
The circuit shown in Figure P21.47 is connected for 2.00 min. (a) Determine the current in each branch of the circuit. (b) Find the energy delivered by each battery. (c) Find the energy delivered to each resistor. (d) Identify the type of energy storage transformation that occurs in the operation of the circuit. (e) Find the total amount of energy transformed into internal energy in the resistors.
Figure P21.47 Problems 47 and 48.
(a)

The current in each branch of the circuit.
Answer to Problem 47P
The current I1 is 0.846 A down in the 8.00 Ω resistor_, I2 is 0.462 A down in the middle branch_ and I3 is 1.31 A up in the right-hand branch_ in the circuit.
Explanation of Solution
Write the expression for the Kirchhoff’s loop rule in second loop from the left of the figure 1.
12.0 V−(4.00 Ω)I3−(6.00 Ω)I2−4.00 V=0(4.00 Ω)I3+(6.00 Ω)I2=8.00 V (I)
Here, I is the current.
Write the expression for the Kirchhoff’s loop rule in first loop from the left of the figure 1.
−(6.00 Ω)I2−4.00 V+(8.00 Ω)I1=0(6.00 Ω)I2+4.00 V=(8.00 Ω)I1 (II)
Write the expression for the Kirchhoff’s junction rule in the figure 1.
I3=I1+I2 (III)
Use equation (III) in (I) and solve the equation.
(4.00 Ω)(I1+I2)+(6.00 Ω)I2=8.00 V(4.00 Ω)I1+(4.00 Ω)I2+(6.00 Ω)I2=8.00 V(4.00 Ω)I1+(10.0 Ω)I2=8.00 V (IV)
Use equation (II) to solve for I2.
I2=(4.00 Ω3.00 Ω)I1−(2.00 V3.00 Ω) (V)
Use equation (IV) in (V) to solve for I1.
(4.00 Ω)I1+(10.0 Ω)[(4.00 Ω3.00 Ω)I1−(2.00 V3.00 Ω)]=8.00 V (VI)
Use equation (VI) to solve for I1.
I1=352 Ω(8.00 V+203 V)=0.846 A (VII)
Use equation (VII) in (V) to solve for I2.
I2=(4.00 Ω3.00 Ω)(0.846 A)−(2.00 V3.00 Ω)=0.462 A (VIII)
Use equation (VIII) and (VII)in (III) to solve for I3.
I3=(0.846 A)+(0.462 A)=1.31 A (IX)
Conclusion:
Therefore, the current I1 is 0.846 A down in the 8.00 Ω resistor_, I2 is 0.462 A down in the middle branch_ and I3 is 1.31 A up in the right-hand branch_ in the circuit.
(b)

The energy delivered to each battery.
Answer to Problem 47P
The energy delivered to 4.00 V battery is −222 J_ and 12.0 V battery is 1.88 kJ_.
Explanation of Solution
Write the expression for the energy delivered to the battery.
ΔUB=PΔt (X)
Here, ΔUB is the energy delivered, P is the power, Δt is the time interval.
Write the expression for the P.
P=I(ΔV) (XI)
Here, ΔV is the voltage of the battery.
Use equation (XI) in (X) to solve for ΔU.
ΔUB=(ΔV)I(Δt) (XII)
Conclusion:
Substitute 4.00 V for ΔV, −0.462 A for I, 2.00 min for Δt in equation (XII) to find the power delivered to 4.00 V battery.
ΔUB=(4.00 V)(−0.462 A)(2.0 min×60 s1 min)=−222 J
Substitute 12.0 V for ΔV, 1.31 A for I, 2.00 min for Δt in equation (XII) to find the power delivered to 12.0 V battery.
ΔUB=(12.0 V)(1.31 A)(2.0 min×60 s1 min)=1.8×103 J=1.8 kJ
Therefore, the energy delivered to 4.00 V battery is −222 J_ and 12.0 V battery is 1.88 kJ_.
(c)

The energy delivered to each resistor.
Answer to Problem 47P
The energy delivered to 8.00 Ω resistor is 687 J_, 5.00 Ω resistor is 128 J_, 1.00 Ω resistor is 25.6 J_, 3.00 Ω resistor is 616 J_ and 1.00 Ω resistor in the right hand branch is 205 J_.
Explanation of Solution
Write the expression for the energy delivered to the resistor.
ΔUR=I2RΔt (XIII)
Here, ΔUR is the energy delivered to the resistor, R is the resistance, I is the current in that branch.
Conclusion:
Substitute 0.846 A for I, 8.00 Ω for R, 2.00 min for Δt in equation (XIII) to find energy delivered to 8.00 Ω resistor.
ΔUR=(0.846 A)2(8.00 Ω)(2.0 min×60 s1 min)=687 J
Substitute 0.462 A for I, 5.00 Ω for R, 2.00 min for Δt in equation (XIII) to find energy delivered to 5.00 Ω resistor.
ΔUR=(0.462 A)2(5.00 Ω)(2.0 min×60 s1 min)=128 J
Substitute 0.462 A for I, 1.00 Ω for R, 2.00 min for Δt in equation (XIII) to find energy delivered to 1.00 Ω resistor.
ΔUR=(0.462 A)2(1.00 Ω)(2.0 min×60 s1 min)=25.6 J
Substitute 1.31 A for I, 3.00 Ω for R, 2.00 min for Δt in equation (XIII) to find energy delivered to 3.00 Ω resistor.
ΔUR=(1.31 A)2(3.00 Ω)(2.0 min×60 s1 min)=616 J
Substitute 1.31 A for I, 1.00 Ω for R, 2.00 min for Δt in equation (XIII) to find energy delivered to 1.00 Ω resistor in the right hand branch.
ΔUR=(1.31 A)2(1.00 Ω)(2.0 min×60 s1 min)=205 J
Therefore, the energy delivered to 8.00 Ω resistor is 687 J_, 5.00 Ω resistor is 128 J_, 1.00 Ω resistor is 25.6 J_, 3.00 Ω resistor is 616 J_ and 1.00 Ω resistor in the right hand branch is 205 J_.
(d)

The type of energy transformation occurs in the operation of the circuit.
Answer to Problem 47P
The chemical energy is transformed to the internal energy in the resistors.
Explanation of Solution
The chemical energy in the 12.0 V battery is transformed into internal energy in the resistors. The 4.00 V battery is being charged, so its chemical potential energy is increasing at the expense of some of the chemical potential energy in the 12.0 V battery.
Conclusion:
Therefore, the chemical energy is transformed to the internal energy in the resistors.
(e)

The total amount of energy transformed into internal energy in the resistors.
Answer to Problem 47P
The total amount of energy transformed into internal energy in the resistors is 1.66 kJ_.
Explanation of Solution
Write the expression for the total amount of energy transformed into internal energy in the resistors.
ΔUT=ΔUB1+ΔUB2 (XIV)
Here, ΔUT is the total amount of energy transformed, ΔUB1 is the energy delivered to 4.00 V battery, ΔUB2 is the energy delivered to 12.0 V battery.
Conclusion:
Substitute −222 J for ΔUB1, 1.88 kJ for ΔUB2 in equation (XIV) to find ΔUT.
ΔUT=−222 J+(1.88 kJ×103J1 kJ)=1.66×103J=1.66 kJ
Therefore, the total amount of energy transformed into internal energy in the resistors is 1.66 kJ_.
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Chapter 21 Solutions
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