Homework Help: An electric potential is given in volts by

1. Aug 8, 2012

zhillyz

1. The problem statement, all variables and given/known data

An electric potential is given in volts by;

$\phi (x,y,z) = 20x - 12y^2 +2yz$,

where x,y and z are measured in metres. Find the magnitude of the electric field $E$ at the point(1,1,3).

2. Relevant equations

$E = -\nabla \phi$

3. The attempt at a solution

$-\nabla \phi = -1*(\frac{\partial \phi}{\partial x} + \frac{\partial \phi}{\partial y} + \frac{\partial \phi}{\partial z})$
$= -1*((20)+(24y +2z)+(2y)) = -1*(20+24+6+2) = -52Vm^-1$

I just wanted to know if I have used the right equation, the process is okay and if there is an occasion I should not use this equation? Thanks!

2. Aug 8, 2012

tiny-tim

hi zhillyz!
no, ∇ is a vector

(and the magnitude of (a,b,c) isn't a + b + c, is it? )​

3. Aug 8, 2012

zhillyz

I'm sorry perhaps you could elaborate but I think that the electric field 'E' is equal to negative grad phi, grad being the gradient of a vector as you say. Do you merely mean I should not add the sums together at the end as this would make it scalar? ie,

$20i +30j+2k$

4. Aug 8, 2012

tiny-tim

no, phi is a scalar, grad phi is the gradient of a scalar, it is a vector

what is the magnitude of a vector (a,b,c) ?

5. Aug 9, 2012

zhillyz

$-20i + -30j + -2k = E, |E|= \sqrt {(-20^2) + (-30^2) + (-2k^2)}$

Better?

6. Aug 9, 2012

tiny-tim

yup!

(i'm assuming that last "k" is a misprint )