
Concept explainers
(a)
Initial magnetic field in the middle of the solenoid
(a)

Answer to Problem 19P
Solution: Initial magnetic field in the middle of the solenoid is 3.77×10−3 T
Explanation of Solution
Given Info: current in the coil is 2A, number of turns is 300, length of the coil is 20 cm.
Formula for initial magnetic field in the solenoid,
Bi=NμoIl
- l is the length of the coil
- N is the number of turns in the coil
- I is the current in the coil
Substitute 2A for I, 300 for N, 20cm for l and 4π×10−7 T.m/A for μo in the above expression to get Bi
Bi=4π×10−7 T.m/A×300×2 A(20 cm)(10−2 m1 cm)=3.77×10−3 T
Conclusion:
Initial magnetic field in the middle solenoid is 3.77×10−3 T
(b)
Final magnetic field in the solenoid
(b)

Answer to Problem 19P
Solution: Final magnetic field in the solenoid is 9.42×10−3 T
Explanation of Solution
Given Info: current in the coil is 5A, number of turns is 300, length of the coil is 20 cm
Formula for final magnetic field in the solenoid,
Bf=NμoIl
Substitute 5A for I, , 300 for N, 20cm for l and 4π×10−7 T.m/A for μ in the above expression to get Bi
Bi=4π×10−7 T.m/A×300×5 A(20 cm)(10−2 m1 cm)=9.42×10−3 T
Conclusion:
Final magnetic field in the solenoid is 9.42×10−3 T
(c)
Area of 4-turn coil
(c)

Answer to Problem 19P
Solution: Area of 4-turn coil is 7.07×10−4 m2
Explanation of Solution
Given Info: radius of coil is 1.50×10−2 m
Formula for area of 4-turn coil,
A=πr2
Substitute 1.50×10−2 m for r in the above expression to get A.
A=π(1.50×10−2 m)2=7.07×10−4 m2
Conclusion:
Area of 4-turn coil is 7.07×10−4 m2
(d)
Change in magnetic flux through 4- turn coil.
(d)

Answer to Problem 19P
Solution: Change in magnetic flux through 4- turn coil is 3.99×10−6 Wb
Explanation of Solution
Given Info: initial magnetic field is 3.77×10−3 T, final magnetic field is 9.42×10−3 T and area of coil is 7.07×10−4 m2
Formula for change in magnetic flux,
Δϕ=ΔBA
Simplify above expression
Δϕ=(Bf−Bi)A
Substitute 9.42×10−3 T for Bf , 3.77×10−3 T for Bi and 7.07×10−4 m2 for A in the above expression to get Δϕ
Δϕ=(9.42×10−3 T−3.77×10−3 T)7.07×10−4 m2=3.99×10−6 Wb
Conclusion:
Change in magnetic flux through each turn of coil is 3.99×10−6 Wb
(e)
Average Induced emf in 4-turn coil.
(e)

Answer to Problem 19P
Solution: Average Induced emf in 4-turn coil is 1.77×10−5 V
Explanation of Solution
Given Info: magnetic flux is 3.99×10−6 Wb
Formula for Average Induced emf in 4-turn coil
ε=N(ΔϕBΔt)
- ΔϕB is the change in magnetic flux
- Δt is the change in time
Substitute 3.99×10−6 Wb for Δϕ , 4 for N , and 0.900 s for Δt in the above expression to get ε
ε=4(3.99×10−6 Wb0.900 s)=1.77×10−5 V
Conclusion:
Average Induced emf in 4-turn coil is 1.77×10−5 V
(f)
Why the contribution of the magnetic field by the current in the 4-turn coil be neglected.
(f)

Answer to Problem 19P
Explanation of Solution
The magnitude of the magnetic field depends on the amount of induced current in the coil. From above calculations, the emf induced in the coil is small according to part (e). When the emf is small, the current in the 4-turn will also be small. Hence, the magnetic field generated by the current is negligible.
Conclusion:
Contributions to the magnetic field by the current in the 4-turn coil is negligible
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Chapter 20 Solutions
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