# Finding value of unknown charge

• ChiralSuperfields
In summary: This is in contrast to the electric field due to a positive charge which would be pointing to the left as a positive test charge would be repelled by the positive charge.

#### ChiralSuperfields

Homework Statement
Relevant Equations
For this problem,

The solution is,

However, why did they assume that the electric field produced by charge q is always pointing to the left at the origin?

Many thanks!

Terrakron
Callumnc1 said:

For this problem,
View attachment 320613
The solution is,
View attachment 320612
However, why did they assume that the electric field produced by charge q is always pointing to the left at the origin?

Many thanks!
They are not assuming that the electric field is pointing to the left.

If ##q## is a positive charge, then indeed, the electric field at the origin, due to ##q##, is to the left (negative).

What if ##q## is negative? Then you have a negative number multiplied by a negative number. In what direction is the electric field, due to ##q##, pointing in this case?

Last edited:
ChiralSuperfields
SammyS said:
They are not assuming that the electric field is pointing to the left.