Hi. It’s Mr. Andersen. And this is AP Physics essentials video 90. It is on the conservation
of charge in reactions, more specifically in nuclear reactions. And so let’s look
at the fission of uranium 235. So if we hit it with a neutron it will quickly break apart
into barium 141 and krypton 92. So the charge we had before that reaction is equal to the
charge that we have after. Now this is a nuclear reaction so we can ignore the electrons that
are so important in chemistry and let’s just write it out. So we have on the left
side a neutron plus a uranium atom equals a barium plus krypton plus 3 of the neutrons.
And so we are ignoring the electrons but we still should include the charge, that is going
to be the positive charge inside the nucleus and then the mass number as well. So we would
write it out like this. And so a neutron has a charge of 0 and it has a mass number of
1. If we look at uranium it has a charge of 92 plus, those are the 92 protons and a mass
number of 235. Remember the atomic number or the number of protons tells us what atom
it is. So this is uranium. Now if we look at the charges on the left side, so it is
a 92 plus a 0, so that is a 92 plus charge, we are going to have an equal charge on the
right side. So we have the plus 56, plus 36 and then we have 0 charge from the 3 neutrons.
Now we also have conservation of mass. So if we look on the left side the mass number
is 235 plus 1, so that is 236. And we are going to have an equal mass number on the
right side. 141 plus 92 plus those 3 mass numbers of the 3 neutrons is going to add
up to 236 as well. And so conservation of charge holds in reactions. And the ones we
are talking about are nuclear reactions. Even if those nuclear reactions create or destroy
elementary particles. If we create an electron, for example or we convert a neutron into a
proton we still conserve our charge. If we make a positive we are going to have to make
a negative. And so the ones that we will study will be nuclear decay. And so in chemical
reactions remember, like if we are burning methane in a bunsen burner, what is most important
is going to be the electrons. That is what chemistry is all about. But if we are dealing
with nuclear reactions we are just dealing with a nucleus itself. So those are going
to be the nucleons, the protons and neutrons on the inside. And so we can pretty much ignore
the electrons unless an electron is created in this nuclear process. And so let’s start
with alpha decay. Alpha decay is the giving off of an alpha particle which is going to
be, an alpha particle is 2 protons and 2 neutrons. No electrons, so it is going to have a 2 plus
charge. It is essentially giving off a helium atom that does not have any electrons. And
so if we write it out, this would be our nuclear reaction. On the left side, let’s say we
have uranium 238, it is undergoing alpha decay, so it is giving off an alpha particle on the
right side. We can build, kind of working backwards, what the atom is that is created
under alpha decay of uranium 238. And so let’s start with a mass number. So the mass number,
on the left we have 238. On the right we have a mass number of 4 plus what is going to be
in this box right here. Well, conservation of mass. That is going to be 234. We could
do the same thing with the charge now. On the left side what is going to be our charge?
A charge of 92 plus on the left side. Again we are ignoring the electrons. What is our
charge on the right side, 2. So what is going to be the charge right here in this box? Conservation
of charge, it has to be 90. So we have a charge of 90 plus. Now that we know there are 90
protons in the middle we also know what the atom is going to be. We know that it is going
to be thorium 234, if we undergo decay of uranium 238. So that is alpha decay. We could
also look at a couple of different types of decay, beta minus decay. So if we look at
beta minus decay, an example could be breakdown of carbon 14. Or the decay of carbon 14 into
nitrogen 14. So if we look at that on the left side what is our charge? It is a 6 plus.
What is the charge on the right side? Well in nitrogen it is 7 plus. Okay, so we have
created a positive charge. Where did that come from? Well, we took that neutron inside
the nucleus and made it into a proton. So since we have made that positive charge we
have to make an equal negative charge. What is that going to be? An electron. And so in
beta minus decay we are giving off an electron. So the electron is important here. What else?
We have an electron antineutrino. Now that does not have any charge. And it does not
have any mass but it is still produced in beta minus decay. And so we have a balance
of charges on the left and the right. So on the left side we have 6. On the right side
we have plus 7 minus 1. So we have a charge of plus 6 on the right side. Charge is conserved.
Now let’s look at beta plus decay. So if we look at the decay of magnesium 23 into
sodium 23, what are we producing? Well we are producing a positron now. And we are producing
an electron neutrino. And so if we look at our charges on the left side, charge of 12
plus. On the right side 11 plus in the sodium. But we are also producing that positive charge
in the positron itself. And so did you learn to analyze electric charge? Know that the
charge before and after a nuclear reaction is conserved.
I hope so. And I hope that was helpful.