MadSci Network: Physics
Query:

Re: How do you measure the mass of an electron?

Date: Tue Jan 16 02:05:51 2007
Posted By: Samuel Silverstein, Lecturer in physics
Area of science: Physics
ID: 1167971999.Ph
Message:

The electron mass (or for that matter, the mass of any charged particle) can be measured by seeing how its trajectory is bent in a magnetic field. A more massive object is harder to accelerate, so its path will bend less than that of a lighter object.

Most undergraduate physics students measure the electron mass as a lab exercise. Here is a nice example of such a lab, including illustrations and calculations.

Strictly speaking, the above is NOT a direct measurement of the electron mass, but rather a measurement of the ratio charge/mass. To find just the mass, you need to make a separate measurement of the electron charge.

I also did this in an undergraduate lab, in the form of Millikan's oil drop experiment. Here, small drops of oil are given an electric charge, and an electric field is used to suspend the drops in the air. By estimating the oil drop mass and measuring the electric field needed to cancel the gravitational pull, the electron charge can be calculated.

Hope this helps!


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