Electron as a Particle
I read that the electron is a point particle. This
makes sense if it
is a fundamental particle, since if it had non-zero volume whatever it
was made out of would be more fundamental.
The problem is, I thought the electron had mass--orders of magnitude
smaller than the proton, but still a measurable mass...
Can a point particle that takes up no space have mass? How?
I suspect my assumptions are wrong somewhere.
Good thinking, you suspect some assumption is wrong, and it is. The
electron -- as a point charge -- is an approximation when observed from
distances are much larger than atomic and molecular distances. At distances
of the order of nanometers, atoms have a finite size. One hydrogen atom e.g.
has a radius of ~5 nm, called the Bohr radius, and a sphere of this radius
is about how much space the electron is occupies in a H atom because the
proton is much much smaller. But when viewed up really close, even atomic
nucleii have a volume.
A "point charge" is an approximation, a mental construction that is quite
good for many cases. Except for the speed of light in a vacuum, I can't
think of any physical quantities that aren't approximations on some scale,
either very small or very large.
I don't know the context in which you learned that the electron is a "point
particle" but I don't think it was intended to imply that the electron has no
volume. The electron is a member of a class of particles called fermions.
Among other things, these particles exclude other fermions from their
i.e. they have volume.
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Update: June 2012