- #1
Frank Castle
- 580
- 23
Having read several introductory notes on Gauss's law, I have found it very frustrating that when the author comes to discussing the standard examples, in which one considers symmetric charge distributions, they do not explicitly discuss the symmetries of the situation, simply stating that, "by symmetry, the electric field must be radial" ,etc.
My question is, for each of the following cases:
1. Spherical symmetry
2. Cylindrical symmetry
3. Planar symmetry
What are the exact arguments, based on symmetry, why the electric field should point in a particular direction?My thoughts on the matter are as follows, please let me know if I've understood it correctly at all:
1. In the case of a spherically symmetric charge distribution, if the electric field (at each point) did not point radially outwards (or inwards), then one could rotate the system and find that the electric field would change in each case. However, the charge distribution is symmetric, and so such a rotation will not change it's electric field, and hence, it must be that the electric field is radial at each point.
2. In the case of cylindrical symmetry, where we assume that the cylinder is infinitely long, if the electric field had a component parallel to the cylinder, then upon reflecting the system (along the axis of the cylinder), this would change the electric field, contradicting the symmetry of the situation, hence there can be no component parallel to the cylinder. Furthermore, if the electric field pointed in a particular direction, at some angle to the surface of the cylinder, then upon rotation of the system, the electric field would change, violating the cylindrical symmetry of the charge distribution. Hence, again, the electric field is radial to the cylinder. [N.B. In this case, if the cylinder were not infinite, then we couldn't use the argument for there being no component parallel to the surface of the cylinder, since in this case, translating along the cylinder would change the amount of charge "above" and "below" and so the electric field would be stronger in one direction than the other.]
3. Finally, in the case of planar symmetry, again assuming that the plane is infinite in extent, if the electric field pointed in a particular direction at some angle to the the plane, then one could rotate the plane and the electric field would change. Furthermore, translating along any particular direction in the plane should not change the electric field, and so there cannot be any components parallel to its surface. Accordingly, the electric field must be perpendicular to the plane. [N.B. If the plane were not infinite in extent, then translating along the plane would mean the there would be more charge in one direction than another and so the electric field would change in this case, and hence, it would not necessarily have to point perpendicularly to the surface.]
My question is, for each of the following cases:
1. Spherical symmetry
2. Cylindrical symmetry
3. Planar symmetry
What are the exact arguments, based on symmetry, why the electric field should point in a particular direction?My thoughts on the matter are as follows, please let me know if I've understood it correctly at all:
1. In the case of a spherically symmetric charge distribution, if the electric field (at each point) did not point radially outwards (or inwards), then one could rotate the system and find that the electric field would change in each case. However, the charge distribution is symmetric, and so such a rotation will not change it's electric field, and hence, it must be that the electric field is radial at each point.
2. In the case of cylindrical symmetry, where we assume that the cylinder is infinitely long, if the electric field had a component parallel to the cylinder, then upon reflecting the system (along the axis of the cylinder), this would change the electric field, contradicting the symmetry of the situation, hence there can be no component parallel to the cylinder. Furthermore, if the electric field pointed in a particular direction, at some angle to the surface of the cylinder, then upon rotation of the system, the electric field would change, violating the cylindrical symmetry of the charge distribution. Hence, again, the electric field is radial to the cylinder. [N.B. In this case, if the cylinder were not infinite, then we couldn't use the argument for there being no component parallel to the surface of the cylinder, since in this case, translating along the cylinder would change the amount of charge "above" and "below" and so the electric field would be stronger in one direction than the other.]
3. Finally, in the case of planar symmetry, again assuming that the plane is infinite in extent, if the electric field pointed in a particular direction at some angle to the the plane, then one could rotate the plane and the electric field would change. Furthermore, translating along any particular direction in the plane should not change the electric field, and so there cannot be any components parallel to its surface. Accordingly, the electric field must be perpendicular to the plane. [N.B. If the plane were not infinite in extent, then translating along the plane would mean the there would be more charge in one direction than another and so the electric field would change in this case, and hence, it would not necessarily have to point perpendicularly to the surface.]