- #1

- 1,097

- 2

## Homework Statement

There are

**two problems attached that illustrate my confusions**, but

**my questions is more general than this specific example.**

## Homework Equations

## The Attempt at a Solution

**For the first problem where we are asked for the potential energy,**

When the dipole moment is parallel to the electric field, it has 0 potential energy.

[tex]U_{i} = - \vec{p} \cdot \vec{E} = 0[/tex]

When the dipole rotates 60 degrees, the potential energy is not 0.

[tex]U_{f} = -\vec{p} \cdot \vec{E} = -pEcos\theta[/tex]

The potential enegry

**given from the formula is negative**, but there are

**only positive answers as possibilities.**

I know it is obvious to choose the positive one that corresponds to our answer, but sometimes both negative a positive quantities are listed as possible answers, and I am often confused on which one to choose.

Why is that negative sign in our formula for potential energy, what's its purpose?

**For problem number 2 where we are asked for the work,**

Initially there is 0 potential energy in the system (perpendicular),

[tex]U_{i} = 0[/tex]

but then the dipole rotates such that it is parallel to the direction of the field so,

[tex]U_{f} = -pE[/tex]

Now,

[tex]W = \Delta U = U_{f} - U_{i} = -pE - 0 = -pE[/tex]

It seems as though the work is negative, but the correct answer is that it is positive.

What am I mixing up?

**What do I need to think about and be consistent with to eliminate these sign errors?**

**Please be as clear as possible. A good example to remember how to determine the sign would be excellent.**

Thanks again!