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Homework Help: Resistor Tolerance/Error

  1. May 25, 2005 #1
    Hey everyone.

    I am doing an Ohm's law lab where there is a variable source, a fixed resistor, a voltmeter across the resistor, and an ammeter in series with the resistor.

    The fixed resistor is an unknown value but has a tolerance of 5%

    You need to figure out its value by plotting Voltage VS Current and the slope of the graph is resistance.

    My question is....I need to add error bars to the graph but I don't know where to put them...the x-axis is current and the y-axis is voltage, so would I put X-error bars of 5%, or Y-error bars of 5%, or both?

    My guess is that since the resistor has tolerance of 5%, and resistance depends on Voltage and current, both x and y axes need to have 5% error bars

  2. jcsd
  3. May 25, 2005 #2


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    Staff: Mentor

    You measured the current and voltage using two meters, right? In that case, the error bars should reflect how well you can measure each of those quantities. If they're analog meters, I usually try to interpolate to 1/10 of the smallest scale division, and use that 1/10 for the uncertainty. If they're digital meters, I usually use 1/2 of the smallest unit on the display. In either case, if the reading fluctuates visibly, I try to eyeball a +/- range.

    The tolerance on the resistor shouldn't figure into the graph itself at all. Measure the slope of the graph to estimate the resistance, and then compare that with the value marked on the resistor. If they're closer than the tolerance, you're OK.

    [added: If you really don't know the "actual" value of the resistance, or at least aren't supposed to know, then I don't see how the tolerance could come into this at all.]
    Last edited: May 25, 2005
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