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HELP needed - using capacitors in the classroom

Discussion in 'Science' started by PinkHelen, Jul 18, 2011.

  1. PinkHelen

    PinkHelen New commenter

    Hi all, I'm still trawling my way through the equipment needed to teach physics year 11 next year, and trying to organise a set of capacitors so that the pupils can observe them charging and discharging.
    I first ordered some excellent electrolytic capacitors (470uF), and these work really well - they take roughly a minute to charge with the suggested circuit, and the pupils could see the voltage slowly creeping up as they charge (and the current slowly decreasing also).
    I've since found out that they can explode if not connected with the correct polarity, and therefore seem to me to be completely un-usable in a classroom (my year 11s would probably take it as a challenge to make them explode).
    I really want to do this as a class practical and not a demo behind a screen, so I sourced some ceramic capacitors, however their capacitance is much lower (47pF) and so they charge pretty much instantly - the pupils would therefore not see much of anything happening.
    I was wondering if anyone out there uses capacitors in the classroom, and if they do:
    • Do you use electrolytic or non-electrolytic?
    • If you use electrolytic how do you manage safety?
    • If you use non-electrolytic, how do you manage to use them so that they have a reasonably long charging time (I've already set my resistance to 470 kiloOhms and it can't go any higher)?
     
  2. PinkHelen

    PinkHelen New commenter

    Hi all, I'm still trawling my way through the equipment needed to teach physics year 11 next year, and trying to organise a set of capacitors so that the pupils can observe them charging and discharging.
    I first ordered some excellent electrolytic capacitors (470uF), and these work really well - they take roughly a minute to charge with the suggested circuit, and the pupils could see the voltage slowly creeping up as they charge (and the current slowly decreasing also).
    I've since found out that they can explode if not connected with the correct polarity, and therefore seem to me to be completely un-usable in a classroom (my year 11s would probably take it as a challenge to make them explode).
    I really want to do this as a class practical and not a demo behind a screen, so I sourced some ceramic capacitors, however their capacitance is much lower (47pF) and so they charge pretty much instantly - the pupils would therefore not see much of anything happening.
    I was wondering if anyone out there uses capacitors in the classroom, and if they do:
    • Do you use electrolytic or non-electrolytic?
    • If you use electrolytic how do you manage safety?
    • If you use non-electrolytic, how do you manage to use them so that they have a reasonably long charging time (I've already set my resistance to 470 kiloOhms and it can't go any higher)?
     
  3. Cosmic_Rainbow

    Cosmic_Rainbow New commenter

    when we use them we put a sticker on one side with a + so they know which way to connect them. also we dont let them turn the powerpacks on untill he circuit has been checked by a teacher to make sure they are the right way round and wont explode.
     
  4. PinkHelen

    PinkHelen New commenter

    Hi Rainbow, I considered that but I'm worried that the class will then turn them around once I've checked them, so they can see what happens when they explode. I considered showing them a video of one exploding to remove the element of mystery, but whilst it would work with some it would probably just spur the others on. It's a shame that I can't trust them :(

    Out of interest, what risk assessment does your department have for if the pupils do then re-connect them incorrectly?
     
  5. lunarita

    lunarita Occasional commenter

    Don't tell them they'll explode, don't eve hint at that, just say they don't work if they're connected the wrong way round.



    And if you're still worried, limit the voltage. Can you give them all 1.5V batteries instead of powerpacks or can you lock the powerpacks at a low voltage? They should still be able to see the pattern in current and voltage.
     
  6. Have a look at your rated voltage - probably 16V or 25V. They will withstand a reverse bias of 10% quite happily (NO PROMISES - you MUST try it out yourself as part of your own Risk Assessment!). So limiting your supply to 1.5V (eg dry cell) should make things pretty safe.
    If you have never seen a capacitor explode, try it! Set the thing up (with a voltmeter for an accurate reading?) and slowly increase the forward bias. You will probably get to 30-50V before one goes. Then try another one, reverse biased. If you are lucky, the end-cap will blow right off with a bang. If you are unlucky, there will be a plop and some "smoke" will emerge. It is actually the gas which is your biggest risk - memory tells me it smells awful and is carcinogenic, but the amounts involved are so small that I didn't use to worry about it! [​IMG]. See if CLEAPSS has any advice (I'm no longer a member).
    There are a whole host of little things you can do with capacitors, but what does your specification expect? Smoothing when connected across a full-wave rectifier? Energy store (disconnect from all electrical supplies, walk to far end of room keeping it in full view, then connect to a small lamp or led; use a low-torque motor to jerk a small weight eg button upwards)? If showing E-> V^2, discharge 1.5V through one lamp. Then discharge 3V through one lamp - brighter and longer. Then 3V through 2 lamps in series - same brightness, but longer discharge (2xRC). Finally 3V through 2 parallel sets of 2 lamps in series - same brightness, same time so 4x energy for 2x voltage! (Wiring 3 parallel sets of 3 lamps in series is more difficult, for 4.5V [​IMG])
    You can always use interesting little tasters to encourage students to join you for an explanation at A level!
    Good luck. Physics is fun!
     
  7. PinkHelen

    PinkHelen New commenter

    Thanks for the advice - I think the ones that we have got do have a tolerance of 10%, but I'll try it out to see.
    Our specification is OCR Gateway Physics - GCSE level so not terribly high level, but your ideas sound like they would make a good demonstration, and as you say a nice lead up to A level.
    Again, thankyou so much for the advice - I'm the only physicist in my department and I've not been teaching very long, so it's difficult with new specifications as I've not got anyone to ask within my school.
     
  8. As an aside - to allow students to see what is going on I show them a giant capacitor that covers the bench.

    2 sheets of tin foil with a thickish piece of plastic - thick bin bag (or survival bag if you can steal one from a friendly DofE bod) works well. Attatch one sheet of tin foil to each side of an EHT power supply. As it charges the students see the tin foil straighten out. If you put enough charge on the plastic will degrade and you will hopefully get a bang. (If no bang throw a lead connected to the Earth onto it to make sure that it has discharged!)
     
  9. piaffe's advice about discharging is very important and should always be the first and last thibgs you do with a capacitor: some can be charged to 1000s of volts and could give a nasty surprise if you touch the terminals so 1) protect yourself, 2) protect the next user.
    10% tolerance refers to the capacitance - a 20pF capacitor will be somewhere between 18pF and 22pF. Because of the manufacturing process, even those from the same batch may well be at the extremes. Resistors (as I'm sure you know) have a colour code for their tolerance, and are usually pretty homogenous. The voltage rating is the maximum voltage that should be applied in the forwards direction ( as denoted by the dimple, or spot, or arrow - see your manufacturers diagram): electrolytic capacitors should be connected with their plates in the correct polarity to provide the expected characteristics. They should withstand 10% in reverse.
     

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