Math  /  Calculus

QuestionThe flow diagram below shows the volume of fluid in a chemostat (a growth chamber for bacteria).
Poll 1: The ODE is: A. dV(t)dt=fd\frac{d V(t)}{d t}=f-d B. dV(t)dt=fV(t)\frac{d V(t)}{d t}=f V(t)

Studdy Solution

STEP 1

What is this asking? We're looking at how the volume of liquid in a bacteria tank changes over time, given a constant inflow and outflow. Watch out! Don't mix up the *rates* of flow with the *volume* itself!

STEP 2

1. Analyze the Flow
2. Set up the ODE
3. Solve the ODE

STEP 3

Alright, so we've got this bacteria tank, right?
It's called a chemostat, which sounds super fancy!
Liquid is flowing *in* at a rate of ff and flowing *out* at a rate of dd.
So, the change in volume is simply the difference between these two rates.

STEP 4

Think of it like a bathtub!
If you turn on the faucet, the water level rises.
If you pull the plug, the water level drops.
The rate of change of the water level depends on *both* the inflow and outflow.

STEP 5

Now, let's translate this into math!
The rate of change of the volume, written as dV(t)dt\frac{d V(t)}{d t}, is equal to the inflow rate ff *minus* the outflow rate dd.
So, our Ordinary Differential Equation (ODE) is: dV(t)dt=fd \frac{d V(t)}{d t} = f - d

STEP 6

Notice that the correct answer to Poll 1 is A!
The rate of change of the volume depends *only* on the difference between ff and dd, not on the volume V(t)V(t) itself.

STEP 7

This ODE is pretty straightforward to solve!
Since ff and dd are constants, fdf - d is also a constant.
Let's call this constant cc, so c=fdc = f - d.
Now our ODE looks like this: dV(t)dt=c \frac{d V(t)}{d t} = c

STEP 8

To solve this, we just integrate both sides with respect to tt: dV(t)dtdt=cdt \int \frac{d V(t)}{d t} \, dt = \int c \, dt V(t)=ct+k V(t) = ct + k where kk is the constant of integration.

STEP 9

We're given that the **initial volume** is V(0)=250mlV(0) = 250 \, \text{ml}.
Let's plug this in to find kk: V(0)=c0+k=250 V(0) = c \cdot 0 + k = 250 So, k=250k = \textbf{250}.

STEP 10

Now we can write the complete solution: V(t)=ct+250 V(t) = ct + 250 Remembering that c=fdc = f - d, we get our **final equation**: V(t)=(fd)t+250 V(t) = (f - d)t + 250

STEP 11

The volume of fluid in the chemostat at any time tt is given by V(t)=(fd)t+250V(t) = (f - d)t + 250.

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