Home Up E1 E2 E3 E4 Topics - E1 Topics E2 Topics E3 Topics E4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 27 28 29 30 31

 

 

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

Should I use meters or inches? - International system of units (SI) 
Set of units that are convenient because they are compatible with one and other.  By agreeing to use the same set of units, comparisons are made easier.  

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
1 kilometer = 1 km = 1000 m  = 1x103 m
1 meter = 1 m = 1 m = 1 m
1 centimeter = 1 cm = 0.01 m = 1x10-2m
1 millimeter = 1 mm = 0.001m = 1x10-3m
1 micrometer = 1 mm = 0.000001m = 1x10-6m

Some reference points:  

1 in = 2.54 cm, 1 km = 0.6 miles, 1 m = 
Cell - 1x10-5m = 10 mm
Distance from  Chicago to LA = 3600 km

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:
1 L cube 10cm x 10cm x 10cm = 1000 cm3
1 mL 1 cm3 

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.

4. Time

SI unit = seconds (s)

Some reference points and some more prefixes
unit relation to seconds distance light travels 
second (s) 1 s 3x108 m
millisecond (ms) 1x10-3 s 3x105 m
microsecond (ms) 1x10-6 s 300 m
nanosecond (ns) 1x10-9 s 0.3 m
picosecond (ps) 1x10-12 s 300 mm  
femtosecond (fs) 1x10-15 s 0.3 mm

5. Temperature

SI unit = Kelvin (K)

Common non-SI units - oC, and oF,  we talked about conversion between these in lecture 1.

6. Energy

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
1 kcal = 4.18 kJ

 

c. In nutrition charts, 1 kcal = 1 Calorie = 1 C (big C)

Example:  1 carrot = 40 Calories =  40,000 cal = 40 kcal
40,000 cal (4.18 J / cal) = 1.7 x 105 J = 170 kJ

 

d. Forms of energy:

-light (solar cells)
-potential energy (energy due to position, chemical bonds)
-kinetic energy (energy due to motion) - the kinetic energy of atoms in a hot         object is high.

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 matter:
burn it -> energy transferred to the surroundings as heat
eat it -> we can do work, also transfer energy to our surroundings as heat (we raise the temperature of what is around us)

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
wood (1kg) 4,300 kcal
coal (1 kg) 7,400 kcal
Gasoline (1 kg) 11,500 kcal
Nuclear reaction (1 kg) - not combustion 20,000,000,000 = 2x1010 kcal

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

DEMO:

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