math/physics help

Post by **werldhed** » Thu Jan 12, 2006 11:21 pm

I need to determine the age of something based on its half-life. I think I have the formula correct, but it's been a long-ass time since I had a math class...

The substance (a protein) has a half-life of 17 hours. I want to figure out how long it's been expressed based on the percent of the original amount of protein.

This is what I came up with:

y=[ln(x)/(-0.693)]*17

Does that look right? Putting in 50% = 17, and 25% = 34, etc. seems to work, but I just wanted to make sure.

Post by **menkent** » Thu Jan 12, 2006 11:42 pm

looks good, not sure your -0.693 shouldn't be positive though.

Guest · Post by **Guest** » Fri Jan 13, 2006 12:14 am

What order is the reaction?

Post by **tnf** » Fri Jan 13, 2006 12:16 am

first

Guest · Post by **Guest** » Fri Jan 13, 2006 12:28 am

Then the time its been exposed would be:

kt = ln(I/F)

Where k is the rate constant
t is the exposure time
I is the starting concentration
F is the final concentration at time t.

To get k you just use this formula:

q = ln(2)/k

where q is the half-life time (17 hours).

Guest · Post by **Guest** » Fri Jan 13, 2006 12:36 am

For a formula to express the whole thing:

q*ln(I/F)/ln(2) = t

17*ln(I/x)/ln(2) = y

where I is the initial concentration, x is the variable for the final concentration, and y is the time elapsed. Use this if you want to plot a graph with x and y axis.

Post by **werldhed** » Fri Jan 13, 2006 1:04 am

That's exactly the same as mine, except that I use -(ln(2)). I'm pretty sure you need that, too, otherwise you'd end up with a negative time.
I dunno, though. Either way it tells me the info I need to know.
thanks

Guest · Post by **Guest** » Fri Jan 13, 2006 1:21 am

Well you wrote x instead of I/x... And you'll only get a negative answer if your I/x is less than 1 since ln(n) where n < 1 is always negative.

Post by **werldhed** » Fri Jan 13, 2006 2:12 am

In that case, I misunderstood what you meant by I/x. What do you mean by the "variable for the final concentration"?

If my original expression intensity is 1.0*, and I'm trying to find out how long the protein has been expressed if its final intensity is 25% of the original, wouldn't I just use .25 as x?

I don't understand what my value for I/x should be.

edit: I/x should be 4, right? So it's the factor by which the original intensity is divided?

*this is a protein, so I'm measuring it in terms of how much fluorescence is given off....see the glowing pig thread

Post by **tnf** » Fri Jan 13, 2006 2:14 am

i think i've done this same exact thing before...

Guest · Post by **Guest** » Fri Jan 13, 2006 2:20 am

werldhed wrote:edit: I/x should be 4, right? So it's the factor by which the original intensity is divided?

yes, exactly. Since I > x all the time, you can't get a negative answer with ln(I/x). Looks like that was the problem.

Post by **werldhed** » Fri Jan 13, 2006 2:22 am

tnf wrote:i think i've done this same exact thing before...

I wouldn't doubt it... I'm measuring RAG2 expression via GFP intensity. It's just goofy coincidence that the glowing pig thread showed up today, too. :icon26:

Anyway, I was preparing for a presentation I'm giving tomorrow and I looked at one of my graphs and though, "Shit! That's not right!"

So now I have to fix it all up.

Post by **werldhed** » Fri Jan 13, 2006 2:26 am

ToxicBug wrote:
werldhed wrote:edit: I/x should be 4, right? So it's the factor by which the original intensity is divided?
yes, exactly. Since I > x all the time, you can't get a negative answer with ln(I/x). Looks like that was the problem.

Gotcha. Thanks.
All of my data is in percents already, so it's easier to just leave it the way I have it, instead of converting it back to a factor. As long as the results are the same, I'm good.
Cheers. :icon14:

Guest · Post by **Guest** » Fri Jan 13, 2006 2:28 am

Glad I could help

Btw, I will most likely never touch learning chemistry/physics again, that stuff isn't my thing.

Post by **werldhed** » Fri Jan 13, 2006 2:36 am

Neither of those are my thing, either. Unfortunately there's always a little of both that creep into everything I do... There's no escape. :icon33:

Guest · Post by **Guest** » Fri Jan 13, 2006 2:44 am

What sort of field are you in?

Post by **werldhed** » Fri Jan 13, 2006 4:20 am

I do research in immunology and cancer immunotherapy (technically I'm a grad student/research fellow right now).
It's all biology, but when when you're doing lab research, a lot of chemistry and physics comes into play.

Unfortunately, some of it I don't know (e.g. this stuff)

Guest · Post by **Guest** » Fri Jan 13, 2006 4:24 am

Cool, maybe you'll help me with biology, since I'm taking Biology I this semester. That and Calculus III and Linear Algebra II

Post by **werldhed** » Fri Jan 13, 2006 4:33 am

NP... excluding the math, that is. I found calc to be quite easy, but that was about 5 years ago, and it wasn't calc III...

Is biology a required course? Because weren't you originally planning to go into engineering or something?

Guest · Post by **Guest** » Fri Jan 13, 2006 4:39 am

Yeah, but here we have 2 years of "pre-university" school, thats called "college", where we have to do one biology course. I wanted to go into engineering, but I've changed my mind since I don't want to do science anymore. Right now I have my mind set on actuary mathematics/finance

Post by **werldhed** » Fri Jan 13, 2006 4:43 am

I see... so is it basic biology, or do you choose a particular subject? (such as micro, or botany, or wildlife biology...)

Guest · Post by **Guest** » Fri Jan 13, 2006 4:45 am

Nah, basic biology, there are no options. People say its hard though :/

Post by **tnf** » Fri Jan 13, 2006 4:47 am

http://www.sinauer.com/detail.php?id=8565
I wrote most of the instructor's manual and about 7 chapters of test questions for that book.

Guest · Post by **Guest** » Fri Jan 13, 2006 4:48 am

So maybe u'll help me with bio? :lub:

Post by **werldhed** » Fri Jan 13, 2006 4:56 am

I've never seen that text, tnf. I've always just used the Cambell & Reece edition.
Did you work on a particular section, or were you all over the board?