# The Naked Scientists Forum

### Author Topic: If energy flows down hill, what will happen when it all reaches the bottom?  (Read 2147 times)

#### Joe L. Ogan

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##### If energy flows down hill, what will happen when it all reaches the bottom?
« on: 07/11/2009 21:26:52 »
If energy flows down hill, what will happen when it all reaches the bottom?  Does the flow down hill have anything to do with gravity?  How did the energy get "up" to begin with?  If you can answer that, will it be possible for it to get back up?

#### JP

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##### If energy flows down hill, what will happen when it all reaches the bottom?
« Reply #1 on: 08/11/2009 07:16:05 »
Energy doesn't flow downhill.

Think of energy as an accounting tool.  Unless something external comes in to add or take away energy from a system, the energy in that system is constant.  Now, energy can have multiple forms.  Potential energy is energy due to where an object is: for example, when you lift an object off the ground, you increase its gravitational potential energy, or when you push down a spring, you store potential energy in the spring.  Kinetic energy is the energy of a moving object.

In you example, the object at the top of the hill has potential energy due to gravity.  When it moves downhill, the potential becomes kinetic energy as it moves faster.  If there's no energy lost due to friction or air resistance, it could continue moving up another hill to the same height it originally had.

How did it get energy to begin with?  Someone gave it enough energy (by pushing it, maybe?) to get to the top of the hill.

#### LeeE

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• Posts: 3382
##### If energy flows down hill, what will happen when it all reaches the bottom?
« Reply #2 on: 08/11/2009 11:54:57 »
In terms of entropy, energy tries to equalise to a common level i.e. energy at the top of a 'hill' is unequal to energy at the bottom and if the two energy states are not isolated they'll try to equalise, with the energy at the top of the hill flowing down to the lower level, and in doing so, raising it a little bit.

Taking this to the extreme, it seems to me that for entropy to always hold true then the expansion of the universe is a necessity, for if the universe is not expanding then once all of the energy in the universe has equalised it will reach a steady state and then stop, as there will be no lower level to equalise with.  In terms of energy density, an expanding universe means that there will always be somewhere at a lower state for the energy to flow to; it can never be said to have equalised if the 'target' is constantly changing.

But having said that, you could argue that the energy can never actually achieve perfect equalisation but only ever approach it, just as we consider it is impossible for an object with mass to ever reach the speed of light, or to cool something to absolute zero.  However, I believe that QM suggests that not only does the new space that is created by expansion have an intrinsically non-zero energy content when created, but that when the difference between two levels becomes small enough they will become equally uncertain, and therefore equivalent, or in other words, equalised.

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##### If energy flows down hill, what will happen when it all reaches the bottom?
« Reply #2 on: 08/11/2009 11:54:57 »