# Finding the y axis on a probability density graph

#### Sorin2225

Problem Statement
I was given a probability density graph and from it I had to figure out what B was in nm^-1/2
Relevant Equations
I think it's just from the graph.
I assumed to find it I would need to find the area under the graph. I also assumed that the part under x would cancel out so I would be left with 2b*10=1 if it was, in fact, true that it had to equal to one. So my final answer was (1/10)/2 nm^-1 but the actual answer was 0.0845 nm^-1/2 and I'm unsure of how this answer was gained.

#### Attachments

• 2.3 KB Views: 57
Related Advanced Physics Homework News on Phys.org

#### RPinPA

Homework Helper
That is not a probability density. A probability density can not be negative.

Judging by the use of the symbol $\psi$ I'm guessing this is a quantum mechanical wavefunction, so the actual probability density is $|\psi|^2$. Your method was correct, that the total probability (total area) must be 1. So make that change and see how it works out.

The units of $b$ are another clue that you're supposed to be squaring these values to get the density, because $b^2 \times$ [length] is unitless.

#### Sorin2225

To get b however I'm doing 3b*10-2b*10+2b*10=1 and rearranging for this but I'm nowhere near the right answer

#### RPinPA

Homework Helper
To get b however I'm doing 3b*10-2b*10+2b*10=1
You shouldn't be. You're integrating $\psi$ with respect to $x$ and I just told you that $\psi$ is not the probability density, not the thing you should be integrating. Therefore trying to get a total probability of 1 by integrating $\psi$ is not a correct thing to do.

Again, you want to integrate the probability density. $\psi$ is not the probability density. Find the probability density and then integrate that.

I know you started from this assumption, in your original post:
I was given a probability density graph...
but what I'm trying to say is that you were not given a probability density graph. A probability density can not be negative. $\psi$ is by inspection not a probability density.

#### Sorin2225

Yes sorry, I realized that my initial assumption about the type of graph was wrong. I am confused on how I am meant to be changing the probability into probability density. To do this I attempted
to square the probability

"Finding the y axis on a probability density graph"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving