

Charles Law Explanation
Name: Lynda B.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 11/6/2004
Question:
We are currently discussing the Ideal Gas Law and
Charles' Law. Using the example of a balloon being heated results in a
larger volume, is very intuitive, and students have no difficulty
grasping the concept. One student then said, well then if you increase
the volume the temperature must increase. I told them, yes it would have
to  the math tells us...but they are looking for something more tangible
to relate to. Do you have a better explanation that can describe why the
temp. should increase if the volume increases?
Charles Law states : V1/T1 = V2/T2. Therefore an increase in
temperature will increase the volume (balloon on a hot summer day).
Another way to look at it is that if you increase the volume, the
temperature must increase  this is harder to explain to students. Would
you use the angle that the act of increasing the volume increases the
kinetic energy of the molecules thus
increasing the temperature?
Replies:
When explaining the gas laws it is necessary to be precise about specifying
not only the units of the measurements but also the conditions of the
changes. The "normal" unit of pressure (P) is atmospheres, of volume (V) is
liters, of temperature(T) is kelvins, of amount of gas (n) is mols. In
those units the gas constant, R=0.08205 literatm/molK and the defining
ideal gas equation is: P*V = n*R*T. In the special case of Charles' law,
the quantity of gas and the pressure of the gas are constant resulting in
V1/T1 = V2/T2 or V2 = V1*(T2/T1). It is possible to change the temperature
of the gas (holding the pressure and amount of gas constant) by adding or
removing heat from the gas. This is an isobaric change. There is no way to
carry out the process you describe, that is increasing the volume
independent of heating it (increasing the temperature). For Charles' law to
apply, if you start with a given volume V1 the only way to get to V2 is to
heat the gas, thus increasing the temperature by a fixed ratio (T2/T1). The
kinetic theory of gases shows that the kinetic energy of the gas depends
ONLY on the temperature. So whatever you do to the gas, if you do not change
the temperature, you do not change its energy. If you expand the gas from V1
to V2 without heating it (called and adiabatic expansion) the temperature
of the gas will decrease, but the pressure also will not remain fixed.
Vince Calder
Lynda,
Preface: Charles / Boyles gas laws should really be taught all at the same
time and be shown, within the same lecture (day) that they can be combined
to form the COMBINED GAS LAW EQUATION and ultimately lead to the Ideal Gas
Law equation of PV = nRT.
Well, this one is a tad bit of a play on words. V1 / T1 = V2 / T2 is very
true. This equation needs to be treated with an understanding that the
"path" is important. State 1 is V1 and T1, and likewise for State 2. But
which came first, the chicken or the egg?
Your student's CHANGE OF STATE (VOLUME) is very path dependent. In other
words...
We both know that the Volume (in this case) IS CAUSAL of the Temperature.
Your very inquisitive student must realize that when he or she magically, or
mathematically rather, increases the volume of the balloon the above
equation should turn into the ideal gas equation PV = nRT ==> T = PV / nR.
==> V = nRT / P. You MAY NOT just "increase volume" without;
1.) Pulling on the balloon ( in all directions ) with some sort of sticky
tape. This may distort the balloon into a larger volume. BUT the above
equation shows that this course of action necessitates that the pressure, P,
drop accordingly. CAUSE AND EFFECT.
OR
2.) You can "add" n moles of molecules of gas (air, etc...). However, this
still means that you have "changed" the system by adding molecules. V1 / T2
= V2 / T2 NOW becomes an equation that no longer fully describes what is
going on.
You asked, "Would you use the angle that the act of increasing the volume
increases the kinetic energy of the molecules thus increasing the
temperature?" The answer is No. I would reverse it, because the
Temperature (heat added to the system) is the perturbation that CAUSEs your
INCREASE in VOLUME. Not the other way around. It IS the increased
molecular motion ( increasing T ) that causes the increase in V, volume.
The point of Charle's Law was to show that the ISOBARIC (constant pressure)
change of the state variables T and V showed their PROPORTIONAL
relationship. T goes up ==> V goes up. (ALL OTHER VARIABLES HELD CONSTANT)
The point of Boyle's Law was to show that the ISOTHERMAL (constant
temperature) change of the state variables P and V showed their INVERSELY
PROPORTIONAL relationship. P goes up ==> V goes down. (all else held
constant)
I hope that I have helped some. If not, please email us back.
Thank you,
Darin Wagner
Lynda,
You present an interesting case where theory tells us something should
happen, but there is not a good real life example to demonstrate the
principle. I admire your students for thinking outside the box.
I agree with your statements. Keep in mind that in your example, when
you compare volume and temperature, you are keeping pressure constant.
If you change the volume, the temperature must increase to maintain
constant pressure. Unfortunately, what typically happens when you
increase the volume is you reduce the pressure while maintaining
constant temperature.
In your balloon example, increasing the temperature actually increases
the pressure in the balloon. The air pressure in the balloon was
balanced with the elasticity force of the balloon which wants to
contract. The temperature change increases the internal pressure which
then rebalances the forces by expanding the balloon.
Bob Hartwell
Rather than approach this from KMT or Thermodynamics which is going to be
more confusing than illuminating for your students (and you could appear to
be giving a handwaving answer), I suggest this approach:
1) Remind them that Charles's Law works only if everything else (P, n) are
held constant.
2) Increasing the volume (perhaps by pulling on the moveable wall of a
cylinder) would have the effect of decreasing the internal pressure (Boyle's
Law).
3) But that is not allowed under Charles's Law  so in order to maintain P
constant, what do you have to do? Heat it up.
I find that this reasoning is more satisfying to the students (and I have
college students), rather than bringing up Thermodynamics which they have
not learned at this point. Hope it helps you.
Greg (Roberto Gregorius)
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Update: June 2012

