Answer to Question #278925 in Differential Equations for Dhel

Question #278925

Tank A initially contains 200 liters of brine containing 225 N of salt. Eight liters of fresh water per minute enter A and the mixture, assumed uniform, passes from A to B, initially containing 200 liters of fresh water, at 8 liters per minute. The resulting mixture, also kept uniform, leaves B at the rate of 8 liters per minute. Find the amount of salt in tank B after one hour.




Application of First Order of Differential Equation

1
Expert's answer
2021-12-13T14:59:46-0500

Let A(t)=A(t) = amount, in N of salt in tank A at time t.t. Then we have

dAdt=\dfrac{dA}{dt}=(rate of salt into tank A) − (rate of salt out of tank A)



dAdt=08A200\dfrac{dA}{dt}=0-\dfrac{8A}{200}

So we get the differential equation



dAA=dt25\dfrac{dA}{A}=-\dfrac{dt}{25}

Integrate


A(t)=Cet/25A(t)=Ce^{-t/25}

Given A(0)=225.A(0)=225. Then



A(t)=225et/25A(t)=225e^{-t/25}

Let B(t)=B(t) = amount, in N of salt in tank B at time t.t. Then we have

dBdt=\dfrac{dB}{dt}=(rate of salt into tank B) − (rate of salt out of tank B)


dBdt=8200(225et/25)8200B\dfrac{dB}{dt}=\dfrac{8}{200}(225e^{-t/25})-\dfrac{8}{200}BdBdt+0.04B=e0.04t\dfrac{dB}{dt}+0.04B=e^{-0.04t}dBdt+0.04B=0\dfrac{dB}{dt}+0.04B=0dBB=0.04dt\dfrac{dB}{B}=-0.04dtB=c1e0.04tB=c_1e^{-0.04t}dBdt=dc1dte0.04t0.04c1e0.04t\dfrac{dB}{dt}=\dfrac{dc_1}{dt}e^{-0.04t}-0.04c_1e^{-0.04t}dc1dte0.04t0.04c1e0.04t+0.04c1e0.04t\dfrac{dc_1}{dt}e^{-0.04t}-0.04c_1e^{-0.04t}+0.04c_1e^{-0.04t}=e0.04t=e^{-0.04t}dc1dt=1\dfrac{dc_1}{dt}=1c1=t+c2c_1=t+c_2B(t)=(t+c2)e0.04tB(t)=(t+c_2)e^{-0.04t}

Given B(0)=0.B(0)=0. Then



B(t)=te0.04tB(t)=te^{-0.04t}

Hence



B(60)=60e0.04(60)5.443 NB(60)=60e^{-0.04(60)}\approx5.443 \ N

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