At what velocity should relativity be taken into account

[moonkey]

Homework Statement

[STRIKE]I have exams coming up and I'm just looking for an idea of when relativity should be taken into account. For example I'm just going over a question involving photoelectric effect and potential fields and de Broglie and all that jazz and the final velocity of the electron turns out to be around 6.5x10⁶ms^-1. Should I take relativity into account for this velocity or is it still safely within the margins of non-relativistic velocity? I'm thinking that it is safe to assume that it is non-relativistic but I would like some advice just in case such a question arises that involves a greater velocity.[/STRIKE]

Sorry, wrong forum. Moving to introductory

Homework Equations

The Attempt at a Solution

 

[L-x]

I think a generally good approach if you're unsure is to work out gamma (http://en.wikipedia.org/wiki/Lorentz_factor). If it's close enough to 1 that you're happy to say gamma=1 then you don't need to to consider relativistic effects. If it's different enough to 1 that you wouldn't be happy rounding it to 1, then you will want to take SR into account.

On that wikipedia page you'll notice that for v=c/5, gamma is still only 1.02, so you can get to pretty big speeds without really needing to worry about relativistic corrections.