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# Anyone good at physics - activity/half-life?

Evening all.

I had some calculations to do for my physics assignment on radioactivity. One of the questions I am stuck on - the actual calculation was very simple but then it asks me to state one assumption made, but I dunno what assumtions I did make LOL!

Please can anyone help? Here's the question (and my calculation):

1. A radioactive source emits β particles. It has an activity of 2.8 × 107 Bq. Estimate the number of β particles emitted in a time interval of 2.0 minutes. State one assumption made.

A = dN / dt

A x dt = dN

(2.8 × 10^7) x 120 = 3.36 x 10^9

Would reeeealy appreciate some help, my teacher doesn't help us whatsoever LOL!

Thank you x

β particles emitted at a constant rate?
Edited by: "cannyscot" 5th Dec 2010

Original Poster

cannyscot

β particles emitted at a constant rate?

Ooh, I think you are right (I feel rather stupid)

Can I ask you one more, please? I only had two corrections through my whole assignment and haven't been able to figure them out LOL!

Original Poster

If anyone can help, pleeeeeeeeeease do - my mind is hurting thinking about this one so I guess I am thinking int he wrong direction or something!

This is a (very) rough graph which I submotted and my teacher simply said that it's not correct but I just can't figure out what is right then

Original Poster

I think I know a man who can help:

I don't know who that is LOL!

Original Poster

starsparkle2311

What are you studying towards Mrs Z? It sounds interesting, sorry I can't … What are you studying towards Mrs Z? It sounds interesting, sorry I can't help btw,lol

It's level 3 (A-level standard) science babe - comprises of chemistry, biology, physics and maths, I am really enjoying it but it is a LOT of extra work outside of college

Btw, don't worry about not being able to help - I don't seem able to help myself

I think that calculation is correct in your title, and the graph both need to be an exponential decay, because nuclear decay is exoponential.

Original Poster

bigfoot100

I think that calculation is correct in your title, and the graph both … I think that calculation is correct in your title, and the graph both need to be an exponential decay, because nuclear decay is exoponential.

Hiya, the title info was given to me - I just had to do the graph
I actually drew the graph in pencil on my assignment but just showed it roughly in Paint to give youse an idea. My lines were exponential but teacher said its wrong

Note how the points on this graph are connected with straight lines not a curved line as you have used.

Would it not be better to use logarithmic axis?

Original Poster

cannyscot

Note how the points on this graph are connected with straight lines not a … Note how the points on this graph are connected with straight lines not a curved line as you have used.

Thanks again cannyscot, but the problem is that I need to show the sodium-24 decay on the same axis, which is why I only did 27 years for the X-axis (one strontium-24 half-life)
I don't understand what I have done wrong oO

Half life % on y axis
time year on x axis

Edited by: "bigfoot100" 5th Dec 2010

cannyscot

Note how the points on this graph are connected with straight lines not a … Note how the points on this graph are connected with straight lines not a curved line as you have used.

Thats because they have plotted using straight lines and data points though, it isn't correct. Its an exponential delay, and therefore a smooth curve.

Original Poster

jah128

Thats because they have plotted using straight lines and data points … Thats because they have plotted using straight lines and data points though, it isn't correct. Its an exponential delay, and therefore a smooth curve.

That's correct, yes - even if I only show one half-life the line should be curved

Original Poster

bigfoot100

Half life % on y axistime year on x axis

I can't get that link to work BF x

Has the image come up? I have reedited my post

Original Poster

bigfoot100

Has the image come up? I have reedited my post

No sorry - the link isn't working either x

Mrs.Z

Note how the points on this graph are connected with straight lines not a … Note how the points on this graph are connected with straight lines not a curved line as you have used.

Thanks again cannyscot, but the problem is that I need to show the sodium-24 decay on the same axis, which is why I only did 27 years for the X-axis (one strontium-24 half-life)
I don't understand what I have done wrong oO[/quote]

OK but on the Strontium graph you have drawn a curve from 100% to 50%.

It should have been a straight line, because you can't assume that decay follows the path of your curve.

Joining points on a graph with straight lines is just a convention used in Physics.

You're right that the graph is exponential but the scale of the X axis won't show it (unless you extended it to 300 years as in the example I've posted.)

I have to say that your tutor/teacher is not being very helpful in providing no feedback - the subject is difficult enough withoug some wazzock being deliberately obtuse!

the sodium 24 line drops off too quickly, at 30 hours the percentage of material is 25%, at 45 hours (roughly 2 days) its 12.5% not 3% as you've shown in your graph. get your teacher fired btw.

zurich

the sodium 24 line drops off too quickly, at 30 hours the percentage of … the sodium 24 line drops off too quickly, at 30 hours the percentage of material is 25%, at 45 hours (roughly 2 days) its 12.5% not 3% as you've shown in your graph. get your teacher fired btw.

X-Axis is in years, not days ;-)

Original Poster

cannyscot

Thanks again cannyscot, but the problem is that I need to show the … Thanks again cannyscot, but the problem is that I need to show the sodium-24 decay on the same axis, which is why I only did 27 years for the X-axis (one strontium-24 half-life)I don't understand what I have done wrong oO

OK but on the Strontium graph you have drawn a curve from 100% to 50%.

It should have been a straight line, because you can't assume that decay follows the path of your curve.

Joining points on a graph with straight lines is just a convention used in Physics.

You're right that the graph is exponential but the scale of the X axis won't show it (unless you extended it to 300 years as in the example I've posted.)

I have to say that your tutor/teacher is not being very helpful in providing no feedback - the subject is difficult enough withoug some wazzock being deliberately obtuse![/quote]

Firstly - thank you soooo much for taking the time to help me, I really do appreciate it
The thing is that first I did draw a straight line from 100% to 50% as I assumed that it was the proportion of one to the other that the question was asking but my teacher said that was wrong.
Then I asked her if I curved the line to 50% would that then be correct, but she said "No"

The teacher is very good at explaining in class but when it comes to assignments, if you get a wrong answer she will simply cross it and refuse to help at all, not even a point in the right direction
I am the best student in my class attitude and grade-wise but she still just won't help at all and if you don't get the right answer by the final draft then you lose that whole grade (pass/merit/distinction)

I can't see what's incorrect with your graph

Original Poster

zurich

the sodium 24 line drops off too quickly, at 30 hours the percentage of … the sodium 24 line drops off too quickly, at 30 hours the percentage of material is 25%, at 45 hours (roughly 2 days) its 12.5% not 3% as you've shown in your graph. get your teacher fired btw.

Thank you for joining
I don't get what you are saying though, sorry - I am trying to fit a 15 hour half-life into 1 year gaps

zurich

the sodium 24 line drops off too quickly, at 30 hours the percentage of … the sodium 24 line drops off too quickly, at 30 hours the percentage of material is 25%, at 45 hours (roughly 2 days) its 12.5% not 3% as you've shown in your graph. get your teacher fired btw.

zurich

the sodium 24 line drops off too quickly, at 30 hours the percentage of … the sodium 24 line drops off too quickly, at 30 hours the percentage of material is 25%, at 45 hours (roughly 2 days) its 12.5% not 3% as you've shown in your graph. get your teacher fired btw.

oops X)

The graph should be a exponential curve, were every 27 years the value of strontium halves and every 15 hours the value of sodium halves.

You've only shown one half life's worth for Strontium.

You have to increase the timespan of your X-axis so that the exponential nature of Strontium's decay is obvious.

Original Poster

bigfoot100

The graph should be a exponential curve, were every 27 years the value of … The graph should be a exponential curve, were every 27 years the value of strontium halves and every 15 hours the value of sodium halves.

Thanks, I thought it was - though its very hard to show the exponential curve for sodium

Thanks very much for the input, you've been most helpful
Edited by: "Mrs.Z" 5th Dec 2010

cannyscot

OK but on the Strontium graph you have drawn a curve from 100% to 50%.It … OK but on the Strontium graph you have drawn a curve from 100% to 50%.It should have been a straight line, because you can't assume that decay follows the path of your curve.Joining points on a graph with straight lines is just a convention used in Physics.You're right that the graph is exponential but the scale of the X axis won't show it (unless you extended it to 300 years as in the example I've posted.)

I don't really agree with this - it is an exponential decay, therefore the convention (when straight lines are desired) is that it should be drawn with logarithmic scales. When drawn with linear scales, you do need to assume the curves...
Edited by: "jah128" 5th Dec 2010

Original Poster

cannyscot

You've only shown one half life's worth for Strontium.You have to … You've only shown one half life's worth for Strontium.You have to increase the timespan of your X-axis so that the exponential nature of Strontium's decay is obvious.

Ok I will try that, but by making the graph longer - I don't want to decrease the size of my gaps as they are as it will make things harder for the sodium line

Thanks sooooo much for your help

Mrs.Z

Thanks, I thought it was - though its very hard to show the exponential … Thanks, I thought it was - though its very hard to show the exponential curve for sodium

I agree seems a pointless exercise they should have chosen more comparable half lives.

Original Poster

bigfoot100

The graph should be a exponential curve, were every 27 years the value of … The graph should be a exponential curve, were every 27 years the value of strontium halves and every 15 hours the value of sodium halves.

bigfoot100

I agree seems a pointless exercise they should have chosen more … I agree seems a pointless exercise they should have chosen more comparable half lives.

bigfoot100

I agree seems a pointless exercise they should have chosen more … I agree seems a pointless exercise they should have chosen more comparable half lives.

I totally agree - when we first got the assignment I asked her if there was a mistake in one of the units

I totally agree - when we first got the assignment I asked her if there … I totally agree - when we first got the assignment I asked her if there was a mistake in one of the units

Well it was a lesson for me lol the first time I did it I treated them both as years.
I have a problem with they way science is taught, most of the stuff is totally irrelevant to anything. I am 3/4 through an Physics degree and I have basically made myself forget A-level physics, and begun again. There needs to be a big shake up in how it is taught.

Original Poster

What image were you trying to show me BIG FOOT?

Original Poster

bigfoot100

Well it was a lesson for me lol the first time I did it I treated them … Well it was a lesson for me lol the first time I did it I treated them both as years.I have a problem with they way science is taught, most of the stuff is totally irrelevant to anything. I am 3/4 through an Physics degree and I have basically made myself forget A-level physics, and begun again. There needs to be a big shake up in how it is taught.

I get what you are saying - we aren't doing a full physics A-level, it is a combination of all the sciences but I do sometimes wonder about what we are learning and the things they put particular focus on oO

The thing is for some who isn't going into a relatable job the syllabus is fairly dry, it should be made more interesting, the stuff we do at uni is mind blowing. It could be easily simplified and would I am sure interest most people.

Mrs Z,

I was niggled about your inability to show the decay of Sodium properly.

So,

Use a timescale of hours for the X axis, say 90 hours, which will show the exponential decay of the Sodium clearly.

The graph for Strontium on that scale will be a straight line parallel to the X axis at 100%.

The object of the exercise to the show how markedly different are the rates of decay of different substances.

Hi there, I might be wrong but I think the Na24 plot should go straight ( vertically) down to the x-axis
In the grand scheme of things if you started off with an equal amount of str-90 & Na-24 , using their half-life decay rates , then using your scale ,in one years time there'll be very very very very little of the original starting Na-24 left whereas the mass of the str-90 would hardly have decayed so graphscale-wise the blue line should only go vertically straight down ( as it can be safe to assume it's decayed to near enough nothing)or if it curves to the right it should be at a right angle where both X&Y axes meet. ( or the curve should go through "zero")
...I might be wrong ..

Original Poster

cannyscot

Mrs Z,Just had another thought.I was niggled about your inability to show … Mrs Z,Just had another thought.I was niggled about your inability to show the decay of Sodium properly.So,Use a timescale of hours for the X axis, say 90 hours, which will show the exponential decay of the Sodium clearly.The graph for Strontium on that scale will be a straight line parallel to the X axis at 100%.The object of the exercise to the show how markedly different are the rates of decay of different substances.

That is actually a very good idea, I am going to give that a go - it would probably look more presentable on paper too. I am making the assumption that I should be showing a half-life for each one but I guess that that is not necessarily the case

Original Poster

darklight

Hi there, I might be wrong but I think the Na24 plot should go straight ( … Hi there, I might be wrong but I think the Na24 plot should go straight ( vertically) down to the x-axis In the grand scheme of things if you started off with an equal amount of str-90 & Na-24 , using their half-life decay rates , then using your scale ,in one years time there'll be very very very very little of the original starting Na-24 left whereas the mass of the str-90 would hardly have decayed so graphscale-wise the blue line should only go vertically straight down ( as it can be safe to assume it's decayed to near enough nothing)or if it curves to the right it should be at a right angle where both X&Y axes meet. ( or the curve should go through "zero")...I might be wrong ..

Hiya, yes on my graph the Sodium line ran practically straight down the y-axis but I wasn't sure if I should cut the line somewhere or take it all the way to 27 years oO

If I use Cannyscots suggestion then this wouldn't be a problem

assume the decays so rapid for a small finite mass of Na24 that you can represent it as a vertical only (the blue line) in your plot with no tail ( comparing it to the str90 time line )

Original Poster

darklight

assume the decays so rapid for a small finite mass of Na24 that you can … assume the decays so rapid for a small finite mass of Na24 that you can represent it as a vertical only (the blue line) in your plot with no tail ( comparing it to the str90 time line )

thank you, I wasn't sure at which point the percentage remaining would be so small that it would be negligible
I am going to have a go at Cannyscots way before anything else
Edited by: "Mrs.Z" 6th Dec 2010