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A solid plastic sphere of radius R1has a charge -Q1on its surface (Figure 15.70). A concentric spherical metal shell of inner radius R2and outer radius R3carries a charge Q2on the inner surface and a charge Q3on the outer surface. Q1, Q2, and Q3are positive numbers, and the total charge Q2+Q3on the metal shell is greater than Q1.

At an observation location a distance rfrom the center, determine the magnitude and direction of the electric field in the following regions, and explain briefly in each case. For parts role="math" localid="1656931802199" a-d, be sure to give both the direction and the magnitude of the electric field, and explain briefly: (a)role="math" localid="1656932347681" r<R1(inside the plastic sphere), (b)role="math" localid="1656932286893" R1<r<R2(in the air gap), (c)role="math" localid="1656932322994" R<r<R(in the metal),(d)role="math" localid="1656932390135" r>R3(outside the metal).(e) Supposerole="math" localid="1656932377163" -Q1=-5nC. What isrole="math" localid="1656932400004" Q2? Explain fully on the basis of fundamental principles. (f) What can you say about the molecular polarization in the plastic? Explain briefly. Include a drawing if appropriate.

Short Answer

Expert verified

(a) The electric field inside the plastic sphere is 0.

(b) The electric field in the air gap is -Q104r2and its direction is negative.

(c) The electric field in the metal is 0.

(d) The electric field outside the metal isQ304r2and it is directed outwards from the sphere鈥檚 center.

(e) The value of the charge Q2is5nC .

(f) The plastic鈥檚 dipole鈥檚 negative part will be outward and the positive part will be towards the center.

Step by step solution

01

Identification of given data

The given data can be listed below as:

  • The radius of the solid plastic sphere is,R1
  • The charge of the solid plastic sphere is,-Q1-Q1
  • The inner radius of the concentric spherical metal shell is,R2
  • The inner surface charge of the concentric spherical metal shell is,Q2
  • The outer radius of the concentric spherical metal shell is,R3
  • The outer surface charge of the concentric spherical metal shell is,Q3
02

Significance of the electric flux

The electric flux is described as the product of the electric field and the special surface鈥檚 area. Moreover, the electric flux is also referred to as the division of the charge of an object with the permittivity.

03

(a) Determination of the magnitude and direction of the electric field inside the plastic sphere

The gaussian surface at the point r<R1will take no charge. However, by the symmetry of the Gaussian surface, the electric field is equal to zero at all the points inside the plastic sphere.

Thus, the electric field inside the plastic sphere is 0.

04

(b) Determination of the magnitude and direction of the electric field in the air gap

A Gaussian surface as a sphere has been considered at the radius R1<r<R2. However, the magnitude of the electric field is the same at all the points on the surface, and also it is directed towards the center.

The equation of the electric flux is expressed as:

=EA

鈥(颈)

Here, Eis the electric field and Ais the spherical surface鈥檚 area.

For the charge -Q1, the equation of the electric flux is expressed as:

=-Q10

鈥(颈颈)

Here, is the charge of the solid plastic sphere and is the permittivity.

Equating equation (i) and (ii),

EA=-Q10

Substitute for in the above equation.

role="math" localid="1656933780132" E.4r2=-Q10E=-Q104r2

Thus, the electric field in the air gap and its direction is negative.

05

(c) Determination of the magnitude and direction of the electric field in the metal

As inside the metal, the value of the inner surface charge of the concentric spherical metal shell and charge of the solid plastic sphere is equal, that is Q1=Q2. Hence, at this point R2<r<R3, the charge inside any type of spherical surface is zero.

Thus, the electric field in the metal is 0.

06

(d) Determination of the magnitude and direction of the electric field outside the metal

For r>R3,

For the charge Q1, Q2and Q3, the equation of the electric flux is expressed as:

=-Q1+Q2+Q30

鈥(颈颈颈)

Here, is the charge of the solid plastic sphere, the inner and the outer surface charge of the concentric spherical metal shell are Q2and Q3and 0is the permittivity.

Equating the equation (i) and (iii),

EA=-Q1+Q2+Q30

Substitute 4r2for Aand Q1for Q2in the above equation.

E4r2=-Q1+Q1+Q30

E4r2=Q30

E=Q304r2

Thus, the electric field outside the metal is Q304r2and it is directed outwards from the sphere鈥檚 center.

07

(e) Determination of the charge Q2

Here the value of the charge -Q1is given as -5nC. As the electric field inside the particular conductor is zero, then for the radius R2<r<R3, the net charge inside the sphere is zero. Hence, the value of the second charge becomes Q1=Q2.

Thus, the value of the charge Q2is 5nC.

08

(f) Determination of the molecular polarization in the plastic

The molecular polarization is described as the ratio of the dipole moment to the local electric field. However, inside a particular crystalline solid, the dipole moment is being considered per unit cell.

Thus, the plastic鈥檚 dipole鈥檚 negative part will be outward and the positive part will be towards the center.

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