Internal Resistance of a Cell

Discussion in 'Physics' started by logearav, Feb 4, 2012.

  1. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    Revered Members,
    Please go through my attachment. When the Cell is in open circuit, voltmeter connected in parallel gives the EMF of the cell and and when the circuit is closed, by adding an external resistance in the circuit, voltmeter gives slightly less value than the previous occasion when measured in open circuit, due to the internal resistance of the cell. V = E - Ir. Here r is the internal resistance of the cell and i have drawn that resistance bellow battery. But why the polarity is opposite to that of battery polarity?
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150

    The polarity you show in your drawing is not correct. It has to be the same as the battery. And your circuit is not complete because it does not show 'r'. Unless you mean that the variable resistor you show at the bottom of your drawing is 'r', but if so it is shown as a variable. Do you really have an ammeter connected between the cell and the internal resistance? Back to the drawing board.

    It does help immensely and especially when you are describing to the forum in order to get help, if you include all details of the circuit including your VM so we can adequately access what you are actually doing and therefore offer an accurate answer. In electronics and many other things in life, accuracy and clarity are important for understanding. We'll help you if you help us understand the details accurately.
     
    logearav likes this.
  3. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    wmodavis,
    Actually, the resistance with the polarity which i have drawn below voltmenter is the internal resistance. My book says, that it happens internally and the resistance is of opposite polarity to that of battery.
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    In that case you show that the battery is short circuited. Not a good thing to do to a battery. On paper it will not make smoke but don't wire it as shown in real life. If you do that the full cell voltage will be across the internal resistance and very high current will flow. Wear a gas mask and flack jacket.
     
  5. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    Thanks for your reply wmodavis.
    Actually that internal resistance is not wired externally. The author of the article, just drawn that internal resistance to explain the things, i.e., to explain what is an internal resistance and mentions the polarity of the internal resistance is opposite to that of a battery. I wanted to know why the polarity of internal resistance is opposite to that of a battery.
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,165
    The battery model polarity is shown as opposite because the voltage drop (like in any serial resistance circuit) across it bucks (reduces) the battery cell voltage. There is no testing point to put a meter on a normal battery cell (some special lab cells have multiple electrodes for ionic resistance measurements) and really measure the resistance, it has to be derived by external measurements.

    http://data.energizer.com/PDFs/BatteryIR.pdf
     
    Last edited: Feb 5, 2012
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