# Is electric field radially inwards or outwards

1. May 7, 2014

2. May 7, 2014

### Staff: Mentor

In general, $\vec E$ points in the direction that V decreases most rapidly.

In this case, V depends only on r, so $\vec E$ points outwards if V decreases with increasing r (i.e. dV/dr < 0), and inwards if V increases with increasing r (dV/dr > 0).

3. May 7, 2014

### Staff: Mentor

The force on a positively charged test particle is in the direction of the electric field; the force on a negatively charged test particle is opposite to the direction of the electric field. If you're careful with your signs, you'll get the right answer just from doing the calculation because you'll write a negative charge as $-Q$ and the negative sign carries through into the sign of the force - but wise people also take a moment to visualize the setup and make sure that their answer has the force pointing in a sensible direction.

4. May 7, 2014

### Sorthal

Alright man so what the symbols mean are: if it is the "x" symbol it means the field is going into the page (away from you) and if it is the other symbol then it means the field is coming out of the page (at you)

5. May 8, 2014

### sophiecentaur

It is better to describe the Field in terms of the Potential - that avoids having to think of a 'page' or where you (or I) happen to be standing. Basically:
E = -∇V or the gradient of the Potential. It's the way that a positively charged 'ball' would 'roll', as jibell was hinting.