Exam 2

Exam grades based on raw scores:

A 85
B 73
C 62
D 50

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Material covered

Material from the textbook and class session and labs related to the following textbook chapters.

  • Chapter 6 - Energy.

    • Work, is a transfer of energy, is equal to force * distance.
    • Gravitational energy = m*g*h.
    • Kinetic energy = 1/2 *m*v^2.
    • Elastic, chemical, radiant, nuclear energies.
    • Thermal energy is the disorganized kinetic energy of atoms/molecules.

    • The idea of energy conservation: that energy is never created nor destroyed, but may get transferred from one place to another, from one body to another.
    • Roller coaster problems, where the total energy is conserved, but balance between different forms of energy may change.
    • Energy flow diagrams.
    • Efficiency = useful output energy / total input energy.

    • Power=energy / time (units are J/s=watt).
    • Amount of power that humans are capable of.
    • Relative costs of electricity vs food calories.

  • Chapter 7 - Entropy and Heat.

    • Temperature, in C and in F, and in K.
    • Thermal expansion.
    • Spontaneous thermal energy flow (from hot to cold).
    • Entropy ("disorder") has to increase. But parts of a system can become ordered (lower entropy) if other parts get more disorganized (higher entropy).

    • The idea of a heat engine: Something that takes in thermal energy, does some work, and exhausts thermal energy.
    • Fundamental limits to how efficient they can be. Efficiency for heat engines gets better as the temperature difference between high and low operating temperatures gets bigger.
    • Refrigerator and heat pump are reverse heat engines.
    • Efficiency not limited in the same way as (forward) heat engines.

    • Calculating efficiencies of vehicles in "passenger * miles per gallon"

    • Exponential growth (or decay) characterizing by a "doubling time" (or half life).
    • Semilog plots: exponentials show up as straight line.
    • On a regular plot, an exponential function looks like a "J-curve".

    • Logistic models: start as exponentials, but then end up at a constant value rather than growing forever.
    • Difference between graphs of renewable and non-renewable resources over time.

  • Chapter 8 - Electromagnetism. Coulomb's law of electric force: $F=9\times 10^9\frac{q_1q_2}{d^2}$. units of charge $q$ are "Coulombs" (C). Meaning of electric field lines: show the direction of force that a positive "test charge" would feel. Current flow as the motion of charged particles. Units for current: Coulombs / sec = Amps. Induced charge, as illustrated by balloons sticking to walls. Induced magnetism. Magnetic fields and forces: always caused by moving charge. Faraday's law: changing the magnetic field inside a loop of wire (whether by moving a magnet, or moving the loop relative to the magnet) causes a current in the loop of wire.

Studying tips

I've suggested that you study particularly the assigned Conceptual Exercises listed at the end of many of our class notes web pages.

  • I will use a handful of the CEs directly in the test.
  • There are answers to all the odd-numbered CEs in the back of your text book, so check yourself with other ones (even if I didn't suggest them) if they relate to material we covered.
  • But the point is not to memorize the answers themselves, but rather to understand *how* you can come up with a particular answer to one of these.
  • ...with a friend: You might not do *all* your studying with someone else, but it's probably worthwhile for everyone to identify the CEs you thought were more difficult (or less obvious) on your own, and then check your answers with a friend, and see how you each approached the tougher problems.

Format of the test

Compared to the quizzes...

  • My intention is that most of you will be finished with the exam in less than the 45 minutes.
  • More in the way of multiple choice, multiple answer, true/false, short answers,
  • Less open ended questions for you to write about.
  • Even with multiple choice questions, it's *still* a fine idea to jot a note about how you're solving a problem. Sometimes that can lead to partial credit.
  • Formulas: I'll give you any needed--you don't need to memorize formulas. But you do need to know what the variables stand for (e.g. acceleration, force and mass in $a=f/m$) and how to manipulate formulas: e.g. given the acceleration and the force...what is the mass?
  • You do need to know the appropriate units for quantities: Energy (Joules = Newton*meters, or kWh), Power (J/s, or watts).
  • You should know how to do...
    • unit conversions,
    • metric conversions, and what the kilo-, micro-, milli-, mega- prefixes mean (answers: 1000, $10^{-6}$, $10^{-3}$, $10^{6}$), and how to use them.
    • how to multiply / divide with scientific notation.