
Difficult Question in Physics from Post UTME
Physics
Chima Sunday (Tutor)
19052016 14:46:00 +0000Hey friends! Checkout this UI past Post UTME questions, about 70% of student that attempted it got it wrong. Try it and see if you can get it right
1. The count rate of an alphaparticle source is 400 per minute. If the halflife of the source is 5days, what would be the count rate per minute after 15days
A. 20
B. 25
C. 50
D. 200Updated Solution
Half life T = 5days
At 0days count rate = 400
After 5days, count rate = 400 ÷ 2 = 200
After 10days, count rate = 100
After 15days, count rate = 50days
Thus correct option is C2. In 24 days a radioactive isotope decreases in mass from 64g to 2g. What is the half life of the radioactive material.
A. 0.75days
B. 2.5days
C. 4.0days
D. 4.8daysUpdated Solution
Happy Solving!
There are actually two approach to this question that can still produce same answer
MethodI
Let the half life be = T_{1/2}
Original mass = 64g
After one T_{1/2}, mass = 64/2 = 32g
After another T_{1/2}, mass = 32/2 = 16g
Another T_{1/2}, mass = 16/2 = 8g
Another T_{1/2}, mass = 8/2 = 4g
After another T_{1/2}, mass = 4/2 = 2g
Then adding up the T_{1/2}, we have
5T_{1/2} = 24days
Since it was after 24days that we have 2g, and it is also 5T_{1/2} gave us 2g
then T_{1/2} = 24/5 = 4.8days
MethodII
We use this formular, N/N_{o} = (1/2)T^{t/T1/2}
were N = 2g, NT_{o} = 64g, t = 24days andT_{1/2} = half life
then 2/64 = (1/2)^{24/T1/2}
since same base, we apply indices to have, 5 = 24/T
by subject formular T_{1/2} = 24/5 = 4.8days
So answer is option D

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