About this video
Potential energy is energy stored in the fields between objects, so it always belongs to a system of two or more objects, never to one object alone. It depends on the arrangement of the system: we choose a convenient arrangement to call zero, then measure how much energy is transferred in to or out of the system. The video applies this to gravitational, electric, elastic and chemical potential energy.
You'll be able to
- explain why potential energy belongs to a system of objects, not a single object.
- predict whether gravitational or electric potential energy increases or decreases when the arrangement of a system changes.
- explain how we choose a zero point and assign values to potential energy.
- explain why elastic and chemical potential energy are really electric potential energy.
Key words
- Potential energy
- Energy stored in the fields between objects in a system, which depends on how the objects are arranged.
- System
- A group of two or more objects we choose to study together, like the earth and a ball.
- Gravitational potential energy
- Potential energy of a system of masses, which grows as the masses get farther apart.
- Electric potential energy
- Potential energy of a system of charges, which depends on how far apart the charges are.
- Elastic potential energy
- Energy stored when a material like a spring is squashed or stretched.
- Joule
- The unit used to measure energy.
Transcript
- [Presenter] Energy is a quantity that is measured in joules, and one kind of energy is kinetic energy that is associated with moving things or motion of an object, but there's another kind of energy called potential energy. What exactly is that? That's what we wanna talk about in this video.
Okay, so what exactly is potential energy? Well, if you take any fundamental interaction, like gravitational force or electric forces or magnetic forces, you probably know that they're mediated by fields: gravitational fields, electric fields, and magnetic fields.
Now, whenever there's another object that enters the field, like another mass or another charge or another magnet, for example, we say that there is energy stored in these fields, and that energy stored is the potential energy. So what's interesting is that potential energy is an energy that is assigned to a group of objects, not just one object.
For example, kinetic energy is an energy that can be assigned to a moving object. But potential energy is not assigned to a particular object. You can't say, you know, "Earth has that much potential energy," or, "This charge has that much potential energy." No, you can always talk about potential energy for a group of objects or a system of objects.
You can say, this system of earth and moon has a specific amount of potential energy. Or this system of magnets has some amount of magnetic potential energy, for example. So the important characteristic of potential energy is that it's an energy that is associated with a system of objects, not a single object.
But now comes a big question. Well, what does this energy depend on? Well, it depends on the specific arrangement. So potential energy depends on the arrangement of the system. But how exactly? Well, let's see. Let's look at gravitational potential energy first.
So consider a system of an earth and ball. This particular arrangement will have some potential energy, but what do you think will happen if I were to raise this ball higher? Will the potential energy increase, decrease, stay the same? Well, let's see. If you have to raise that ball higher, you have to push it and then move it.
In doing so, you would have transferred some energy into the system. Ooh, because you're adding some energy into the system, the gravitational potential energy increases. And so when the ball moves higher up, then the distance between the ball on the earth increases, the gravitational potential energy increases.
See, this is how the gravitational potential energy depends on the arrangement. The more the distance between the ball and the earth, the more the gravitational potential energy. Similarly, if the distance were to reduce, the gravitational potential energy reduces.
But how do we know exactly how much is the gravitational potential energy? How do we assign a number to it? Well, the first step is to assign the gravitational potential energy of some arrangement to be zero. That's the first step. So for example, we could say, "Hey, when the ball is on the ground, let's assign this arrangement to be zero gravitational potential energy."
It's completely your choice. We are completely free to choose what arrangement we want to have zero gravitational potential energy. But once we do that, now, take this ball and raise it up. In doing so, we would've transferred some amount of energy into it. Let's say we transfer 15 joules of energy.
Now, as a result, the gravitational potential energy has increased the 15 joules. So it goes from zero to 15. And so now we know that this arrangement of the earth and ball system should have a potential at your 15 joules. Isn't it amazing? This is how we assign numbers to it. Okay?
But wait a second. Is it always necessary to choose the ground to be zero potential? Not necessarily. For example, if you are doing some experiment in an apartment, for example, now, if you were to choose the zero potential to be when the ball is on the ground, that would be so inconvenient.
Instead, this time, what we'll do is we'll choose the zero potential to be, you know, when the ball is on the floor. That's more convenient. So now, we'll say when the ball is on the floor, that arrangement of the earth and ball, that, let's call it zero potential energy.
And then we can do it just like before, figure out how much energy we transferred in moving it from here to here, and figure out the potential energy of this particular arrangement. So you see, we choose whichever arrangement is convenient for us to assign zero, and then from there, we assign values for every other arrangement.
Now, what's cool is that the same thing applies to other forms of potential energy as well. For example, consider electric potential energy. If you have two unlike charges like this, separate by some distance, this particular arrangement will have some potential energy.
Now, what do you think will happen if we were to move the charges farther apart? What do you think will happen to the potential energy? Why don't you pause and think about it. Same way we did gravitational potential energy, think in the same way. All right? I know light charges attract each other naturally, and therefore, to pull them apart, I have to put a force and move them.
In doing so, I would transfer energy into the system and therefore, the electric potential energy increases. Similarly, if the charges were to come closer to each other, the electric potential energy would reduce. And again, just like before, we can choose a particular arrangement to have zero electric potential energy.
And then from there, every other arrangement can be assigned a particular value for electric potential energy. Okay, let's consider one last example. What if you have light charges? What do you think will happen if I were to move light charges farther apart? Do you think electric potential energy would increase or decrease?
Again, pause and think about this. All right, this time I know light charges naturally tend to go further apart. Ooh, which means if I have to push them together closer, now, you know, I'll end up transferring energy into the system. And so this time, pushing them closer increases the potential energy, and therefore, when they go farther apart, the potential energy this time reduces.
And we can actually see electric potential energy in action in many places. For example, during a thunderstorm, we have massive electric potential energy due to separated charges, and as a result, we get lightning strikes. But we can see them in other places as well.
You probably know that matter contains lots and lots of charges, which can attract or repel each other. And some arrangements of these charges can actually make materials deform. For example, consider a spring. You can easily compress a spring, and while compressing, look, you're pushing on it and you're making it move.
You're transferring energy into that spring. And so when you let go, that energy is released and the spring relaxes back and exact. This is exactly how, for example, a pogo stick works, right? You compress it, store energy into it, and the energy gets released.
We call this elastic potential energy. But guess where this elastic potential energy fundamentally comes from? Well, it comes from the distribution of charges, arrangement of charges. I mean, it's really complicated, but here's how we can think about it. If you zoom in, we can model that there are, let's say for example, a couple of positive charge particles over here, there at some distance.
And you know, when you have the relaxed length, there is some potential energy. Now, when you compress it, look at what happens to these charges. The charges come closer, and so the electric potential energy of the system increases. That's what's really happening when, you know, a spring is compressed, and so when you let go of it, that energy is released.
So elastic potential energy is fundamentally just electric potential energy. Similarly, chemical potential energy is also fundamentally electric potential energy. For example, we say that food has chemical potential energy. What we really mean is that certain arrangement of charges will have certain electric potential energy.
And after eating it, that arrangement changes. As a result, the electric potential energy reduces, for example. And in doing so, some energy is released and the body can use it to do useful things, like, for example, move from one place to another. That's pretty amazing, isn't it?
So long story short, potential energy is the energy associated with the system of objects. Remember, it's always defined for a system, meaning a group of two or more objects. And what does it depend on? Well, the potential energy purely depends on their arrangement.
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