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
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.