1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Heisenberg's uncertainty principle question

  1. May 30, 2012 #1
    1. The problem statement, all variables and given/known data
    The position and momentum of a 1.00 KeV electron are simultaneously determined. If the position is located to within 0.100 nm, what is the percentage of uncertainty in its momentum?

    (Arthur Beiser - Concepts of modern Physics, 3rd Part exercise 33)

    2. Relevant equations

    3. The attempt at a solution

    [tex]Δx\cdotΔp≥\frac{\hbar}{2}\rightarrow \; Δp≥ \frac{\hbar}{2\cdotΔx}[/tex]
    Given that Δx=0,100 nm, we can evaluate the uncertainty in its momentum Δp.
    The momentum of the particle should be of roughly equal magnitude, and evaluating the fraction [tex]\frac{Δp}{p}%[/tex] gives me 98%, where the answer is 3,1 %. What did I do wrong?
  2. jcsd
  3. May 30, 2012 #2


    User Avatar
    Gold Member

    What did you plug for p?

    It should be p=1[Kev] /c where c is the speed of light.
  4. May 30, 2012 #3

    [tex]p=\frac{1,6\cdot10^{-16} J}{3\cdot10^{8} \frac{m}{s}}=0,533 \cdot 10^{-24} kg\cdot\frac{m}{s}[/tex]
    and because [tex]Δp=\frac{1,05\cdot10^{-34}J\cdot s}{2\cdot 10^{-10}m}=0,525\cdot 10^{-24} kg\cdot \frac{m}{s} [/tex]
    we get

  5. Jun 3, 2012 #4
    Could I get a bumpity-bump 'cause these exercises are crucial for the finals?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook