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Calculating the velocity of a molecule

  1. May 31, 2017 #1
    Hi, I'm stuck on a problem. I have this function that represents the velocity:
    f(v)=v2e(-mv2)/(2kT)

    T is the temperature and equals 293.15K
    m = 4.65 .10-26 kg
    k = 1.38 .10-23 J/K


    The problem wants us to find the derivative of the function. I found:
    df/dv= (2v) e(-mv2)/(2kT)+v2e(-mv2)/(2kT)(-2mv/2kT)
    = ve(-mv2)/(2kT) (2- (mv2/kT))


    I know the derivative I found is right, but now, I have to find the velocity for df/dv=0

    We know v>0 and ex>0
    So, df/dv=0 => (mv2)/kT = 2
    => v= √(2kT/m)


    That's where I'm stuck. The book tells me the answer is 1500km/h.
    I don't understand this solution. I calculated many many times and didn't found the right solution. Can someone please help me? :cry:
     
  2. jcsd
  3. May 31, 2017 #2
    What answer did you get? Did you get 417? What are the units of that?
    If you got a different answer, can you show us exactly how you got it? Your formula looks right.
     
  4. May 31, 2017 #3
    Yes, I got 417!
    I just plugged in all the values in the last equation that I wrote, v= √(2kT/m).
    Since k is in J/K, T in K and m in kg, the unit of v is J/kg.
    J also equals kg m2 s-2 so I assume the unit of v can also be m2s-2?
     
  5. May 31, 2017 #4
    Oh, I got it! My answer is then in m.s-1 and I just need to multiply by 3.6.
    417*3.6=1501 km.h-1

    Thank you for the help :)
     
  6. May 31, 2017 #5
    Can't emphasise enough the importance of thinking UNITS, UNITS, UNITS; ALWAYS, ALWAYS, ALWAYS! It will save you a lot of trouble and avoid a lot of mistakes, both with homework questions like this and real problems in doing science.
    For example, if you are asked for a velocity, the answer CANNOT be (e.g.) 417. It must be 417 SOMETHING - m/s, km/h, furlongs per fortnight or whatever. If you did that you would immediately see that your answer was not necessarily wrong, but another step was needed.
     
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