Homework #3 will be posted on Wednesday It will be due Tuesday, Sept. 29, 5:00 pm Exam #1, Wednesday, Sept. 30

Kirchhoff’s Law #1 1. A hot, dense glowing object (solid or gas) emits a continuous spectrum.

Rules for Thermal Emission by Opaque Objects Hotter objects emit more total radiation per unit surface area. Hotter objects have their peak radiation at shorter wavelengths (they will appear “bluer”)

Absorption & Emission Line spectra

Electron Energy Levels Electrons cannot have just any energy while orbiting the nucleus. Only certain energy values are allowed (like the floors of an aprtment building). Electrons may only gain or lose certain specific amounts of energy (equal to differences in energy levels).

Electron Orbits / Absorption & Emission Electrons can gain or lose energy while they orbit the nucleus. When electrons have the lowest energy possible, we say the atom is in the ground state. When electrons have more energy than this, we say the atom is in an excited state. When electrons gain enough energy to escape the nucleus, we say the atom is ionized.

This diagram depicts the energy levels of Hydrogen. Each element has its own distinctive set of energy levels for its electrons. This diagram depicts the energy levels of Hydrogen. 1 eV = 1.60 x 10-19 joules

Emission/Absorption Spectra Each electron is only allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. Hydrogen Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed.

Kirchhoff’s Laws #2 2. A hot, low density gas emits light of only certain wavelengths – an emission line spectrum.

Kirchhoff’s Law #3 3. When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum – an absorption line spectrum.

Absorption Spectra If light shines through a gas, each element will absorb those photons whose energy match their electron energy levels. The resulting absorption line spectrum has all colors minus those that were absorbed. We can determine which elements are present in an object by identifying emission & absorption lines.

Molecules have rotational & vibrational energy levels (less energetic than electron energy levels, energies correspond with infrared, microwave, and radio radiations)

The Doppler Shift: A shift in wavelength due to a wave emitter moving towards (shorter wavelength) or away (longer wavelength) from an observer.  v  c =

The Doppler Effect BLUESHIFT REDSHIFT 1. Light emitted from an object moving towards you will have its wavelength shortened. BLUESHIFT 2. Light emitted from an object moving away from you will have its wavelength lengthened. REDSHIFT 3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

Measuring Radial Velocity We can measure the Doppler shift of emission or absorption lines in the spectrum of an astronomical object. We can then calculate the velocity of the object in the direction either towards or away from Earth. (radial velocity)  v  c =

Measuring Rotational Velocity

Phases of Matter solid liquid gas plasma the phases solid liquid gas plasma depend on how tightly the atoms and/or molecules are bound to each other As temperature increases, these bonds are loosened:

In thinking about phases of matter, recall that temperature measures the average kinetic energy of particles. Faster particles can escape electrical bonds easier.

Matter, Forces and Motion

Scalars and Vectors Scalar: a quantity described solely by its size (and units) Vector: a quantity described by its size AND direction

speed – rate at which an object moves [m/s; mi/hr]. A scalar quantity. velocity – an object’s speed AND direction, e.g. “10 m/s moving east”. A vector quantity. acceleration – a change in an object’s velocity, i.e., a change in speed or direction [m/s2]. A vector quantity.

Force, momentum, and acceleration are all vectors Momentum (p) – the mass of an object times its velocity (p=mv) Force (f) – anything that can cause a change in an object’s momentum As long as the object’s mass does not change, the force causes a change in velocity, or an acceleration (a) Force, momentum, and acceleration are all vectors

Forces and Newton’s Laws of Motion

Sir Isaac Newton (1642-1727) Invented calculus Invented the reflecting telescope Connected gravity and planetary forces Philosophiae naturalis principia mathematica

Newton developed the field of physics now referred to as “classical mechanics”, which describes how and why things move. To put this on a quantitative basis, he developed the field of mathematics referred to as the calculus.

Newton’s Laws of Motion Newton’s First Law A body in motion remains in motion and a body at rest remains at rest unless acted upon by an outside force.

If the net force acting on an object is zero, then there is no change in the object’s motion.

What happens when there are forces?

F = rate of change of momentum Newton’s Second Law of Motion The change in a body’s velocity due to an applied force is in the same direction as the force, and is proportional to the force, but is inversely proportional to the body’s mass. F = ma Or F = rate of change of momentum

Do not confuse mass and density Mass = amount of matter Density = amount of matter per volume Higher density means more matter packed into same volume

Acceleration is Inversely Proportional to Mass a = F / m

Because force is a vector, as is velocity, forces only affect motion in the direction of the force. Motion perpendicular to the force is unchanged.

Both p and v are vector quantities Momentum: p = mv Both p and v are vector quantities

Law of Conservation of Momentum If the net force acting on an object is zero, then the total momentum of a system remains constant.

Newton’s Third Law of Motion “For every applied force, a force of equal size but opposite direction arises” or For every action there is an equal and opposite reaction

F = ma (= rate of change of momentum) Newton's Laws of Motion A body in motion remains in motion and a body a rest remains at rest unless acted upon by an outside force. F = ma (= rate of change of momentum) For every applied force, a force of equal size but opposite direction arises.