
Concept explainers
(a)
ToCalculate: The potential difference between the cylinders.
(a)

Answer to Problem 90P
V=2kQln(b/a)κL
Explanation of Solution
Given information :
Radii of coaxial
Charge of the inner cylinder = +Q
Charge of the outer cylinder = −Q
Formula used :
Electric potential:
V=QC
Where, Q is the charge stored and C is the capacitance.
C=2π∈0κLln(b/a)
Where, κ is the dielectric constant, ∈0 is the absolute permittivity and L is the length of the capacitor, a and b are radii of the
Calculation:
The potential difference between the cylinders to their charge and capacitance is,
V=QC
The capacitance of a cylindrical capacitor as a function of its radii a and b and length L:
C=2π∈0κLln(b/a)
V=Qln(b/a)2π∈0κLV=2kQln(b/a)κL
Conclusion:
The potential difference between the cylinders is V=2kQln(b/a)κL .
(b)
ToCalculate: The density of the free charge of on the inner cylinder and the outer cylinder.
(b)

Answer to Problem 90P
σf(a)=Q2πaL
σf(b)=−Q2πbL
Explanation of Solution
Given information:
Radii of coaxial conducting thin cylindrical shells = a & b
Charge of the inner cylinder = +Q
Charge of the outer cylinder = −Q
Formula used:
Charge density:
σ=Q2πrL
Where, Q is the charge, r is the radius and L is the length of the cylinder.
Calculation:
Surface charge density is:
σf(a)=Q2πaL
σf(b)=−Q2πbL
Conclusion:
The density of the free charge of on the inner cylinder and the outer cylinder are:
σf(a)=Q2πaL
σf(b)=−Q2πbL
(c)
ToCalculate: The bound charge density σb on the inner cylindrical surface of the dielectric and on the outer cylindrical surface of the dielectric.
(c)

Answer to Problem 90P
σb(a)=−Q(κ−1)2πaLκ
σb(b)=−Q(κ−1)2πbLκ
Explanation of Solution
Given information :
Radii of coaxial conducting thin cylindrical shells = a & b
Charge of the inner cylinder = +Q
Charge of the outer cylinder = −Q
Formula used :
Bound charge can be expressed as:
Qb=Q(κ−1)κ
Where, κ is the dielectric constant.
Bound charge density:
σb=QbA
Where, A is the area.
Calculation:
Qb(a)=−Q(κ−1)κ
Qb(b)=Q(κ−1)κ
σb(a)=Qb(a)Aσb(a)=−Q(κ−1)κ2πaLσb(a)=−Q(κ−1)2πaLκ
σb(b)=Qb(b)Aσb(b)=−Q(κ−1)κ2πbLσb(b)=−Q(κ−1)2πbLκ
Conclusion:
The bound charge density σb on the inner cylindrical surface of the dielectric and on the outer cylindrical surface of the dielectric.
σb(a)=−Q(κ−1)2πaLκ
σb(b)=−Q(κ−1)2πbLκ
(d)
ToCalculate: The total stored energy.
(d)

Answer to Problem 90P
U=kQ2ln(b/a)κL
Explanation of Solution
Given information:
Radii of coaxial conducting thin cylindrical shells = a & b
Charge of the inner cylinder = +Q
Charge of the outer cylinder = −Q
Formula used:
The energy stored in the capacitor:
U=12QV
Where, Q is the charge and V is the electric potential.
Calculation:
The potential difference between the cylinders is V=2kQln(b/a)κL .
The total stored energy in terms of the charge stored and the potential difference between the cylinders:
U=12QVU=12Q[2kQln(b/a)κL]U=kQ2ln(b/a)κL
Conclusion:
The total stored energy is,
U=kQ2ln(b/a)κL
(e)
ToCalculate: Mechanical work that is required to remove the dielectric cylindrical shell.
(e)

Answer to Problem 90P
W=kQ2(κ−1)ln(b/a)κL
Explanation of Solution
Given information:
Radii of coaxial conducting thin cylindrical shells = a & b
Charge of the inner cylinder = +Q
Charge of the outer cylinder = −Q
Formula used:
Work done in terms of potential energy of the system:
W=ΔU
Calculation:
The work required to remove the dielectric cylindrical shell in terms of the change in the potential energy of the system:
W=ΔUW=U′−U
The potential energy of the system with the dielectric shell in place is,
U′=κU
W=κU−UW=U(κ−1)W=kQ2(κ−1)ln(b/a)κL
Conclusion:
Mechanical work that is required to remove the dielectric cylindrical shell is W=kQ2(κ−1)ln(b/a)κL .
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Chapter 24 Solutions
EBK PHYSICS FOR SCIENTISTS AND ENGINEER
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