- Hello students
and welcome to the pre-lab video for
thermal expansion.
So suppose we have a long thin rod like this
of a pure metal.
In this case,
this one is copper.
It has a particular length at room temperature
it turns out if we raise the temperature
the rod will increase it's length.
And it turns out this change in length,
this increase in length
is directly proportional to the increase in temperature.
And the constant of proportionality is something
we call the coefficient of linear expansion.
So this is an important property of many solid
materials and today we are going to experimentally
determine the coefficient of linear expansion
for a particular pure metal.
So you may have copper,
you may have iron,
you may have brass or aluminum.
Now we're going to use
the synco linear expansion apparatus
and I have one right here and we see there's a housing
like this and inside there
there is one of these thin rods
of a particular metal.
Now one thing you will have to do is measure
the length of the rod at room temperature
so you will need that,
so you will need to disassemble this
pull this out,
measure it,
and put it back together.
There are two rubber stoppers at either end
and
the metal rod is inside the two rubber stoppers
and it's centered into the housing there.
So a few features of the housing,
there is a little screw at this end,
and at this end there is a screw attached to a dial,
and this allows you to record
the initial length,
not really the initial length,
but the initial position of the rod on one end
and then we can when it expands,
take another measurement and between the two
we can determine the change in length,
and let me show ya how we would do that.
So you'll see there are two measuring scales
on this end of the expansion apparatus,
there's one here,
in millimeters, a linear straight one there,
but there's a dial scale
that's in one hundredths of millimeters.
And every time I turn this around one time,
it will go in or out exactly one millimeter.
So I'm able to get a measurement down to
a change in distance down to a one hundredth of millimeter
so that's very good,
because the change in length
will not be relatively speaking a lot,
it will be probably less than a millimeter.
So,
I would at room temperature,
put my rod in there,
and then I want to make contact with the screws
at both ends so that I'm sure that I'm getting
an initial reading here at the initial length
of the rod at room temperature.
So to be sure that you're correctly doing that,
there is an electrical circuit in here
and when
I make contact with the dial here
and with the screw here,
a little light bulb comes on right here.
So you'll notice that,
that tells you hey you're ready to take the measurement
down here,
so I would go down here to the dial,
and to the linear scale,
and suppose on the linear scale,
the front face of my dial is between,
one and two millimeters,
and then the dial the one hundredths is maybe at 20,
I would record that hey I'm between one and two
millimeters in linear scale,
dial is at 21 hundredths of millimeters.
I would record that.
And then it's very important that we back out,
several turns here,
to allow for expansion.
Now I want to heat the linear rod up,
so you'll see you have a thermometer right here,
a little manual thermometer.
And we're gonna heat it up using a device
we call a steam generator,
and it's a very simple device,
and we simply put water in it,
there's a sight glass here
that will let you know the level of the water inside
if you use too much water, you'll have to wait
too long to get your steam.
Too little water, it will evaporate right before
the experiment is done.
Probably fill it about half full and you can tell that
by looking at the sight glass.
Then you would simply plug it into a receptacle
and it will generate steam.
So I would attach the hose from the steam generator,
to this end of the tube,
so it will take five to 15 minutes depending
upon your steam generator.
And you will eventually see steam coming out the exit
point of the cylinder right here.
So you want to let it heat for maybe a good five or ten
minutes like that and then you can look at the thermometer
right here,
and get a temperature, a final temperature
above room temperature.
It will be about 100 degrees C obviously.
Now another way to get the final temperature is
if you know the barometric pressure,
you can go to a steam table and get the temperature
for steam,
for that given pressure situation
and sometimes that's a little more accurate.
So I don't know what will be available to you,
you would need a barometer and a steam table,
so perhaps it would be available,
maybe you would just use the thermometer right here.
So once we get it fully heated up,
we let the steam flow through it five or ten minutes,
and we record our final temperature,
we want to get a new reading on the dial down here,
so I would tighten it up again,
and we'd bring it in with it at the elevated temperature
and still until I see the bulb illuminate down there,
and at that point, I would go down here again,
I would look where the front face of the dial is,
on the linear scale, and maybe it's a little bit
beyond two millimeters now,
between two and three millimeters,
and my dial is say at ten.
I would record that
and that would tell me, hey, I had an increase
of 90 one hundredths
of a millimeter.
And that would give me my delta L or my linear expansion
and now with the original length,
and my change in temperature,
I have everything I need to determine
the coefficient of linear expansion
for that material.
I can compare it with a known value,
and compute a percent difference.
So very easy setup to use,
and if you're very careful,
and you record the measurements here correctly,
you'll get very very good results
and you should get a low percent error.
So I hope you would review this before you come
to the lab to study thermal expansion.
So I look forward to seeing you in the lab.