# Concept question

1. Feb 4, 2014

### nil1996

(Warning, a dumb question incoming )

Just got curious while reading an article on photoelectric effect.
Why doesn't the mass of electrons increase(because electrons already revolve at the speed of light) when it absorbs photons instead of increasing radius of orbit?

(Sorry for my English.It isn't my first language.)

2. Feb 4, 2014

### Staff: Mentor

Electrons don't revolve at the speed of light.

3. Feb 4, 2014

### nil1996

But they may be moving at near speed of light.And when objects near speed of light absorb energy they increase their mass,isn't it?

4. Feb 4, 2014

### phinds

Only from the point of view of a remote observer. An object's mass in its own frame of reference is called the invariant mass, and as the name implies, it doesn't change.

5. Feb 4, 2014

### Staff: Mentor

I thought that might be what you're getting at, and the answer is "no, not really". Mathematically the relationship you're looking for is $E^2 = (m_0c^2)^2 + (pc)^2$ where $m_0$ is the mass of the object at rest and $p$ is the momentum. If the energy increases, the momentum must increase... and if the momentum increases enough, the electron will be able to escape.

The idea that mass increases with speed isn't outright wrong, but it has limited usefulness, and orbiting electrons are one of the situations where it doesn't apply. You'll hear a lot about it in the popular press and in some introductions to special relativity, but it's mostly a holdover from earlier days. The equation above (which, interestingly, reduces to $E=mc^2$ when $p=0$) is more flexible and generally applicable.

6. Feb 4, 2014

### nil1996

ok thanks a lot guys