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Finding out internal resistance of a lemon when used as a power supply

  1. Dec 13, 2007 #1
    Hi There,

    Right I recently did an experiment in Physics to find out the internal resistance of a lemon when used as a power supply.

    We connected the lemon to a voltmeter, ammeter and variable resistor, changed the resistance on the variable resistance and measured the voltage and current against each other.

    So i have a table of voltage against current.

    I have drawn a graph of voltage against current, voltage on y axis, and current on the x axis. And drawn a line of best fit. Voltage is in milli volts, 10^-3 and current is in micro volts 10^-6

    How would i go about finding the EMF and eventually the Internal resistance of the lemon when used as a power supply.

    Also how would i go about finding the intercept of the graph if the line of best fit does not go through the y axis on the scale i have used.

    Anything to help would be fantastic

    Thank you in advance.
  2. jcsd
  3. Dec 13, 2007 #2


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    Science Advisor
    Homework Helper

    The intercept is the point at which the line crosses the Y axis.
    This would be the voltage with no current (hint what's the definition of the emf of a cell?)
    The slope of the graph is voltage/current, ( hint what's ohm's law?)
  4. Dec 13, 2007 #3


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    Staff: Mentor

    A straight line must pass through the y-axis somewhere if it's extended far enough, unless it's vertical (and therefore parallel to the y-axis).

    I suspect you need to "zoom out" your graph scale so the intercept is actually visible on the graph.
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