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Very Very simple question on dimensions of equation

  1. Nov 8, 2011 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    My teacher said

    e2 c x ħ/R

    Where e is the electric charge, c is speed of light and h is reduced constant and R is radius.

    My teacher said this has dimenions of energy, is this right?
  2. jcsd
  3. Nov 8, 2011 #2

    D H

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    c ħ/R has dimensions of (distance/time)*(energy*time)/(distance), or energy.

    Multiplying by the square of the electrical charge yields (charge)2*(energy), which does not have dimensions of energy.
  4. Nov 8, 2011 #3
    Ok thank you. I have another question. My teacher was showing us a way to check the dimesnions of equations. He showed us this


    So if something has mass x t dimensions, it can be given as


    it can also be written as L-1MT is this right?

    If I wanted to multiply two values together, let us say MT times T this is just



    How do you calculate something like


  5. Nov 8, 2011 #4
    What the teacher is telling you is fine so long as you don't mix up, say, seconds and hours or meters and centimeters, etc.

    MT times T is MT^2

    LMT/M^2 = L(M^-1) T

    In my opinion the better way to do it is put the units into the expression and handle them just like the units are typical algebraic variables and see if the units remaining in the expression are what you seek.
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