potential energy curve Watch

tazmaniac97
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On the y axis why does it say -436 kJ/mol and -104 kcal/mol? What does this mean and why is the part where I drew a dot the stability point (where attractive and repulsive forces cancel), shouldn't the stability point be the part that's touching the x axis as this would show 0 energy?

I'm so confused can someone please help me
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tazmaniac97
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also does U stand for potential energy in this formula  F = - \frac{dU}{dr}

I'm confused because my lecturer wrote that U(r) stands for potential energy :confused:
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teachercol
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U stands for potential energy

U(r) indicates that it is a function of r

KJ and Kcal are alternative units for the same thing.
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teachercol
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And youre confusing force/separation with energy separation.

Min energy = zero force
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tazmaniac97
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(Original post by teachercol)
U stands for potential energy

U(r) indicates that it is a function of r

KJ and Kcal are alternative units for the same thing.

(Original post by teachercol)
And youre confusing force/separation with energy separation.

Min energy = zero force
Why is the energy negative?
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teachercol
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Potential energy is negative because of how we define zero energy.

Zero PE is when the two particles are infinitely separated and there is no force

You have to put energy in to separate particles (aka break bonds) so PE is negative.
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tazmaniac97
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(Original post by teachercol)
Potential energy is negative because of how we define zero energy.

Zero PE is when the two particles are infinitely separated and there is no force

You have to put energy in to separate particles (aka break bonds) so PE is negative.
I still don't get it, the part where the dot is (in the diagram) is the stability point and it's where (according to my lecturer) is where the attractive and repulsive forces cancel out, but if they cancel out then shouldn't the energy be 0?
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Stonebridge
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(Original post by tazmaniac97)
I still don't get it, the part where the dot is (in the diagram) is the stability point and it's where (according to my lecturer) is where the attractive and repulsive forces cancel out, but if they cancel out then shouldn't the energy be 0?
Why should it? Potential energy is something gained or lost as a result of a force moving its point of action.
If I applied a constant force to a spring to stretch it, it would expand until the force it was pulling back with equalled the force I was pulling it out with. At that point the forces are balanced. At that point there is potential energy in the spring as a result of the force having done work. The fact that the forces are balanced (me vs the spring) doesn't mean the potential energy is zero. You are confusing force and energy.

In the potential vs distance graph, the gradient (actually negative potential gradient) is equal to the force. To the right of the dip the gradient is positive (attraction) and to the left it's negative (repulsion). At the stable point at the bottom of the dip the gradient is zero and force is zero.
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tazmaniac97
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(Original post by Stonebridge)
Why should it? Potential energy is something gained or lost as a result of a force moving its point of action.
If I applied a constant force to a spring to stretch it, it would expand until the force it was pulling back with equalled the force I was pulling it out with. At that point the forces are balanced. At that point there is potential energy in the spring as a result of the force having done work. The fact that the forces are balanced (me vs the spring) doesn't mean the potential energy is zero. You are confusing force and energy.

In the potential vs distance graph, the gradient (actually negative potential gradient) is equal to the force. To the right of the dip the gradient is positive (attraction) and to the left it's negative (repulsion). At the stable point at the bottom of the dip the gradient is zero and force is zero.
Oh I think I get it, so in order to get to that point a certain amount of potential energy is lost.

Thanks
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tazmaniac97
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(Original post by teachercol)
...
Thanks for your help too
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