The distance of observation point on the axial line, from the mid point of short electric dipole is doubled. By what factor does the electric field due to dipole changes?
Answers
Explanation:
Electric field due to an electric dipole at a point on its axial line: AB is an electric dipole of two point charges −q and +q separated by small distance 2d. P is a point along the axial line of the dipole at a distance r from the midpoint O of the electric dipole.
The electric fiedl at the point P due to +q placed at B is,
E
1
=
4πε
0
1
(r−d)
2
q
(along BP)
The electric field at the point P due to −q placed at A is,
E
2
=
4πε
0
1
(r+d)
2
q
(along PA)
Therefore, the magnitude of resultant electric field (E) acts in the direction of the vector with a greater, magnitude. The resultant electric field at P is
E=E
1
+(−E
2
)
E=[
4πε
0
1
(r−d)
2
q
−
4πε
0
1
(r+d)
2
q
] along BP
E=
4πε
0
q
[
(r−d)
2
1
−
(r+d)
2
1
] along BP
E=
4πε
0
q
[
(r
2
−d
2
)
2
4rd
] along BP
If the point P is far away from the dipole, then d≪r
∴E=
4πε
0
q
−
r
4
4rd
=
4πε
0
q
r
3
4d
E=
4πε
0
1
r
3
2p
along BP
[∵ Electric dipole moment p=q×2d]
E acts in the direction of dipole moment.
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