- WELCOME TO THE VIDEO TUTORIAL ON PRESSURE.
SO PRESSURE IS IMPORTANT TOPIC WHEN WE'RE LOOKING AT GASES.
SOLIDS AND LIQUIDS AT AMBIENT CONDITIONS,
THERE'S NOT A BIG DEAL WITH PRESSURE
BUT GASES HAVE A HUGE EFFECT.
SO THE FIRST THING WE WANT TO DO IS,
WHAT'S THE DEFINITION OF PRESSURE?
PRESSURE IS A FORCE OVER AREA.
SO AN EXAMPLE OF THIS WOULD BE IF WE--
IF I'M--
IF I'M PRESSING MY HANDS TOGETHER,
WE WOULD HAVE EQUAL FORCE BETWEEN THEM.
AND THAT WILL--
AND SO THERE'S THE PRESSURE I FEEL.
BUT NOW WITH ONE HAND,
IF I CHANGE MY FORCE TO THE TIP OF MY FINGER,
NOW THIS HAND FEELS THE PRESSURE
BECAUSE THE FORCE IS BEING APPLIED OVER A SMALL AREA
VERSUS IF I HAVE APPLIED THE PRESSURE,
THE FORCE OVER A LARGE AREA.
SO HERE IT WOULD BE-- WE'D HAVE EQUAL PRESSURE
BECAUSE WE HAVE EQUAL FORCE OVER EQUAL AREA.
THIS WOULD BE UNEVEN PRESSURE
BECAUSE THE FORCE IS BEING APPLIED OVER A SMALLER AREA.
SO THERE'S THE DEFINITION OF PRESSURE.
AND SO FOR EXAMPLE, PSI,
IF YOU'VE EVER FILLED YOUR TIRES WITH AIR,
PSI STANDS FOR POUNDS PER SQUARE INCH.
SO THE POUNDS IS THE FORCE AND THEN
THE INCHES SQUARED IS THE AREA.
SO THERE ARE--
A WIDE OF RANGE OF WAYS TO MEASURE PRESSURE.
ONE OF THE FIRST WAYS
WE MEASURED PRESSURE WAS WITH A BAROMETER.
SO THE WAY A BAROMETER WORKS IS IT'S FILLED WITH MERCURY.
AND IN THE OLDEN DAYS, HUMAN BEINGS, WE LOVED MERCURY.
WE'VE USED IT FOR ALL TYPES OF THINGS.
BUT HERE WE'RE GOING TO USE IT TO MEASURE PRESSURE.
AND SO WHAT WE HAVE HERE IS A SEALED AND EVACUATED TUBE.
ALL RIGHT.
SO ALL OF THE GAS MOLECULES HAVE BEEN REMOVED FROM THIS TUBE
AND IT'S SEALED.
AND SO THEN IF WE STICK IT INTO A TUB OF MERCURY,
SO THE MERCURY SITTING HERE,
AND SO, WHAT HAPPENS?
WELL, WE'RE ALL EXPERIENCING ONE ATMOSPHERE OF PRESSURE.
SO THAT ONE ATMOSPHERE
OF PRESSURE PRESSES DOWN ON THE MERCURY
AND PUSHES IT UP THE EVACUATED TUBE TO FILL THAT EMPTY SPACE.
AND SO THEN
WHAT HAPPENS IS IF WE MEASURE THIS COLUMN OF MERCURY,
WE WILL SEE THAT IT MEASURES 760 MILLIMETERS.
SO THE PRESSURE UNITS CAN--
THEY SEEM VERY STRANGE.
HERE'S ONE ATMOSPHERE, THAT COULD MAKE SENSE.
SO NOW WE CAN SEE
HOW ONE ATMOSPHERE EQUALS 760 MILLIMETERS OF MERCURY.
THAT'S THE HEIGHT EQUIVALENT OF THE MERCURY COLUMN
FROM PRESSING DOWN WITH ONE ATMOSPHERE OF PRESSURE.
WHEN THIS--
WHEN THE BAROMETERS WERE FIRST USED,
WE DIDN'T HAVE THE METRIC SYSTEM YET.
SO AT THAT TIME, YOU KNOW, SEVERAL HUNDRED YEARS AGO,
THEY USED THE UNIT TORR.
SO WE CAN SEE THAT MILLIMETERS
OF MERCURY AND TORR ARE THE SAME SIZE UNIT.
SO THEY CAN BE INTERCHANGED.
THEN IF YOU DID IT--
IF WE DID A UNIT CONVERSION,
GUESS HOW MANY INCHES ARE IN 760 MILLIMETERS?
YOU GOT IT, 29.92 INCHES.
SO THIS WAS JUST A MILLIMETER TO INCH UNIT CONVERSION, RIGHT?
SO WE COULD SAY THAT OR 29.92 INCHES OF MERCURY.
AND HERE'S THE CONVERSION TO PSI WHICH IF YOU THINK ABOUT TIRES,
SEEMS LIKE TIRES USUALLY, WHAT,
30 TO 40 PSI OF PRESSURE WHICH WOULD BE ABOUT TWICE
THE ATMOSPHERIC PRESSURE,
SO THAT SEEMS REASONABLE.
AND THEN PA, THIS STANDS FOR PASCALS.
AND THIS IS FROM THE SI UNIT SYSTEM
WHICH WE ARE NOT WORKING WITH BUT I WANTED TO LET--
HAVE IT THERE FOR THE SAKE OF COMPLETION.
SO PRESSURE IS PRIMARILY GOING TO BE USED IN GAS CALCULATIONS.
AND SO IT'S IMPORTANT
THAT YOU RECOGNIZE ALL OF THE DIFFERENT UNITS
THAT CAN BE USED TO COMMUNICATE PRESSURE.
JUST TO KIND OF FILL IN SOME CONTEXTS.
BLOOD PRESSURE IS THE PRESSURE EXERTED BY BLOOD
ON THE WALLS OF THE BLOOD VESSELS.
SO THE SYSTOLIC PRESSURE, THAT'S THE MAXIMUM PRESSURE.
AND NOTICE THAT WHEN WE GET OUR BLOOD PRESSURE READINGS,
THEY'RE GIVEN TO US IN MILLIMETERS OF MERCURY.
SO A NORMAL SYSTOLIC IS IN THE 90 TO 130 DEGREE RANGE.
THE DIASTOLIC IS THE MINIMUM BLOOD PRESSURE.
AND SO THAT'S ALSO GIVEN IN MILLIMETERS OF MERCURY
WITH A NORMAL RANGE 50 TO 90.
SOME OTHER PLACES
THAT WE'RE GOING TO ENCOUNTER PRESSURE UNITS
WILL BE WHEN WE TALK ABOUT VAPOR PRESSURE
AND THE BOILING POINTS OF SUBSTANCES.
SO THE MORE VOLATILE A COMPOUND, RIGHT,
WILL HAVE HIGHER VAPOR PRESSURE.
SO VOLATILE COMPOUNDS HAVE HIGHER VAPOR PRESSURE.
AND SO--
AND THAT WILL BECOME IMPORTANT IF YOU--
IF YOU WILL--
IF YOU'RE CURIOUS ABOUT THE RELATIONSHIP
BETWEEN VAPOR PRESSURE
AND INTERMOLECULAR FORCES AND BOILING POINTS.
WE DO HAVE A VIDEO TUTORIAL TO HELP YOU WITH THAT.
AND THE MAIN POINT OF THIS PAGE HERE,
THOUGH, IS TO SEE THAT--
HERE'S OUR TEMPERATURE,
THAT AS WE INCREASE THE TEMPERATURE,
WE INCREASE THE VAPOR PRESSURE.
AND THEN THE LAST--
THE LAST PLACE THAT PRESSURE SHOWS UP IN A BIG WAY
AND IN OUR COURSE MATERIAL IS THAT THE ATMOSPHERIC PRESSURE
DOES CHANGE WITH ALTITUDE.
AND SO WHAT WE CONSIDER A NORMAL PRESSURE
AT SEA LEVEL DROPS SIGNIFICANTLY AS WE RISE ABOVE SEA LEVEL
AND INCREASES AS WE GO BELOW SEA LEVEL.
SO THIS IS--
CONCLUDES OUR BRIEF INTRODUCTION TO PRESSURE
AND ITS UNITS AND IT GIVES YOU A BIT
OF A CONTEXT FOR WHERE YOU WILL USE THIS INFORMATION
FOR FUTURE ASSIGNMENTS.
SO IF YOU'RE WORKING ON YOUR GAS LAW HOMEWORK RIGHT NOW,
IT'D BE GREAT TO WORK IF YOU PRACTICE PROBLEMS
TO REINFORCE YOUR UNDERSTANDING
OF PRESSURE UNITS AND HOW IT PLAYS INTO GAS LAWS.