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• # Difficult Question in Physics from Post UTME

Physics

Chima Sunday (Tutor)
19-05-2016 14:46:00 +0000

Hey 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 alpha-particle source is 400 per minute. If the half-life of the source is 5days, what would be the count rate per minute after 15days
A. 20
B. 25
C. 50
D. 200

Updated 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 C

2. 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. 4.8days

Updated Solution
There are actually two approach to this question that can still produce same answer
MethodI
Let the half life be = T1/2
Original mass = 64g
After one T1/2, mass = 64/2 = 32g
After another T1/2, mass = 32/2 = 16g
Another T1/2, mass = 16/2 = 8g
Another T1/2, mass = 8/2 = 4g
After another T1/2, mass = 4/2 = 2g
Then adding up the T1/2, we have
5T1/2 = 24days
Since it was after 24days that we have 2g, and it is also 5T1/2 gave us 2g
then T1/2 = 24/5 = 4.8days
MethodII
We use this formular, N/No = (1/2)Tt/T1/2
were N = 2g, NTo = 64g, t = 24days andT1/2 = half life
then 2/64 = (1/2)24/T1/2
since same base, we apply indices to have, 5 = 24/T
by subject formular T1/2 = 24/5 = 4.8days
So answer is option D

Happy Solving!

• Damian Nonsy

1) C. 50 2) D. 4.8

2 19-05-2016 17:16:00 +0000

• Rosemary Ekelekhomen

how is it 50

2 19-05-2016 18:39:00 +0000

• Rosemary Ekelekhomen

plis solve

2 19-05-2016 18:39:00 +0000

• Majiri E

1.C 2.C

2 19-05-2016 19:14:00 +0000

• Jerome Veronica

can someone solve it pls

2 19-05-2016 20:09:00 +0000

• Rosemary Ekelekhomen

plis can someone solve it

2 19-05-2016 20:31:00 +0000

• Damian Nonsy

N/N°=(1/2)t/T

2 19-05-2016 20:41:00 +0000

• Chima Sunday (Tutor)

Thumb up to you guys @Damain Nonsy, Rose Ekelekhomen, Majiri E. and Jerome Veronica for that wonderful attempt. More especially @ Damain Nonsy for having both questions right. The questions have been updated with its solutions

2 23-05-2016 14:32:00 +0000