# Centre of charge ?

1. Feb 27, 2006

### gandharva_23

like centre of mass can there be a concept of centre of charge . To apply newtons laws of motion one can assume a point on the body to represent the whole body and assume the entire mass to be concentrated at that point . to apply coulomb's law can i assume all the charge of the body to be concentrated at a given point ?

2. Feb 28, 2006

### Eivind

Assume that the entire force is concentrated in one point of the body, at the center.

3. Feb 28, 2006

### gandharva_23

you probably dint get the question or i m unable to understand what u r trying to say .if i have 3 unequal point charges located on the vertices of an equilateral triangle then can i assume the whole charge to be concentrated at one point to represent the whole system ? i.e one can assume the entire charge to be concentrated at that point for the purpose of calculating the electric field intensity ? can we derive a general expression for coordinates of center of charge like center of mass ?

4. Feb 28, 2006

### Physics Monkey

Hi ghandharva, if I understand your question correctly then the answer in general is no. Consider the following the situation. I have a positive and a negative charge fixed at either end of an uncharged rod which lies on the x axis. This system has no net charge but it still has a nonzero electric field. Furthermore, a positive charge placed further down the x axis will exert a non-zero force on the system. As Astronuc talked about in your other thread on this topic, there is a sense in which only the total charge is relevant. If the total charge of a system is nonzero, then far away from the system the electric field of the system will look like the field of a single point charge whose magnitude is the sum of all the charges in the system. This is called the monopole part of the field, but in our case this term is zero since the total charge is zero. The result is that far away from the system the electric field looks like a dipole field which falls off as $$1/r^3$$ with distance unlike the $$1/r^2$$ dependence of the monopole term. Because many interesting systems are charge neutral, the dipole term is very important for many real world applications. See here for more: http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/dipole.html

Hope this helps.

5. Feb 28, 2006

### gandharva_23

the explaination was pretty useful .... thanx a lot .