Electrostatics – How an Object Placed in an Electric Field Changes the Field

Question

Lets say I have a point charge of magnitude $+q$, All around it I would have a symmetric radial electric field. Now if I place a neutral object lets say a sphere (doesn't matter insulating or conducting) in this field some distane away from the point charge. A negative charge will be induced on the object near the point charge and a positive charge on the opposite side.

No matter how small this induced charge is, due to the radial distance of the two (positive and negative) there must be an increase/decrease in net electric field on either side of the object and mostly everywhere else too !

I hope that what I am thinking is wrong, because we have not been taught that anything placed in electric field would affect the field itself regardless of it's nature. But I can't figure out what am I thinking wrong, how to solve this dilemma ?

Answer 1

If the material placed in the field of the positive charge is a conductor, the field will be distorted and the method to see the field is the image charges method. It will depend on the boundary conditions.

For a grounded conducting sphere

Field lines outside a grounded sphere for a charge placed outside a sphere.

For a non grounded conductor:

This illustration shows a spherical conductor in static equilibrium with an originally uniform electric field. Free charges move within the conductor, polarizing it, until the electric field lines are perpendicular to the surface. The field lines end on excess negative charge on one section of the surface and begin again on excess positive charge on the opposite side. No electric field exists inside the conductor, since free charges in the conductor would continue moving in response to any field until it was neutralized.

If the field is created by a point charge the geometry will change but the physics is the same.

If you have a positive point charge and bring into its field a dielectric, then the field lines will change again depending on constants as :

Figure 6.6.6 Electric field intensity in and around dielectric rod of Fig. 6.6.5 for (a) e_b > a and (b) e_b< e_a.

One can again imagine the geometric changes for a field from a sphere.

In summary, the field distorts with the presence of matter, differently for a conductor or dielectric .