# Mass of an electron?

1. Jul 28, 2010

### curiousphoton

Just a quick (hopefully not too dumb) question :

How do physicists determine the mass of an electron? Is there a device or experiment used? I am just confused because of its particle / wave properties.

2. Jul 28, 2010

### mgb_phys

JJ Thompson (discovery of the electron) measured the ratio of mass/charge by seeing how electrons bent in a magnetic field.
Then Robert Millikan measured the charge with the famous oil drop experiment - so having the charge and mass/charge you can get the mass.

3. Jul 28, 2010

### talk2glenn

The mass of anything is defined by the force it takes to make that particle accelerate in some direction relative to an observer, by F=ma, or M=f/a. If we apply some force f, and observe an acceleration of the electron a, we can easily deduce its mass m.

In practice, they do this in labs with electromagnetic fields. Electrons are negatively charged, and get pushed by magnetic fields (just like bringing positive poles on a magnet near negative poles on other magnets pushes them away from one another); we can observe this movement, and deduce mass.

4. Jul 29, 2010

### curiousphoton

Thanks for the refresher.

The reason I asked was because I was thinking about how a hydrogen atom could weigh less than when it's proton and electron are considered individually. This is due to negative binding energy, meaning you have to put energy into the atom to seperate the proton from the electron.

I was just thinking how this applies to real life. I mean if I have 2 bowls of mashed potatoes and I combine them, the 1 bowl of mash potatoes should theoretically weigh the same as the 2 individual bowls of mashed potatoes, correct (neglecting the bowls of course)? Now to seperate the one bowl potatoes back into two bowls, I have to put some sort of work, or energy, into the system, correct?

I guess I just confused on how the hydrogen system is different than the mashed potatoes?

5. Jul 29, 2010

### Staff: Mentor

But you didn't release any energy when you combined the two bowls of potatoes originally, right? This is different from combining an electron and proton, which does release energy.

6. Jul 29, 2010

### curiousphoton

So why isn't energy released when combining the two bowls of potatoes? I mean each bowl is a combination of protons and electrons and neutrons, right? So we combine both and don't release energy, but when we combine an electron and proton we do release energy?

7. Jul 29, 2010

### Bob S

Here is a picture of an electron beam being bent in the field of a Helmholtz coil in an undergraduate physics lab. The diameter of the orbit is ~0.5 cm, and is perpendicular to the direction of the magnetic field.

Bob S

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