# Homework Help: Question about electirc field

1. Mar 14, 2013

### Miike012

In my book it says that the electric field (E) is equal to the following

E = -dV/dx <0
where V = Volatage and x = position

However if you look in the paint doc on one of my homework problems you will see that
E>0

They say that E = dV/dx = Δv/Δx (where Δx = d).
So E = Δv/Δx > 0 (because E is UNIFORM therefor dV/dx = Δv/Δx)

Questions:
1. When should I use the equation E = -dV/dx?
2. Why did they not use E = -dV/dx?
3. Is E a vector in this equation? Hence the neg sign indicates direction

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2. Mar 14, 2013

### SammyS

Staff Emeritus
I believe in this case they are using "E" as the magnitude of the electric field.

3. Mar 15, 2013

### rude man

Always.

But the real equation is E = - del V where
del V = ∂V/∂x i + ∂V/∂y j + ∂V/∂z k
where i , j and k are unit vectors in x,y,z direction resp.

Reason: the E field may point in a direction with x, y and/or z components, not just x.

Last edited: Mar 15, 2013