Very Very simple question on dimensions of equation

  • #1

Homework Statement





Homework Equations





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?
 

Answers and Replies

  • #2
D H
Staff Emeritus
Science Advisor
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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.
 
  • #3
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.
Ok thank you. I have another question. My teacher was showing us a way to check the dimesnions of equations. He showed us this

LMT

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

LMT/L

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

MT2

right?

How do you calculate something like

LMT/M2

Thanks
 
  • #4
1,198
5
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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