You’ve heard the word pressure being used in relation to blood pressure or the weather (high- and low-pressure weather systems).
Pressure is defined as how much force is on a certain area.
Pressure = Force / Area P = F / A
There are different units for measuring pressure.
Units of pressure
One atmosphere = the weight of all the air in the Earth’s atmosphere, weighing down over just one square inch of the Earth’s surface.
The standard unit for pressure is the Pascal, in the metric system.
1 Pascal = 1 Newton of force distributed over 1 square meter.
1 Pa = 1 N/m^2
Bar and millibar
These units are often used in meterology.
1 bar = 100,000 Pascals
This is slightly less than the average atmospheric pressure on Earth at sea level.
We often use this when measuring tire pressure.
Millimeters of mercury
Okay, this unit sounds odd. Mercury is a metal that is liquid at room temperature.
First, let’s simplify by using common abbreviations. “millimeters of mercury” will now be written as mm Hg.
And why mention “millimeters?” A millimeter is just a tiny length. Liquid drops of mercury obviously have no special length, they just act like any other liquid, filling volumes.
So what does this mm Hg mean? It comes from the original way that scientists first measure air pressure.
In 1644 Evangelista Torricelli put mercury in a glass tube. When the air pressure increased, the mercury in the glass tube moved one way, and when the air pressure decreased, the mercury moved the other way.
Torricelli used a ruler marked in inches (although in later years, people began marking it in millimeters.) The unit of mm HG is still used when measuring blood pressure.
How does force work?
Use your finger to put a force on your skin. There might be a lot of force in your finger, but it is spread out over a wide-ish area, over 1 square centimeter. So the force per area, pressure, is small.
But now put exactly the same amount of force over a very tiny area. Now the force per area, pressure, is large enough to puncture the skin.
Gas pressure is exerted perpendicular to any surface. Here is the interior of a tire.
Swimmers feel pressure from the water on all sides.
Hmm, note anything funny about the length of the red force lines here? The longer the line, the more that the force is. Are the forces on the swimmer balanced or not? Think about the concept of buoyancy!
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?
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
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?
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 )
At constant temperature, the product of a gas’s pressure and volume is constant.
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.