Lecture 5: Perspectives/Cosmic Forces
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Goals
  • Conclude Perspectives Discussion
    • Pressure, Energy, Power, Luminosity
  • Discuss properties of matter
    • Atoms, Isotopes, and Molecules

Pressure
Pressure = force/area
  • 1 Pa = kg/(m*sec2) = N / m2 (after Blaise Pascal)
1 atmosphere
= 14.7lb/in2
  ~100 kPa
  = 1 bar
  = 760 mm Hg (mercury)
  = 29.92 inches Hg

 

Aside [In case you are interested]:
Blaise Pascal (1623-1662), for whom the pressure unit is named, was a child prodigy who worked on geometry, hydrostatic and atmospheric pressure, hydrodynamics, and discovered what is now known as Pascal's law of pressure. Pascal instigated and performed experiments that showed that atmospheric pressure decreases with altitude and concluded that a vacuum existed above the atmosphere. He also invented the hydraulic press and the syringe, and constructed a (mechanical) digital calculator. At the age of 31 he left physics and mathematics to work in the areas of philosophy and theology. In this context he is famous for his rationalization for believing in God known as Pascal Wager, that it is a better "bet" to believe in God than not.

 

Pressure
Examples

Pressure depends not just upon the force but also the area. A small force can produce a large pressure if the contact area is small. An ordinary needle penetrates objects because of the small area of the tip - thus it is easy to generate high pressures.

Some examples are given below.

50,000 Pa:
 Pressure on the floor of a 50 kg person in ordinary shoes
5x106 Pa:
 Pressure on the floor of a 50 kg person in high heels
5x106 Pa:
 Pressure on the highway from a 5 ton truck

Energy
Energy: force*dist
  • The capacity for doing work.
    • Kinetic: Energy of motion (KE)
    • Potential: Stored energy (PE)
  • Units of Energy
    • 1 Joule (J) = N-m (after James Joule)
    • 1 erg = dyne-cm = 10-7 J
    • 1 calorie = 4.186 J
    • 1 kilocalorie = 1 Calorie (nutrition)

Energy
Examples

Some everyday and not so everyday example of energies.

8.4x106 J
 = 2000 Calories
= Energy the body uses in one day
330,000 J
 = energy to boil 1 quart of water
= energy to run a 60 W light bulb for 1.5 hours
400,000 J
 = energy of a 1 ton car at 60 mph
 
  
1 erg
 = a snow flake hitting the ground
1000 J
= energy a match produces
4x109 J
 = 1 ton TNT
1015 J
 = nuclear explosive (250 kilotons TNT)
1025 J
 = solar flare

Power
Power: energy/time
  • The rate at which energy is used (or work is done).
    • Watt (W) = 1 Joule/sec = 107 ergs/sec
  • Examples
    • 1 kW/m2 = Solar power hitting the earth
    • 100 W = Rate at which the body expends energy
    • 100 W light bulb - only 1/5 of power goes into light, the rest goes to heat

Luminosity

Luminosity is the power radiated by an object (energy/sec).

Total Energy radiated by an object of constant luminosity is given by:

  • Total energy = Luminosity x Lifetime

People
Example
A person radiates ~ 100 W.

So that the energy output in a day is


Object
Energetics

In many cases the luminosity of an object is quoted in solar luminosities. Below is a table that will help make some connection to from (large) earth based to galaxy size luminosities.

Object

Luminosity
(Watts)

Total Lifetime
Energy Output
(Joules)

Nuclear bomb

1021

1015

Solar Flare

1023

1025

Star

1026

1044

Supernova

1037

1044

Milky Way

1037

1055


Units are
a Must

Using Units for Calculations

  • Always use units.
    • Can't understand the answer without them.

  • Check if an answer makes sense:
    • Are the units correct?
    • What "order of magnitude" do you expect for the answer

Prefixes
for Units

Some Example Densities

Name

Symbol

Factor

Length
Example (meters)

Giga

G

109

109

Mega

M

106

106

kilo

k

103

103

centi

c

10-2

10-2

milli

m

10-3

10-3

micro

m

10-6

10-6


Atoms
  • Atom => "indivisible"

  • Matter is made of atoms.
    • Very small particles in constant motion.
    • About 1024 atoms in your thumb

  • Different atoms => different elements

Simple
Picture of
an Atom
A simple picture consists of a nucleus made up of protons and neutrons surrounded by a cloud of electrons.


Simplest
Atoms

Hydrogen and helium are the simplest, lightest atoms in the universe.

  • Hydrogen: Most abundant and simplest atom

  • Helium: Next most abundant element
The schematic atoms above are not to scale!

Properties
of Atoms
  • Atoms are intrinsically neutral
    • # protons = # electrons
  • Atoms are small and mostly space.
    • Size of nucleus = 10-15 m
    • Size of atom = 1 x 10-10 to 2 x 10-10 m
  • If a nucleus were 1 mm in size then the electrons would be over 100 m away!
Note: Atoms consist of mostly empty space!

Other
Atoms

A few additional example atoms are given below:

  • Carbon - important for life
    • 6 p+, 6 n, 6 e-
  • Iron - most stable element
    • 26 p+, 30 n, 26 e-
  • Uranium
    • 92 p+, 146 n, 92 e-
    • Heaviest naturally occurring element.
    • radioactive - unstable

 


Isotopes

Isotopes of an element have the same number of protons, but a different number of neutrons.

For example:

  • U-238: 92 p+, 146 n, 92 e-
  • U-235: 92 p+, 143 n, 92 e-

 

Molecules

  • Chemically combined atoms
  • Example -
    • Water: H2O
    • 2 Hydrogen atoms and 1 Oxygen atom
  • Electrons are shared between the H-atoms and the O-atom.
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