I. Scientific Measurements
A. Making a measurement - accuracy vs. precision
accuracy - how close a measurement comes to the true value
precision - reproducibility of a measurement
Example: make a
measurement with a yard stick divided into many increments - carefully measure
that a piece of wood is half the length of the yard stick - get this every
single time so very good precision. What if the yard stick is only
2'11", not very accurate. Good accuracy often requires callibration.
Can have good precision but lousy accuracy - "I am a very precise free throw shooter, I hit the front of the rim every single time!"
B. Units of measurements
A measure means nothing without a unit: I am 31 could be said by me and by a toddler roughly midway through their third year of life. It all depends on units.
1. The Measurement of Length
I use meters or inches? - International system of units (SI)
The SI unit of length is the meter (m) 1m = 3.3 ft
The SI system also incorporates the ideas of the metric system - based on 10s
in referring to the size of cells (1x10-5 m) or distance between Chicago and LA (3.6x106 m) , it is not convenient to use meters, although scientific notation helps.
To aid in expressing a wide range of lengths, a series of prefixes are introduced. Here are some common ones used for length
Some reference points:
1 in =
2.54 cm, 1 km = 0.6 miles, 1 m =
As the system is based on 10, it is easy to convert between units (unlike inches, yards, and miles)
2. Volume - space occupied by matter
No basic SI unit - unit can be derived from basic SI unit of length - volume of a cube 1m on a side is 1m x 1m x 1m = 1m3
Typical units of volume:
Some reference points:
1 mL approximately 20 drops of water
1 L approximately 1 quart, 4 L approximately 1 gallon
5 L amount of blood in the body
3. Mass - Quantity of matter
SI Unit = kilogram (kg)
A reference point - a paper clip weighs approximately one gram
Note: Mass is not weight - weight depends on gravity and is a force (ice skater from Lecture 1). Your mass is the same on earth and on the moon.
Law of conservation of mass - In a physical or chemical reaction mass can neither be created or destroyed
Don't I lose mass when I burn propane in my barbecue?
propane + oxygen -> carbon dioxide + water (net result no change in mass)
Later we will show that in a nuclear reaction matter is created and destroyed.
SI unit = seconds (s)
Some reference points and some more prefixes
SI unit = Kelvin (K) -
Common non-SI units - oC, and oF, we talked about conversion between these in lecture 1.
a. What is energy? - capacity to do work.
SI unit - Joule (J) (also unit of heat which technically refers to the transfer of energy although it is often used synonymously with energy)
b. Common non-SI unit: calorie (cal)
1 cal = 4.18 J
c. In nutrition charts, 1 kcal = 1 Calorie = 1 C (big C)
Example: 1 carrot = 40 Calories
= 40,000 cal = 40 kcal
d. Forms of energy:
-light (solar cells)
e. Energy can be in several forms that can be interconverted but it is never lost
-Drop a brick - energy converted from potential energy to kinetic energy and finally transferred to my toe, ouch!
-Light + carbon dioxide + water -> plant matter + oxygen (energy stored in chemical bonds, potential energy")
-Can release the heat of plant
In all of these processes - energy is conserved
Law of conservation of energy - in any chemical or physical process, energy can be converted from one form to another but is neither created nor detroyed.
7. Heat - Physical and chemical processes absorb or release heat
exothermic - process that releases heat to surroundings
e.g. combustion - ideally carbon compound + oxygen -> carbon dioxide + water
endothermic - process that absorbs heat from the surroundings
e.g. evaporation - we sweat to cool off because the evaporation of water on our skin takes heat away from our bodies
Hot packs -
sodium acetate dissolved in water -> solidifies to sodium acetate crystals + heat gives off heat (exothermic) and raises the temperature of the surroundings
Cold packs -
heat + ammonium nitrate (used as fertilizer) + water -> dissolved ammonium nitrate
absorbs heat (endothermic) and lowers the temperature of the surroundings