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Suppose you are doing an experiment to determine the work function of a metal.  You get $KE_1$, $\nu_1$ and $KE_2$, $\nu_2$.   We know that $KE = h\nu - W$ but when you solve the simultaneous equations, we have a $h$ slightly different from the textbook value.

Question: In such a situation should you

  1. Use the textbook $h$ then average {$W_1,W_2$}; or

  2. Simply solve the simultaneous equations for $W$

I believe that the values obtained will be different.

Thank you.

Added: Assume you don't get to do more repeats etc.

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Look up work funktion values for metals in eg CRC Handbook. You will see those are given for just one digit after the point. This shows the accuracy of the experiments. Which h to choose then is secondary. –  Georg Oct 14 '11 at 16:45
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1 Answer

up vote 4 down vote accepted

You should definitely use the textbook value of $h$. In any experiment there are always (hopefully) small errors in measurements so the fact that you get two different values for the work function $W$ from two different experiments is to be expected. Average those $W$s and consider their difference to be a rough estimate of the potential error.

You say assume you don't get to repeat the experiment, but that is exactly what experimenters should do to try to get the most accurate estimate of the value they are measuring and of the potential error in that value.

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Definitely agreed. By using the textbook value of Planck's constant, in a sense you get to incorporate all the thousands of experiments that have been done to determine that value to ~7 decimal places, which is kind of like a "poor man's version" (or should that be "poor grad student's version"?) of increasing your sample size. –  David Z Oct 14 '11 at 18:07
    
Thanks guyz! :) –  danz Oct 14 '11 at 21:55
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