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Resistors, potential dividers and ohms law

  1. Apr 19, 2012 #1
    Here is my question: When I look a p.d divider circuit I get slightly confused, because the way I was reading the explanation it says: that when the a thermistors has 0 resistance it has 0V across, so when the resistance increase the voltage increases and the will turn on the base emitter to allow current to flow though a turn on the light for example. So when they say that its 0V do they mean that it has 0V drop across and their is still a current flowing through it as the thermistor is basically acting as a normal part of the circuit?

    Say for example I have a 12v supply, and 2 ohm thermistor, and their is 0 resistance on the thermistor, when I apply ohms law and do 12/0 I get a paradox basically the current has disappeared some where. Or is there just 12 Amps flowing through but in that case I would still need a resistance of 1 ohm resistance.

    I have left out the other resistor due to simplifying my question to what I don't understand. I do know that any resistor put in a circuit there will be a volt drop across the resistor.

    I would appreciate any input available
     
  2. jcsd
  3. Apr 19, 2012 #2

    sophiecentaur

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    A real thermistor is just a variable resistor as far as the rest of the circuit is concerned. When it gets really hot, the resistance is nearly zero.
    Your idea of connecting a thermistor which is (hot enough to be 0 Ohms - not actually achievable, in real life aamof) straight across a 12V supply would not be realistic. It would be a short circuit, allowing an infinite current to flow.
    You can't just 'leave out' the other resistor because it is a vital part of the circuit. It doesn't "simplify" the question - it makes it harder in fact.

    The current through the two resistors in series is governed by the total resistance. I=V/R

    R = the sum of the two resistors and changes as the thermistor changes temperature - so the current will also vary.
    But, for one particular value of thermistor resistance, you can work out 1. The current and 2. the voltage across each resistance in turn (the volts will, of course add up to your 12V).
    When the thermistor is at 0Ohms, there will be 12V across the other R.

    BTW your arithmetic is wonky!! 12/0 = INFINITY!!!!
     
  4. Apr 20, 2012 #3
    First thank for the input and clearing it up, for me. I think I have wrapped my head a round the idea near. I still cant see how 12/0= infinity, but I will study up on that.

    Thanks again.
     
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