# Homework Help: Determine the value of a decay constant

1. Aug 26, 2015

### Kingyou123

1. The problem statement, all variables and given/known data

What's up guys, I'm stuck on #2 of this worksheet,http://luc.edu/faculty/dslavsk/courses/phys111/homework/phys111-2015hw1.pdf
The half - life of C - 14 is 5730 years. Determine the value of its decay constant (and express the answer in appropriate MKS units).

2.The attempt at a solution
I'm not sure if I'm doing this correctly , but I set up my formula like this , 5730=14e-(constant of decay)5730

Last edited: Aug 26, 2015
2. Aug 26, 2015

### SammyS

Staff Emeritus
Hello . Welcome to PF !

You will get a better response if you display the question so that it can be read directly. The following is what I was able to obtain from your pdf file using Window's "Snipping Tool" & easily display here.

Better yet, you could type out the problem. I makes it easier to respond to, and it shows initiative on your part.

As far as your attempt at a solution goes, it looks like you simply picked out some numbers at random from the problem statement and then plugged them into the given equation.

3. Aug 26, 2015

### Kingyou123

I've been using this fourm. https://www.physicsforums.com/threads/decay-constant.24169/ , as a guide and I can't figure out what to do... it's the first assignment of the semester and we haven't gone over this...

4. Aug 26, 2015

### SammyS

Staff Emeritus
C-14 is carbon 14. That's just an identifier for the type of nucleus being addressed. The number 14, will not be involved anywhere in the solution.

The problem statement gives many hints, maybe even telling you much of what you need to do to solve the problem.

If you start with N0 C-14 nuclei, then after 5730 years (one half-life), how many C-14 nuclei remain in your sample?

That quantity is what you put in for N(5730) .

5. Aug 26, 2015

### Kingyou123

Omg, you just made it so easy... so the formula would be 5370=(.5)e^(-(decay)5370) or would I set the entire thing to zero?

6. Aug 26, 2015

### SammyS

Staff Emeritus
No.

If you start with N0 nuclei, then how many will remain after one half-life has elapsed?

7. Aug 26, 2015

### Kingyou123

Half of the N0 nuclei would remain.

8. Aug 26, 2015

### SammyS

Staff Emeritus
Correct.

That's what goes on the left hand side of the equation.

9. Aug 26, 2015

### Kingyou123

So the formula would start looking like this .5=(.5)e^-(decay)5730

10. Aug 26, 2015

### SammyS

Staff Emeritus
How did you get that?

Where does the .5 come from on both sides?

Isn't the left side N0/2 , and the right side N0 times the exponential ?

11. Aug 26, 2015

### Kingyou123

I assumed that .5 is half, I get confused when I don't enter values in. So No/2=N0e^-(decay)5730? The thing that confuses me is what values to put in the fourmla that I have now.

12. Aug 26, 2015

### SammyS

Staff Emeritus
You should use another set of parentheses to make that technically correct.

0.5⋅N0=N0e^(-(decay)5730) .

Just solve for the decay constant. First step is to divide both sides by N0 . Right ?

13. Aug 26, 2015

### Kingyou123

Okay I divided both sides by No and then took the natural log of both sides. Afterwards I divided by -5730 to get my answer, correct?

14. Aug 26, 2015

### SammyS

Staff Emeritus
Yes, but 5730 is in units of years. That gives you the decay constant in units of years-1. MKS would be in units of seconds-1.