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Ionisation chamber - doubt about current nature and residue

  1. Nov 28, 2008 #1
    1. The problem statement, all variables and given/known data
    This is from Advanced Physics by Adams and Allday, section 8, practice exam questions, question 25.

    An α-source with an activity of 150 kBq is placed in a metal can. A 100 V d.c. source and a 109 Ώ resistor are connected in series to the can and the source. This arrangement is sometimes called an ionisation chamber.

    b) Describe how the nature of the electric current in the connecting wire differs from that in the air in the can.

    d) With the α-source removed from the metal can, the voltmeter still registers a potential difference of 0.2 V Suggest two reasons why the current is not zero.

    2. Relevant equations
    None.

    3. The attempt at a solution
    b) Describe how the nature of the electric current in the connecting wire differs from that in the air in the can.
    In the wire current is carried by electrons.
    In the air current is carried by both electrons and positively charged ions.

    I believe these answers are true but are they what the examiner was looking for?

    d) With the α-source removed from the metal can, the voltmeter still registers a potential difference of 0.2 V Suggest two reasons why the current is not zero.

    Background radiation creates some ionisation in the can.
    Leakage?

    I think "Background radiation" is a good answer but "Leakage" is weak. Is there something else the examiner might have been looking for?
     
  2. jcsd
  3. Nov 28, 2008 #2
    I don't follow this question at all. The can is made of metal. What is to stop the current flowing through the walls of the can?
     
  4. Nov 29, 2008 #3
    Sorry - I do not know how to include the diagram. The diagram shows the -ve terminal of the battery connected via the resistor (with a voltmeter across it) to the source and the the +ve terminal of the battery connected to the can. The source is electrically insulated from the can.
     
  5. Feb 12, 2009 #4
    Hello :smile:

    It's taken a while ... here's the diagram that makes my original question make sense.

    Best

    Charles
     

    Attached Files:

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