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Homework Help: Determine the energy of the electromagnetic radiation

  1. Dec 24, 2005 #1


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    Please i really need help im sorry to have had to make a second thread but i really need to finish this.

    2. The electron and positron each have a rest mass of 9.11 x 10^-31 kg. In a certain experiment, an electron and positron collide and vanish, leaving only electromagnetic radiation after the interaction. Each particle is moving at a speed of 0.20c relative to the laboratory before the collision. Determine the energy of the electromagnetic radiation.

    First of all do i have to solve this question like a collision problem, if i do there is no after situation.

    Anyways i think i have to solve for Ek.

    So Et=Ek + Erest

    Et=9.11 x 10^-31 x (3.00 x 10^8)^2/sqrt(0.96)
    Et=8.37 x 10^-14
    Erest = 8.2 x 10^-14
    Ek= 8.37 x 10^-14 - 8.2 x 10^-14
    Ek= 1.7 x 10^-15

    But this answer isnt right. The books has 0.615 Mev

    2. The Big Bang, which is a theory predicting the origin of the universe, is estimated to have released 1.00 x 10^68 J of energy. How many stars could half this energy create, assuming the average star's mass is 4.00 x 10^30 kg.

    I did this but again my answer is different from the back of the book.

    m=1.00 x 10^68/(3.00 x 10^8)^2
    =1.11 x 10^51

    =1.11 x 10^51/4.00 x 10^30
    =2.78 x 10^20 kg

    The book's answer is 1.39 x 10^20 stars. What did i do wrong.

    3. A supernova explosion of a star with a rest mass of 1.97 x 10^31 kg, produces 1.02 x 10^44 J of kinetic energy and radiation.
    a) How many kilograms of mass are converted to energy in the explosion?

    This one i have no idea how to do.

    Please i really need help with all these questions i really have to get these done soon.
  2. jcsd
  3. Dec 24, 2005 #2


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    Staff Emeritus
    Science Advisor

    1. The rest mass of the electron/positron is 0.511 MeV, and that is the minimum photon energy when annihilation occurs. Just at the relativistic kinetic energy from each electron to each photon. Think total energy.

    2. How many stars could half this energy create, . . . ?

    3. Again assume total energy. Some of supernova's rest mass is converted to radiation and kinetic energy of the material.

    What is the mass equivalent of 1.02 x 10^44 J of kinetic energy and radiation?
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