Video: SAT Physics Exam 1 Q01

Consider the following types of energy. (a) Gravitational potential energy (b) Electrical potential energy (c) Elastic potential energy (d) Nuclear potential energy (e) Kinetic energy. Which of the above is the type of energy that would decrease the most in the fission of a plutonium nucleus?

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Video Transcript

Consider the following types of energy. (a) Gravitational potential energy, (b) electrical potential energy, (c) elastic potential energy, (d) nuclear potential energy, (e) kinetic energy. Which of the above is the type of energy that would decrease the most in the fission of a plutonium nucleus?

Okay, so, in this example, we have a nucleus of plutonium. Let’s say that nucleus is right here. We know that a nucleus of this element consists of protons and neutrons. Protons have a positive charge and repel one another, while neutrons have no electrical charge.

Now, since a nucleus does have all these positively charged particles in it pushing one another away, we might expect that force of electrostatic repulsion to be enough to break a nucleus apart. However, in an atomic nucleus, this force is countered by another force called the strong nuclear force. The strong nuclear force is attractive like gravity is and, for stable atomic nuclei, is largely responsible for keeping that nucleus together.

In our example though, we’re talking about a nucleus that doesn’t stay together, but rather it splits apart. That is, it undergoes fission. Nuclear fission takes place when the forces pushing the protons and neutrons in the nucleus apart are greater than the forces pulling them together. Fission is especially common in larger atomic nuclei such as this plutonium nucleus. Fission involves the breakdown of the nuclear core into smaller nuclei and sometimes the emission of other particles such as neutrons as well.

Since fission involves the splitting of an atomic nucleus, in terms of energy exchange, it leads primarily to a loss of nuclear potential energy, that is, the potential energy stored in the constituents of the plutonium nucleus before it split. Now, our question asks us which of these five types of energy decreases the most in such a fission event? Let’s consider the other options besides nuclear potential energy.

First, gravitational potential energy. This has to do with the position of an object or objects in Earth’s gravitational field. Typically, the farther an object is from the core of the Earth, the greater its gravitational potential energy. When we think about a nucleus splitting though, we don’t know the directions that the parts of the nucleus will go. They may go up or down or left or right or any other direction. So, we can’t say offhand whether gravitational potential energy will actually decrease when a plutonium nucleus goes through fission.

Even more than that though, at this subatomic scale, the distances that the split-off portions of the plutonium nucleus move after fission occurs are so small that the change in gravitational potential energy from when the nucleus was whole to when it was split into pieces is negligibly small. For that reason, we’ll cross option (a) off of our list of answers.

Now, let’s consider electrical potential energy. When a nucleus undergoes fission, it’s this kind of energy, thanks to electrostatic repulsion between like charges, that overcomes the attractional forces of the nucleus and causes it to split. Interestingly, when one larger nucleus splits into two smaller nuclei, electrical potential energy does decrease overall. This is because all the positively charged protons that were pushing so hard against one another in the original nucleus are now separated somewhat from one another. So, the electrical potential energy has gone down.

This decrease though isn’t as significant as the loss of nuclear potential energy in fission. More nuclear potential energy, energy having to do with the attractional force between nucleons, is lost compared to electrical potential energy, which is energy having to do with the repulsive force between protons. So, even though electrical potential energy does decrease in a fission process, it doesn’t decrease as much as nuclear potential energy. And so, we’ll cross option (b) off our list too.

This brings us to option (c) elastic potential energy. This is energy associated with something like a spring that is either stretched or compressed beyond its natural length. Unlike a spring though, a nucleus is not an elastic object; it’s not one that stores energy based on being stretched or compressed. So, since essentially no elastic potential energy is involved in fission, we can’t say that this is a type of energy that will decrease.

Lastly, let’s consider option (e) kinetic energy. If we imagine that our plutonium nucleus is initially at rest before it splits, then we can expect the products of that split to be moving off with some speed in various directions. These products will each have kinetic energy because they each have mass as well as speed. So, rather than decreasing, we would expect the kinetic energy of this plutonium nucleus to increase after it goes through fission. Since our question asks for the type of energy that decreases the most, we’ll cross off option (e).

And so, our original assessment is confirmed. It’s nuclear potential energy that would decrease the most in the fission of a plutonium nucleus.

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