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High school physics - NGSS
Course: High school physics - NGSS > Unit 3
Lesson 3: Energy and fieldsPotential energy and position
The potential energy between two objects due to long-distance forces can be thought of as being stored in a field. When the objects move due to the field forces, the energy stored in the field decreases. Created by Sal Khan.
Want to join the conversation?
- Shouldn't the electric potential energy remain constant as we increase the distance since increasing the distance would decrease the magnitude of the force?(3 votes)
- no because it's talking about the POTENTIAL(1 vote)
- how does the energy get into the field in the first place?(3 votes)
- How does energy cause so much stuff to happen in short amount of time?(1 vote)
Video transcript
- [Presenter] In this
video, we're gonna talk about how energy is stored in field and in particular, how if we
change the position of things within that field, how it
might change the energy. So, just as a bit of a refresher, let's remind ourselves what energy is. It is the capacity to do work. And we've also seen that work, we can view it as equal
to the magnitude of force times the displacement in
the direction of that force. And then we can also remind
ourselves what a field is. And I'm not just talking about a big lawn or something like that, a football field. I'm talking about a general
idea in physics that's used. It's really just a concept
that allows us to predict and explain how to things that
are not touching each other are still interacting, are still able to exert
forces on each other. And in other videos, I've
also talked about that. Really nothing in this
universe is touching. We just conceptualize
that sometimes they are. But just as an example of a field, we have an electric field here. I could have done another type of field, we could have done a magnetic field, we could have done a gravitational field. Although when you study general relativity which Einstein gave us, we realize that it might
not exactly be optimal to think about it as a field. But in an electric field, right over here, we have a positive charge,
we have a negative charge. We know from experience that these two things attract each other and the convention is to
draw these field lines that go from the positive
towards the negative. Now, if we were to just let
go of these two point charges over here, what would happen? Well, we know that due to the
electric field constructed or created or that we imagined was created by this negative point charge, this positive charge would have a force acting on it towards the negative charge and vice versa due to the electric field that is created by the positive charge. The negative charge is
going to also be attracted to the positive charge. They're both going to
move towards each other. And so, when we talk
about energy in fields or energy stored in fields,
in our initial configuration, how is there energy in this field? How is their capacity to do work? Pause this video and think about that. Well, think about it this way. Imagine if each of these
charges were attached to some type of a mass, let me do this in a color
you can actually see. So, let's say that this is
towing some type of a mass and this is towing some type
of a mass right over here. Well, when you let go and
the forces are exerted on each of these point charges, assuming that the forces are large enough, they're going to be able
to pull these masses towards each other. So, there's a potential amount
of work that could be done and it would essentially keep happening until these point charges touch. And I used air quotes with my hand even though you can't see it, or until they can't get
any closer to each other or some other is keeping
them from getting any closer to each other. So now, let me think about how
could I increase the energy that is stored in this field. Pause the video and think about that. Well, what if I were to
keep the positive charge where it is, but if I were
to take the negative charge and if I were to move it
in a direction opposite from the force direction that
the field is trying to exert. So, instead of the negative charge there, what if I moved it all the way out there? Once again, let me do that in
a color you can actually see. Well, if I moved it out here and we're still towing
some type of a mass, you can see that when I moved
it against the direction that the force of the
field is trying to exert, that I've increased
the energy in the field because now I can do more work. I can drag this potential
mass over a larger distance. So, we have a general principle here. If we let these charges go in
the direction of the forces that are being exerted
on them due to the field, we're going to reduce
the energy in the field. But then if we are able to
move them against the forces of the field, and as you can imagine, we're going to have to put energy into the system to do that. But if we do that, then
we're storing more energy in this field because in that case, they're going to be further
away from each other and so they could drag their little masses that they're towing even further.