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phetsims / charges-and-fields

"Charges And Fields" is an educational simulation in HTML5, by PhET Interactive Simulations.
GNU General Public License v3.0
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User reported issue: E-field does not follow inverse-square law #117

Closed arouinfar closed 7 years ago

arouinfar commented 7 years ago

A teacher wrote into phethelp saying

I collected data using the field strength meter and a single positively charged particle, and the values did not follow the inverse square law. Is there something else I am missing? image image image This last one should show a direct proportionality (straight line through the origin). Can you help me?

I took a few quick measurements, and there may be something to the teacher's claim. For example, if I calculate the E-field for a +1nC charge for the scenario shown in the screenshot, I find that the equipotential voltages should be roughly 16V, 6V, and 1.5V.

image

I'll need to dig into this more carefully tomorrow.

arouinfar commented 7 years ago

I've re-reviewed the user's data, and have realized that he plotted the Electric Potential versus 1/r^2, not the Electric Field. The electric potential of a point charge behaves as kq/r.

For a sanity check, I collected the voltage data using the distances in the user's table. Plotting the voltage vs. 1/r results in a linear trend. image

I've already replied to the user and cc'ed phethelp, so this issue can be closed.