# Equipotential line and electric field

1. Jun 2, 2012

### Saitama

1. The problem statement, all variables and given/known data
The equation of an equipotential line in an electric field is y=2x, then the electric field strength vector at (1,2) may be
a)4i+3j
b)4i+8j
c)8i+4j
d)-8i+4j

2. Relevant equations

3. The attempt at a solution
Since its an equipotential line, the electric field vector should perpendicular to this line. Therefore, the slope of line perpendicular to y=2x is -1/2. The equation should be y=$\frac{-1}{2}$x+k, where k is any constant. Now i am stuck, can somebody help me proceed in the right direction?

2. Jun 2, 2012

### Infinitum

You cannot find the exact vector, as there can be many answers satisfying the given condition. But, you sure can check the options to see which of them would give you a vector perpendicular to y=2x

I believe the options were specifically made that way so that you can get the answer.

3. Jun 2, 2012

### Saitama

How can i check that the vector is perpendicular to line y=2x?

4. Jun 2, 2012

### Infinitum

Check the slopes of the vectors!

5. Jun 2, 2012

### Saitama

Lol, then the answer is d), its slope is -1/2.
But then what's the use of (1,2)?

EDIT: Got it why the points are given. :)

Last edited: Jun 2, 2012
6. Jun 2, 2012

Good going!