# Negative Joule

1. Nov 21, 2004

### DB

Is it possible to have a negative joule?

2. Nov 21, 2004

### Staff: Mentor

Not sure what you are really asking, but the joule is a unit of energy or work. So, is it possible to have a negative energy or do negative work? Yes.

3. Nov 21, 2004

### DB

Ya that's what I meant, so what would be an example of a Negative Energy/Work

Thanks

4. Nov 21, 2004

### Staff: Mentor

Two random examples:

The work done to stop a car (by applying the brakes) is negative work since the force and displacement are opposite.

If you take the top of a hill as your reference (zero) point, then the gravitational potential energy of an object at the bottom of the hill would be negative.

And don't forget that usually what is interesting is the change in energy. And that can certainly be negative.

5. Nov 21, 2004

### DB

Got it, ty

6. Nov 21, 2004

### tfleming

another interesting example depends on which way two (oppositely) charged particles are rotating around each other, i.e. clockwise or anticlockwise. we have either negative or positive potential energy

7. Nov 24, 2004

### Astronuc

Staff Emeritus
Negative energy or work arises from comparing one statepoint to some reference.

Think of temperature scales, such as Centrigrade and Fahrenheit. They both have negative temperatures. But when we consider temperatures in the Kelvin and Rankine scales - with absolute zero as the reference - the temperatures are always positive.

Compared to absloute zero (correct me if I am wrong), everything has positive energy.

Negative energy is a 'relative' term.

One could ask is there such a thing as negative mass. As far as I know, mass is always positive.

Which reminds me of solving the diffusion equation for neutron flux in a reactor. One could select a function and boundary conditions that would give the possibility of a negative flux (or negative numbers of neutrons passing through a unit area per unit time). But a negative number of neutrons does not make sense physically - and a such a solution would clearly be wrong.