• 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!



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