Matter & Energy
1. Energy
* Feynman: " . . . we have no knowledge of what energy is."
* Expenditure of energy
* Varieties of energy
> Kinetic
- Heat
> Potential
- Elastic, graviational, etc.
> Radiative
* Kinetic Energy: KE = (1/2)mv2
* Heat & Temperature
* Conservation of energy: energy never destroyed
* Conservation of energy + mass
> E = mc2
2. Matter
* Atoms
> Old picture: Electrons orbit nucleus (Bohr Atom)
> New picture: Electrons smeared-out into 'cloud'
- 'Probability distributions' of electrons
- Electron energy levels
> Ions
> Isotopes
> Molecules
* Phases of Bulk Matter
> Solid
> Liquid
> Gas
> Plasma
> Phase depends on temperature
Questions
1. Ques. #3, pg. 79.
2. Ques. #7, pg. 79.
3. What are the differences between an atom, an ion, an isotope and a molecule?
4. Ques. #9, pg. 79.
5. Ques. #10, pg. 79.
6. Ques. #12, pg. 79.
7. Ques. #15, pg. 79.
8. Ques. #16, pg. 79.
9. Prob. #2(a), pg. 80.
10. In what way is the solar system a poor model of the atom?
11. Which of the following situations requires expenditure of energy? Explain your answers.
a) Lifting a rock from the ground to a height of 2 meters.
b) Pushing the same rock along the ground.
c) The same rock sliding on a flat stretch of perfectly frictionless ice.
Answers
1. Temperature measures the average kinetic energy of the atoms in a substance. Thermal energy is a measure of the total kinetic energy of the atoms in a substance.
2. According to the law of conservation of energy, energy is neither created nor destroyed in any process - it is merely converted from one form to another. Ultimately, the energy that keeps you alive came from the conversion of mass into energy in the center of the Sun! This release of energy warms the Sun, which allows it to radiate light. Upon reaching Earth, this sunlight is absorbed by green plants; the green plants store this energy as chemical potential energy. When the plants are eaten by humans (or animals), this energy is stored in the body, and it's this energy that is keeping you alive. This energy 'goes' into powering your body: breathing, moving, hearing, thinking, etc.
3. atom: smallest particle of a chemical element; consists of nucleus (protons, neutrons) + electrons that occupy region surrounding nucleus.
ion: atom that has gained or lost one or more electrons.
isotope: atom of a substance containing a varying number of neutrons. All atoms of a substance are identical in the number of protons they have, but can differ in the number of neutrons. These differing types of the atoms of an element are called isotopes of that element.
molecule: smallest particle of a compound; consists of two or more atoms bonded together.
4. Electrical charge is a fundamental property of matter. Protons carry positive electrical charge; electrons carry negative electrical charge; neutrons carry zero electric charge. Unlike charges attract; i.e., it's the electrical attraction between protons (+) and electrons (-) that hold the atom together. Like charges repel.
5. An atom consists of a very small, but heavy nucleus plus a much larger region occupied by electrons. The nucleus consists of protons and neutrons. Electrons occupy the space surrounding the nucleus. A typical diameter for an atom is 10-10 meter. The diameter of the nucleus is about 100,000 times smaller than the diameter of the whole atom.
6. The phase of a sample of matter is dictated by how tightly its atoms are bound to each other. Solids: bonds are very strong; atoms tend to remain fixed in location relative to other atoms (although they may wiggle a bit, as if they bound to each other by springs). Liquids: Bonds are weaker; atoms can now slide around each other, though they never get too far apart, on average. Gases: Atoms are now completely detached from each other; they typically interact only when they collide.
7. A plasma is a gas in which some of the atoms are ionized. As temperature increases, atoms tend to have more kinetic energy (on average), thus move faster. When atoms collide at these high speeds, they tend knock electrons off each other, thus becoming ionized; the higher the temperature, the more violent the collisions, thus the more likely that atoms will lose electrons when they collide. At sufficiently high temperatures, all atoms lose all of their electrons. Such conditions prevail at the center of the Sun and other stars.
8. (i) An atom has energy by virtue of moving (kinetic energy). (ii) An atom has energy stored in its electrons; this is electrical potential energy, which resembles gravitational potential energy. (iii) Energy is stored in the nucleus of the atom; this is effectively mass-energy. It is this energy that is released in nuclear power reactors and in 'atomic' bombs. In the excited state, an electron occupies a higher energy level (and is, on average, farther from the nucleus). It's rather like saying that a baseball at the top of a stairway possesses more gravitational potential energy than does a brick at the bottom of the stairs.
9. E: energy; m: mass: c: speed of light. Einstein's equation proclaims that mass and energy are interchangeable: mass can be converted into energy, and energy can be converted into mass.
10. Modern physics claims that we cannot exactly specify the position of an electron in the neighborhood of a nucleus, thus cannot specify its 'orbit.' On the other hand, we can describe the orbits of the planets with great accuracy (and consequently predict their positions far into the future). So, we're left with the notion that the position of any electron can only be specified in terms of its probable location. It is quite true that electrons with higher energy have greater probability of being far from the nucleus; but this isn’t enough information to specify the electron’s orbit.
11. a) Expenditure of energy is require because we have force and motion.
b) Again, force is required to create motion, so energy must be expended.
c) No energy expended. Nothing is required to keep the rock going (in principle).