# B Sign of potential energy

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1. Sep 8, 2016

2. Sep 8, 2016

### Staff: Mentor

It depends on the reference point chosen. In some cases, there is a 'standard' reference point that makes things easier to describe. For example: In determining the PE of a system of two charges, we usually take PE = 0 when they are infinitely far apart. In that case, the sign depends on whether the force is attractive or repulsive.

3. Sep 17, 2016

### lychette

You know that in gravitation work must be done to lift an object, ie to increase its potential energy. when an object is at an infinite distance from ,lets say the Earth, the force on the object is zero and therefore no more work need be done to 'lift' the object.
In gravitation this is true for all objects regardless of their origin (Earth, Moon, Saturn...etc). It makes sense to then call infinity the zero of potential energy.
This means that ALL gravitational potential energies are negative. The potential at the Earth's surface is -63Mj/kg.

4. Sep 17, 2016

### Staff: Mentor

Only if you use infinity as your zero point. That is not always convenient. It's perfectly OK to measure gravitational PE from any convenient point, such as the ground. And from such a reference point, gravitational PE can certainly be positive.

The important point is that only changes in potential energy are meaningful.

5. Sep 23, 2016

### lychette

'The important point is that only changes in potential energy are meaningful.'
I think this is an over simplification.
I lift object A from the floor to a height of 1m and object A has potential energy.
My friend on the floor above me lifts an identical object A to a height (measured by him) of 1m and his object A has potential energy.
Both objects have the same change in potential energy.
I do not believe that my object A has the same potential energy as his object A, we can only compare the potential energy of our objects if we decide on a sensible zero of potential energy. There is more to it than 'changes in potential energy'

6. Sep 23, 2016

### Staff: Mentor

If for some reason you wanted to compare the potential energy of the two objects, you would of course use the same reference point. That zero point is arbitrary.

If the objects just sit there at their respective positions, who cares? Again, only changes in potential energy are meaningful.

7. Oct 23, 2016

### donaldparida

@Doc Al , so in conclusion:
We cannot measure the actual amount of potential energy of a system. We can only measure the change in potential energy of the system between two configurations.We first define an arbitrary reference configuration(which is usually the objects at infinite distance from each other) and measure the work done in changing the configuration of the system of objects to the required configuration(by measuring the work done in moving one of the objects from its position in the reference configuration also known as reference point to its position in the changed configuration) and this work done is the change in potential energy of the system in changing it's configuration. The sign of the work done and hence the change in potential energy of the system depends on the nature of forces existing between them(attractive or repulsive) and if we consider the system in the reference configuration to have zero potential energy we can drop the "change in" in "change in potential energy of the system" and simply call it the "potential energy of the system". Right?

Last edited: Oct 23, 2016
8. Oct 23, 2016

### Staff: Mentor

It isn't an oversimplification, but perhaps he should have said that only differences in potential energy are meaningful. That phrasing covers your example too.

9. Oct 24, 2016

### lychette

Good point, like me you also seem to be one of those 'who cares'

10. Oct 24, 2016

### Staff: Mentor

Agreed. That's a better way to put it.

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