistor' from a teacher - turns out their class were putting 12V through a 1W 10R resistor - only 12 times its rated value - durr! (tho' WHY 10R resistors were even in the box is a whole different kettle of fish). I've kept one - tis pretty funny
     
  14. Christine Broadbent

    Christine Broadbent magic maiden

    Thanks PhysicsSimon, that's very helpful and reassuring. :)
     
  15. Nick Mitchener

    Nick Mitchener COMMITTEE

    I usually buy high wattage resistors because they will be abused by kids and teachers not following instructions. I also use batteries and locked power supplies to limit voltage when required.
     
    Christine Broadbent likes this.
  16. I have just been following this thread in order to answer my own question! We have just bought a few new rheostats for the I-V characteristics practical and the kids said they wasn't working....I'm wondering is this because they are too high a resistance?..they are 125 Ohms 1.8A and they measure the length of a 30 cm ruler but the light bulb has extinguished after they have moved the slider 3/4 cm...should we be looking at a lower value Ohm ? Any advise would be most welcome!
     
  17. Nick Mitchener

    Nick Mitchener COMMITTEE

    Use them as a potential divider with batteries or a low power supply they will be brilliant as you will get much better resolution than with a shorter rheostat. However in general use I would have thought you would want a higher current rating.
     
    Samantha Hull likes this.
  18. Our ray box bulbs are 12V 24W) resistance works out to be a couple of ohms, so adding 125 ohms in the circuit will extinguish the bulb before the slider has moved very far..
    Our biggest pea bulb is 12V 2.2W) the resistance of the bulb is 65ohms which should be ok with your 125 rheostats.
    Our rheostats are 15ohm.
    As Nick for using a potential divider,,,, just cant persuade the teachers to do it that way.
    I refuse to use batteries, they dont last long ....when the little b***** short circuit them.
     
  19. Thank you !
     
  20. Nick Mitchener

    Nick Mitchener COMMITTEE

    Presumably there is a risk of damaging the rheostats when used as variable resistors with the 12V24W bulbs as at the lowest setting of say 2 ohms you will have a current of 3 amps. They will be much more protected with the higher resistance bulbs in the circuit.