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brainyman89
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what makes half life(T) of radiation independent of the quantity of radioactive substance? why is it constant whatever the amount of the radioactive substance is?
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mathman said:By definition, half life is the time it takes for half of the sample to decay. Since the decay of an individual nucleus is independent of the size of the sample, the half life doesn't depend on the size of the sample.
pallidin said:Uh... why not just use "full life"?
Wouldn't that make more sense?
gmax137 said:you're kidding, right??
the 'full life' does depend on the quantity
mathman said:By definition, half life is the time it takes for half of the sample to decay. Since the decay of an individual nucleus is independent of the size of the sample, the half life doesn't depend on the size of the sample.
The half-life is the time it takes for 50 g of a 100 g sample to decay. Which is the same time it takes for 500 mg of a 1000 mg sample to decay. Which is the same time it takes for 1 lb of a 2 lb sample to decay.pallidin said:Half of the sample?
What the heck is that?
Why not just use the full sample? As, you said, the result is independent of the size of the sample.
Let's just use half of DNA to convict criminals.
DrGreg said:The half-life is the time it takes for 50 g of a 100 g sample to decay. Which is the same time it takes for 500 mg of a 1000 mg sample to decay. Which is the same time it takes for 1 lb of a 2 lb sample to decay.
The time it takes for 100 g of a 100 g sample to decay is theoretically infinite.
pallidin said:That makes no sense AT ALL.
If half of the product decays in a predicitable manner, yet 100% does not, there is something seriously wrong.
I don't think that first sentence is quite right: it's just the average time it takes for one to decay. In other words, if you do the experiment over and over again with one particle at a time, half the time it will decay in that amount of time and half the time it won't.Drakkith said:The half life is the average time it takes for any atom of that element/isotope to have a 50% change of decaying. That means that if you look at every atom of that element that was created at the same time, 50% of all those will have decayed at the half life point.
Drakkith said:The half life is the average time it takes for any atom of that element/isotope to have a 50% change of decaying.
russ_watters said:I don't think that first sentence is quite right: it's just the average time it takes for one to decay.
When you get down to the last nucleus that hasn't changed, there is still no defined time for that single nucleus to decay. So, even in a practical sense, there is no such thing as "full life". And, if you are talking in terms of a mathematical model - the exponential function never reaches zero.gmax137 said:you're kidding, right??
the 'full life' does depend on the quantity
sophiecentaur said:... Half life is only (afaik) used in the context of radioactive decay...
sophiecentaur said:I was discussing this very thing with a student, yesterday. Half life is only (afaik) used in the context of radioactive decay. It's the one instance where the 'general Public' need an appreciation of what exponential decay applies to their lives. All other situations involving exponential decay seem to use a 'decay constant' or 'time constant', both of which represent the time for decay to fall to 1/e. This is because the Maths comes out without having to introduce an extra constant in the calculations.
QuantumPion said:Half life is also used in biology. The biological half-life is the time needed for an organism to process some chemical. E.g. the biological half life of iodine (which is completely different from the radioactive half-life) is about 100 days.
Atoms have no memory. They don't "know" how old they are. A 100-year-old undecayed atom is just as likely to decay as a 0-year-old undecayed atom. So after 100 years your remaining 8oz of undecayed material behaves as if you had started with 8oz of material in the first place. And it takes 100 years for 4oz of it to decay. And then another 100 years for 2oz of the remaining 4oz to decay, and another 100 years for 1oz of the remaining 2oz to decay, and so on.pallidin said:Hmmm...
Let's see: A 1-pound sample half-life decays in 100 years(theorectically speaking)
Therefore 8oz decays, presenting a remaining 8oz.
That remaining 8oz should decay in an additional 100 years.
Full-life... 200 years.
If that is not true, how is the first half of the sample somehow subject to different rules than the second half?
pallidin said:Hmmm...
Let's see: A 1-pound sample half-life decays in 100 years(theorectically speaking)
Therefore 8oz decays, presenting a remaining 8oz.
That remaining 8oz should decay in an additional 100 years.
Full-life... 200 years.
If that is not true, how is the first half of the sample somehow subject to different rules than the second half?
pallidin said:Hmmm...
Let's see: A 1-pound sample half-life decays in 100 years(theorectically speaking)
Therefore 8oz decays, presenting a remaining 8oz.
That remaining 8oz should decay in an additional 100 years.
Full-life... 200 years.
If that is not true, how is the first half of the sample somehow subject to different rules than the second half?
sophiecentaur said:There is no such thing as "full life".
Why?pallidin said:Well, there had better be, else reality has a huge problem.sophiecentaur said:There is no such thing as "full life".
pallidin said:Well, there had better be, else reality has a huge problem.
brainyman89 said:what makes half life(T) of radiation independent of the quantity of radioactive substance? why is it constant whatever the amount of the radioactive substance is?
pallidin said:Huh, well, how about this:
I have 1 gallon of gasoline in an engine.
With a constant load, 1/2 gallon is "burned" in x amount of time.
The remaining 1/2 gallon will "burn" at the same rate.
The "full life" is 2x.
The above is obviously true, to me anyway.
Of course, nuclear decay is obviously a different process than combustion.
I'm OK with that, so I must be missing something here.
Vanadium 50 said:The rate of combustion is constant.
The rate of nuclear decay is proportional to the amopunt of material.
QuantumPion said:Nuclear decay is not a constant load...