
Concept explainers
(a)
The magnetic field as function of perpendicular distance r from the central axis of cable for 0<r, r1 , r1<r<r2 and r>r2 .
(a)

Answer to Problem 80P
The magnetic field as function of perpendicular distance r from the central axis of cable for 0<r, r1 is 0 , for r1<r<r2 is μ0I2πr and r>r2 is 0 .
Explanation of Solution
Formula Used:
The relation for the magnetic is given by,
um=B22μ0
Calculation:
The magnetic field inside the cylinder (r<r1) and outside the cylinder (r>r2) is zero because the net enclosed current is zero.
Br<r1=0
And,
Br>r2=0
The magnetic field between the cylinder is calculated as,
(2πr)B=μ0IB=μ0I2πr
Conclusion:
Therefore, the magnetic field as function of perpendicular distance r from the central axis of cable for 0<r, r1 is 0 , for r1<r<r2 is μ0I2πr and r>r2 is 0 .
(b)
The proof that the magnetic energy density in the region between the cylinder is by um=12(μ0/4π)I2/(πr2) .
(b)

Answer to Problem 80P
The proof that the magnetic energy density in the region between the cylinder is by um=12(μ0/4π)I2/(πr2) is stated below.
Explanation of Solution
Formula Used:
The expression for the magnetic energy density in the region between the cylinder is given by,
um=B22μ0
The magnetic field between the cylinder is given as,
B=μ0I2πr
Calculation:
The magnetic energy density in the region between the cylinder is calculated as,
um=(μ0I2πr)22μ0=μ0I28π2r2=12(μ04π)(I2πr2)um=12(μ0/4π)I2/(πr2)
Conclusion:
Therefore, the proof that the magnetic energy density in the region between the cylinder is by um=12(μ0/4π)I2/(πr2) is stated above.
(c)
The proof that the total magnetic energy in a cable of volume of length l is given by Um=(μ0/4π)I2lln(r2/r1)
(c)

Answer to Problem 80P
The proof that the total magnetic energy in a cable of volume of length l is given by Um=(μ0/4π)I2lln(r2/r1) is stated below.
Explanation of Solution
Formula Used:
The magnetic energy density in the region between the cylinder is given by,
um=12(μ04π)(I2πr2)
The magnetic energy dUm in the cylindrical element of volume dV is given by,
dUm=umdV=(12(μ04π)(I2πr2))(l2πrdr)=μ0I2l4π⋅drr ……. (I)
Calculation:
Integrate equation (I) over the limits r1 to r2 .
∫dUm=r2∫r1μ0I2l4π⋅drrUm=μ0I2l4π∫r2r11rdr=μ0I2l4π[lnr]r2r1=μ0I2lln(r2/r1)4π
Conclusion:
Therefore, the proof that the total magnetic energy in a cable of volume of length l is given by Um=(μ0/4π)I2lln(r2/r1) is stated above.
(d)
The proof that self inductance per unit length of the cab arrangement is given by L/l=(μ0/2π)ln(r2/r1) .
(d)

Answer to Problem 80P
The proof that self inductance per unit length of the cab arrangement is given by L/l=(μ0/2π)ln(r2/r1) is stated below.
Explanation of Solution
Formula Used:
The expression for the total magnetic energy is given by,
Um=μ0I2lln(r2/r1)4π …… (II)
The energy in the magnetic in terms of L and I is given by,
Um=12LI2 …….. (III)
Calculation:
The self inductance per unit length is calculated as,
Um=12LI2L=2(Um)I2=2(μ0I2lln(r2/r1)4π)I2Ll=μ0ln(r2/r1)2π
Conclusion:
Therefore, the proof that self inductance per unit length of the cable arrangement is given by L/l=(μ0/2π)ln(r2/r1) is stated above.
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Chapter 28 Solutions
Physics for Scientists and Engineers
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