Q:A bacteria culture initially contains 2500 bacteria and doubles every half hour. What is the size of the baterial population after 20 minutes?
A:
Known: Ao = initial bacterial quantity of 2500; and the bacteria quantity is doubling every half hour.
Unknown: We need to find the amount of bacteria after 20 minutes of growth.
To do this we must first find a function of the bacteria quantity for a time: A(t).
The first step is to express the relationship between quantity and rate of change measured in bacteria per hour as a differential equation:
dA/dt = kA. where A is the quantity of bacteria at a given time and k is the relative growth rate.
The formula for finding the relative growth rate is:
(ln(y2)ln(y1))/(x2x1)>(ln(2Ao)ln(Ao))/(1/2)=2ln(2)= 1.386
This is a seperable equation so: dA/A=kdt. Integrating both sides yields > ln (A) = kt + C where C is a constant.
Taking the exponential of both sides gives us: A = Ce^kt.
We have a function for the quantity of bacteria but we must find the constant. We can find the constant by using the initial conditions at time zero.
2500 = (C*e^0) > C=2500=Ao.
A = Ao*e^kt where Ao = 2500 bacteria and k = 1.386
So plugging in 20 minutes or 1/3 hour into the function gives us:
A=2500*e^(1.386*1/3)= 3968 bacteria.
3/7/2014

Adam S.