
Concept explainers
a.
To draw: A direction field for the differential equation.
a.

Explanation of Solution
Given information:
Differential equation: RdQdt+1CQ=E(t)
R=5Ω , C=0.05F , E=60V
Concept used :
If dydx=F(x,y) , F(x,y) gives the slope of the solution curve at point (x,y) . If small line segments with slope F(x,y) are drawn on several points (x,y) , the results are called direction field.
Graph:
RdQdt+1CQ=E(t)
R=5Ω , C=0.05F , E=60V
5dQdt+10.05Q=60⇒5dQdt+20Q=60⇒dQdt+4Q=12⇒dQdt=12−4Q
Interpretation: The graph is the direction field of the differential equation dQdt=12−4Q
b.
To find: The limiting value of the charge.
b.

Answer to Problem 27E
The limiting value of the charge is 3 Coulomb.
Explanation of Solution
Given information:
It is clear from the direction field that all the solutions approaches to the value 3 .
i.e. limt→∞Q(t)=3
c.
To find: Whether there are any equilibrium solution or not.
c.

Answer to Problem 27E
Q(t)=3is an equilibrium solution.
Explanation of Solution
Given information:
dQdt=12−4Q
Concept used :
If for a particular solution , dQdt be zero, then that solution will be equilibrium solution.
Calculation:
From the direction field it appears that Q=3 is an equilibrium solution of dQdt=12−4Q .
dQdt=12−4Q=12−4×3=12−12=0
d.
To draw: The graph of the solution when Q(0)=0C using direction field.
d.

Explanation of Solution
Given information:
Q(0)=0 C
Graph :
Q(0)=0 C
Interpretation:
The graph shows the particular solution when Q(0)=0 C and hence it passes through origin.
e.
To estimate: The charge after half a second using Euler’s method.
e.

Answer to Problem 27E
The charge after half a second is 2.7672 C
Explanation of Solution
Given information:
dQdt=12−4Q ; Q(0)=0 C
h=0.1
Formula used :
Euler’s Method: Approximate values for the solution of the initial-value problem y′=F(x,y) , y(x0)=y0 with the step size h , at xn=xn−a+h .
yn=yn−1+hF(xn−1,yn−1) n=1,2,3.....
Calculation:
h=0.1
dQdt=12−4Q and Q(0)=0 C
Here, Q′=F(Q,t)=12−4Q
Q0=0
Q1=Q0+hF(Q0,t0)=0+0.1×12=1.2Q2=Q1+hF(Q1,t1)=1.2+0.1×7.2=1.92Q3=Q2+hF(Q2,t2)=1.92+0.1×4.32=2.352Q4=Q3+hF(Q3,t3)=2.352+0.1×2.592=2.612Q5=Q4+hF(Q4,t4)=2.612+0.1×1.552=2.7672
Q(0.5)=2.7672
Therefore, the charge after half a second is 2.7672 C
Chapter 7 Solutions
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