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Electromotive force

  1. Mar 14, 2015 #1
    sorry for the drawing.Hope you guys can bear with it.
    The electromotive force can be calculated by opening the switch and the voltmeter will show the exact voltage of the battery. When the switch is closed,there will be a voltage drop due to the internal resistance of the battery. The question is, there will be a potential different across the two points between the battery when the circuit is incomplete which will show the exact voltage of the battery,but does the same goes to reading of the voltage given that the circuit is complete ? Why ? (please refer to the drawing)
    another question, if there is an internal resistance within the battery why do they requirre the two points ? and why not just connect the voltmeter to the positve and negative terminal of the battery ? Thanks !
  2. jcsd
  3. Mar 14, 2015 #2


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    Science Advisor

    More correct: A voltage drop due to the current through the battery and the internal resistance of the battery.
    The internal resistance is not a resistor, it is due to the construction of the battery. The internal resistance is defined as the drop in battery voltage divide by the current drawn from the battery.
    Sorry, I have no idea what you are asking.
  4. Mar 14, 2015 #3
    Best to have a multimeter to measure the respective values before trying to understand the completion of a circuit. Electrons flow from positive to negative between two points upon the closing of a circuit. <link deleted>
    Last edited by a moderator: Mar 14, 2015
  5. Mar 14, 2015 #4
    To measure the emf of a voltage source you must not draw any current (or very little current) from the voltage source during the measurement. When using a voltmeter use one with a high input impedance ( resistance) usually rated by the number of ohms per volt measured. Best above 1 megaohm per volt. For the best accuracy use the potentiometer circuit below the accuracy of which is only limited by the accuracy of the standard cell and the ratio of the resistance arrived at in its adjustment.

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