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Homework Help: Exponential growth differential equation

  1. Feb 25, 2012 #1
    1. The problem statement, all variables and given/known data

    A biomedical company finds that a certain bacteria used for crop insect control will grow exponentially at the rate of 12% per hour. Starting with 1000 bacteria, how many will they have after 10h?

    2. Relevant equations

    Topic is application of differential equations.

    3. The attempt at a solution

    This question is from the topic on application of differential equations.
    I stumbled on the solution but i need to understand why it works and whether it is the correct method.

    at T=0, y=1000, dy/dt=0


    Total T=y+dy/dt

    from here i just substitute in t=10 to get answer.

    question is why exponential growth in this case uses base e and not other base? why does it work when C is initial value?

    also is this the correct method when using differential equations to solve this problem? don't want the working to be affected just cause I know the formula is y=Ce^kt
  2. jcsd
  3. Feb 25, 2012 #2
    You are making things more complicated than they have to be.

    y=C exp(k t) is of course the right starting point.

    y(t + 1 hr) = y(t) + 12% = y(t) * 1.12.

    Plug that into above equation. C will drop out and you can solve for k.

    Next, you have y(0) = 1000, which gives you C=1000 as you have already noted.

    The final step is to calculate y(10 hrs).

    Another way is to say that each hour the number is multiplied by 1.12, so after 10 hours,
    you multiply by (1.12)^10.

    Note that you get the wrong answer (you underestimate the number of bacteria) if you just say +12% per hour x 10 hours = +120%. This is linear growth instead of exponential.
  4. Feb 26, 2012 #3
    i would use formula directly but the topic is on differential equations. Need to derive the formula from differential equations first. How do you do that?
  5. Feb 26, 2012 #4


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    Is the question exactly as you worded it? The phrase 'will grow at a continuous rate of 12%' or 'will grow continuously with a relative rate of 12%' or something similar is often used to denote that the population satisfies the differential equation
    dP/dt = kP where here k=.12. I strongly suspect that different courses will use slightly different terminology, so you should check carefully with how the words have been used in your class before
  6. Feb 28, 2012 #5
    ok. now the problem is modified.

    Number of bacteria is assumed to grow exponentially at the rate given by dN/dt = kN, where N is the number of bacteria at time t hour. The bacteria grow to double the original amount in 0.5 hour.

    Find the equation for N as a function of t and the original amount N0.

    dN/dt = kN

    ∫1/kN dN = ∫dt

    ln (kN) = t+C

    kN = et+C

    kN = Cet

    at t=0.5 , k=2

    2N = Ce0.5

    C = [itex]\frac{2N}{e^0.5}[/itex]

    substitute back C

    N = 2N0et-0.5

    don't know the correct answer, so i'm posting here if anyone can tell me if im correct... thanks!
  7. Feb 28, 2012 #6
    Nope, check your second step, the integration. Should be ln(N)=kt+C. Then N=Cexp(kt), the standard expression for exponential growth or decay.
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