This discussion is now closed.

Check out other Related discussions

Can someone explain this physics question in idiot terms please!

Uranium - 235 and uranium -238 (proton number=92) are isotopes. According to a popular theory, there were equal numbers of each isotope when the earth was formed. Assuming this to be true, estimate the ratio R = (number of U-235)/(number of U-238) at time 4.5x10^9 years after formation.

Please can someone show me how to do this, I don't even know how to work out ratios.

Also, do beta particles have anything unique to them that make them dangerous or is there only danger their ionising ability?

Reply 1

[_Z_]

Are the half lives given?

Reply 2

Datura

OP

1

sorry yeah they are, half life or U-235 is 7x10^8y and that of U-238 is 4.5x10^9y

Reply 3

Willa

11

this question is to do with half life of U-235 (which decays radioactively - random and spontaneous decay).

So use the equation:

N = N0*e^-lt

where N0 is number of atoms at start = we pick a number e.g. 1
so N = e^-lt

so now you just look up l (the decay constant) for u-235 and plug that and the time given into the equation. You'll need to have l in terms of decay per year since the time is in years. Otherwise convert time to seconds and use probability of decay in one second.

Do the same for u-238

then R = (e^-[l235]t)/(e^-[l238]t) where the bits in square brackets are the decay constants. Given from the equation: decay constant = (ln 2)/half life

Beta particles have medium penetrative abilities - they can get through paper for example, but not a sheet of aluminium. It is the ionising effect that does the damage, but it wont do damage to humans unless it ionises stuff inside the body - that's where the penetrative effect coems in. Alpha particles arent really dangerous, because they cant get through the skin (although it's bloody dangerous to inhale them - e.g. radon gas - will cause lung cancer/throat cancer)

Reply 4

Datura

OP

1

Thanks for your help! I've got so much physics to get through today and tomorrow.

Reply 5

[_Z_]

Bebop

sorry yeah they are, half life or U-235 is 7x10^8y and that of U-238 is 4.5x10^9y

after 4.5x10^9 yrs.... u238.. will have halved...because the time passed is exactly equal to half life.... for u235 approx... 2.68 (take it as 2) half lives fit in this time period.... so u238=50%left u235= 25% left

ratio is u235:u238
1:2 i think this is right... not sure..... it says estimate..

Reply 6

Datura

OP

1

in the book it says U238 has halved in given time; while u235 has gone through 6.43 half live, a factor of 86(1/2^6.43) Ration is 2/86=0.023

I can't seem to get this answer or understand what they have done to get it.

Reply 7

[_Z_]

Bebop

in the book it says U238 has halved in given time; while u235 has gone through 6.43 half live, a factor of 86(1/2^6.43) Ration is 2/86=0.023

I can't seem to get this answer or understand what they have done to get it.

ok... heres how i did it b4. but made one silly mistake....... u know that u238 will half during this time.........to calculate how many half lives u 235 goes through...:

(4.5x10^9)/(7x10^8)=~6.43 which is the no of half lives U235 goes through...suppose u had 100 atoms of u238 and 100 atoms of u235........ no of atoms of u238 left=(100/2^1) and no of atoms of u235=(100/2^x) where x=no of half lives=6.43 so u would get U235 :u:

238
50:1.16093 which is 0.023

Reply 8

minkailin

let initial U-235 and U-238 amount b X.
then after 4.5*10^9 years:

amount of half lives passed for U-235 is (4.5*10^9/7x10^8)=6.43
so amount of U235 left is X(0.5^6.43)

the amount of U238 left is 0.5X, since one half life passed.

so U235/U238=X(0.5^6.43)/0.5X=0.5^5.43=0.0232

Reply 9

kenkennykenken

I am confused and I have no idea what is going on. Can anyone explain it in a bit more detail i.e. more explanation along with the calculations...

Can someone explain this physics question in idiot terms please!

Related discussions

Latest

Trending

Trending

Articles for you