KaiserScience

Home » Physics » Fluids » Pressure

Pressure

Objective: Students will be able to

* explain what causes atmospheric pressure
* explain how a braomter works
* describe the relationship between the pressure and volume of a gas (for a given mass, at a constant temperature)
* explain what causes an object to rise in the air around it

Evaluation
Level 1: Memorization of vocabulary; ability to answer true/false questions; matching questions
Level 2: Show comprehension of physics concepts, at the Mass Learning Standards level, by running PhET simulations of gas, pressure and volume, and then explaining the observed phenomenon in complete sentences, at grade level grammatical accuracy.

Where does air pressure actually come from?

How does air pressure change as a function of altitude?

Why aren’t we crushed by the air pressure around us? That pressure is pointing inwards, on every point of our body.

Because we have an equal amount of pressure inside our bodies!
Fluids and gases inside our body cells have their own pressure.
Internal pressure outward = air pressure inward

What would happen if we removed this internal pressure inside us, or inside any object? Let’s take a large object and evacuate it: As the level of gas pressure inside the object decreases, what occurs?

Air pressure demonstration imploding tanker train

Atmospheric pressure versus altitude

Changes in atmospheric pressure with height.

Atmospheric pressure is the weight of the air above.

At sea level, average air pressure =

1000 millibars = 1 kilogram / cm^2 = 14.7 pounds / in^2

One half of the atmosphere lies below an altitude of 5.6 kilometers.

Above 100 km = 0.00003 percent of the atmosphere

Picture1

chap 17

Graph reading skills

We have 2 different Y-axes:
Both are for altitude, but one is in km while the other is in miles.

We have 2 different X-axes:
Both are for temperature, but one is in degrees C while the other is in degrees F.

At what altitude do air molecules have the warmest temperature?

Higher level question: Why would it nonetheless feel cold if you were in this warmer region of the atmosphere?

Thermal structure of the atmosphere

figure 6 Tarbuck Lutgens

Troposphere – bottom layer – temperature decreases with an increase in altitude. This is where our weather occurs.

temperature drops, to a height of about 12 kilometers

Stratosphere – here temperature remains constant to about 20 kilometers. Temp then gradually increases until the stratopause, at 50 km

Temp increase here because ozone is concentrated here: ozone absorbs ultraviolet radiation from the sun.

Mesosphere – temp decreases with altitude, until 80 km. Air temperatures approach −90°C.

Thermosphere – contains only a tiny fraction of the atmosphere’s mass.

Temp increases here because O2 and N2 gas molecules absorb short-wave, high-energy solar radiation.

Yet it would feel cold if you exposed yourself to these hot air molecules! Why?

Because although the air molecules are vibrating faster (‘hotter’) there are far fewer of them.

What would keep you warmer? Having only one huge fireplace, for a very large apartment building? Or having small fireplaces in every single apartment?

Heat = ( temp of air molecules ) x ( # air molecules )

Gas laws

At constant temperature, the product of a gas’s pressure and volume is constant.

Boyle's law pressure temp

Learning Standards

2016 Massachusetts Science and Technology/Engineering
HS-PS1-3. Cite evidence to relate physical properties of substances at the bulk scale to spatial arrangements, movement, and strength of electrostatic forces among ions, small molecules, or regions of large molecules in the substances. Make arguments to account for how compositional and structural differences in molecules result in different types of intermolecular or intramolecular interactions.

HS-PS2-8(MA). Use kinetic molecular theory to compare the strengths of electrostatic forces and the prevalence of interactions that occur between molecules in solids, liquids, and gases. Use the combined gas law to determine changes in pressure, volume, and temperature in gases.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

PS1.A Structure of matter (includes PS1.C, nuclear processes)

That matter is composed of atoms and molecules can be used to explain the properties of substances, diversity of materials, how mixtures will interact, states of matter, phase changes, and conservation of matter. States of matter can be modeled in terms of spatial arrangement, movement, and strength of interactions between particles. Characteristic physical properties unique to each substance can be used to identify the substance.

PS1.A: STRUCTURE AND PROPERTIES OF MATTER

The arrangement and motion of atoms vary in characteristic ways, depending on the substance and its current state (e.g., solid, liquid). Chemical composition, temperature, and pressure affect such arrangements and motions of atoms, as well as the ways in which they interact. Under a given set of conditions, the state and some properties (e.g., density, elasticity, viscosity) are the same for different bulk quantities of a substance, whereas other properties (e.g., volume, mass) provide measures of the size of the sample at hand.

 

%d bloggers like this: