PHY100 Module 17: Ideal Gas Law
Introduction
Of the three states of matters – solid, liquid, and gas. The behavior of gas is easier to study.
Normally there are three measurable quantities for a sample of gas: temperature, pressure,
and volume. In this module you will learn how these three quantities affect each other.
Objectives
After success completion of this module, you should be able to:
 Convert between Celsius and Kelvin temperature
 Calculate pressure from an object on a contact surface
 Describe the relationship of pressure and volume of ideal gas with the temperature
fixed.
 Describe the relationship of temperature and pressure of ideal gas with the volume
fixed.
 Describe the relationship of volume and temperature of ideal gas with the pressure
fixed.
Activities
17-1: Pressure “Bed of Nails” video
What the Bed of Nails video to find out what happens to pressure when force is spread over a
large area.
Pressure is defined as the force exerted on a surface divided by the area of the contact
surface:
P
The unit for pressure is
F
A
N
 pascal ( Pa) . For comparison, 1 Pascal is about 1.5 x 10-4 psi.
m2
In the previous “Bed of Nail” video,
By lying on the bed of nails, the person spread his weight (let’s assume 200 lb, which is
equivalent to 890 N) onto (possibly) a 2.5 m x 1.3 m bed, the pressure on the person is:
P
890 N
 274 pascal
(2.5  1.3)m2
17-1Q: The Pressure on the person standing on a single nail
Estimate the pressure on the person if he were to stand on one single nail.
17-2: Temperature Scales
There are three common types of temperature scale: Fahrenheit, Celsius, and Kelvin. Kelvin
scale is also called absolute temperature scale. All three use the freezing point and boiling
point of pure water as low and high standards:
Fahrenheit uses 320F for freeing point and 2120F for boiling point
Celsius uses 00C for freezing point and 1000C for boiling point
Kelvin uses 273 K for freezing point and 373 K for boiling point.
Both Kelvin and Celsius scales have 100 divisions between freezing and boiling points of
water. However, for the same temperature, Kelvin scale reads 273 degree higher than Celsius
Textbook reference: pp.214-221
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PHY100 Module 17: Ideal Gas Law
scale:
K  C  273
The zero Kelvin degree (0 K) is called absolute zero; so called because it is the lowest
possible temperature.
Celsius temperature is the standard temperature scale for the metric unit system, Kelvin
temperature is used by the scientists. Fahrenheit is used in the US. The conversion table
between Fahrenheit and Celsius is generally provided in work place where it’s needed. The
conversion between Kelvin and Celsius is stated above.
Example
Normal human body temperature is about 370C, it is equivalent to
K  37  273  310K
17-2Q: Kelvin Temperature
What is the Kelvin degree of a room temperature at 20 0C?
17-3Q: “Boyle’s Law (P vs. V)” video
The relation of P and V while holding T constant (how the pressure and volume of a gas
change when its temperature is fixed) was discovered by Robert Boyle (1627 – 1691):
a. Record the P-V values from the video into the table below:
Pressure
Volume
Temperature
b. In the data table above, which quantity is kept constant – P, V, or T?
c.
According to the experiment, when the volume of gas increases, its pressure
______________; when volume of gas decreases, its pressure ___________.
This means P and V of an ideal gas are ______________ to each other when
T is fixed.
17-4Q: “Gay-Lussac’s Law (P vs. T)” Video
The relationship of P and T while holding V constant (how the temperature and pressure of a
gas change when volume is fixed) was discovered by Joseph Gay-Lussac (1778-1850):
a. Measure the temperature and pressure of the metal sphere and record your
data into the following table:
Pressure
Textbook reference: pp.214-221
Temperature
Volume
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PHY100 Module 17: Ideal Gas Law
b. In the data table above, which is held constant – P, V, or T?
c.
The data shows when temperature of gas increases, its pressure __________;
when temperature of gas decreases, its pressure _______________. This
means the P and T of an ideal gas are ____________________ to each other
when V is fixed.
d. The data, when graphed, is a straight line. When you extrapolate the data, you
will find that zero pressure happens when temperature is _________ 0C, a
temperature deemed ______ degree in Kelvin scale and is called
_______________________.
17-5Q: Charle’s Law (V vs. T)
a. In this demonstration, which quantity is held constant – pressure (P),
temperature (T), or volume (V)?
b. What is the relationship between V and T of an ideal gas when P is
fixed – proportional or inversely proportional?
17-6: Ideal Gas Law
The relationships between P, V, and T of an ideal gas can be described in one law – the Ideal
Gas Law:
PV  nRT
Where n is the number of moles of the gas and R is the universal gas constant.
Homework
#1) Liquid Nitrogen can be used to cool things down to 77 K. How many Celsius degree is
that?
#2) Your right rear tire has to support a weight of 3000 N. The contact area of the tire with the
road is 0.02 m2. What is the pressure from this tire on the road?
#3) Use Ideal gas law to explain why the pressure inside the tires increase after a car has been
driven. (Note: the volume of tire can be considered fixed)
Textbook reference: pp.214-221
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