How to Calculate Uncertainty

$\Delta p \Delta x\geq \frac{h}{4\pi }$

Posts: 48
Joined: Wed Sep 30, 2020 10:03 pm

How to Calculate Uncertainty

One of the last things we went over in lecture that will be on Midterm 1 is the Heisenberg Indeterminacy Equation. I’m still a little confused on how one would find the change in p and change in x of this equation. If anyone knows how to and can help I would really appreciate it. Thank you!

Brandon Pham 1K
Posts: 39
Joined: Wed Sep 30, 2020 9:59 pm
Been upvoted: 1 time

Re: How to Calculate Uncertainty

For reference, (delta x)*(delta p) >= h/4pi

Generally, a question will either give you a value for one of the variables and ask you to find the other. If given delta x (which will usually be a range), simply rearrange the question to solve for delta x. Also note that delta p (change in momentum) can also be written as (m * delta v)

Claire_Kim_2F
Posts: 50
Joined: Wed Sep 30, 2020 10:02 pm

Re: How to Calculate Uncertainty

For me I just use the equation and plug in for delta p and delta x with delta x usually being the diameter away from the atom and delta p equaling m and delta v usually. Then reading the problem and plugging into the equation. I wish I could put the equation here but I do not know how to attach images. Sorry if this explanation was vague.

Juliet Cushing_2H
Posts: 49
Joined: Wed Sep 30, 2020 9:39 pm

Re: How to Calculate Uncertainty

I echo the above statement! Because the Heisenberg uncertainty principle equation has two unknowns, you will have to be given one of them (or be given the tools to solve for one of them). Either you know Δx, or you know Δp, or you know Δv which can be used to find Δp given p=mv. Plug in your known value to the equation and pop out the other!

Wasila Sun 1K
Posts: 49
Joined: Wed Sep 30, 2020 10:00 pm
Been upvoted: 2 times

Re: How to Calculate Uncertainty

Heisenberg's Uncertainty/Indeterminacy equation is (delta p)(delta x) $\geq$ h/4$\Pi$. Usually in a question, like the one done in class, they would give you either the uncertainty in position (delta x) or the uncertainty in momentum (delta p). From there you can use the equation to solve for the variable that they did not give you. Because there are only 2 variables and h and pi are constant numbers, if given one variable we can manipulate the equation to solve for the other. If they are asking you to solve for the uncertainty in velocity and not uncertainty in momentum we can use p=mv or v=p/m to find v.

Gabriel Nitro 1F
Posts: 45
Joined: Wed Sep 30, 2020 9:32 pm
Been upvoted: 1 time

Re: How to Calculate Uncertainty

Hi,

With regards to uncertainty in position, if you are not solving for it, the problem will most likely give you the value. However, they could give it to you in a variety of ways. For instance, in terms of the "atom in a box" model, they could give you a statement such as "The uncertainty in position of the particle is a diameter of 150 pm." In this case, assuming you've converted the 150 pm into m, that diameter would be your range of uncertainty.

However, if they said "The uncertainty in position of the particle is a radius of 150 pm," you would need to double it to get the diameter, convert to m, then plug that into the Heisenberg Indeterminacy Equation.

Hope this helps! :)

Neel Sharma 1G
Posts: 41
Joined: Wed Sep 30, 2020 9:32 pm
Been upvoted: 1 time

Re: How to Calculate Uncertainty

Gabriel Nitro 1F wrote:Hi,

With regards to uncertainty in position, if you are not solving for it, the problem will most likely give you the value. However, they could give it to you in a variety of ways. For instance, in terms of the "atom in a box" model, they could give you a statement such as "The uncertainty in position of the particle is a diameter of 150 pm." In this case, assuming you've converted the 150 pm into m, that diameter would be your range of uncertainty.

However, if they said "The uncertainty in position of the particle is a radius of 150 pm," you would need to double it to get the diameter, convert to m, then plug that into the Heisenberg Indeterminacy Equation.

Hope this helps! :)

I saw this question online and was wondering if the uncertainty is .5nm or 1nm. The position of an electron is known to be within 0.500 nm. What is the minimum uncertainty in its momentum?