Physics 7A FAQ

• What topics are covered in this class?
  • conservation of energy
  • various types of energy and ways to graph them
  • heat and work
  • behavior of molecules as temperature changes and as bonds form or break
  • behavior of ideal gases as pressure, volume, and temperature change
  • entropy
  • thermodynamics


• What math skills will I need?
  • using algebra to manipulate and solve an equation
  • graphing and interpreting graphs
  • familiarity with basic functions (linear, quadratic, reciprocal, exponential, logarithmic) and their graphs and derivatives
  • how changing a function changes its graph
  • conceptual meaning of derivative and integral


• What materials should I have for this class?
  • Beyond the usual pencil and paper, you will definitely want graph paper and a straightedge. You will be doing a lot of graphing and you want it to be as precise as possible. If you don't think you'll be using a whole pack of graph paper, split one with some friends. Alternatively you can print out custom graph paper of whatever grid size you want at Incompetech.
  • A few different colors of pen/pencil (or a fancy click-pen with several colors) can be a very good way to write your notes in a more organized way, and to distinguish between several curves on the same graph.
  • Doing arithmetic in your head is impressive, but to speed things up you'll probably want a calculator. A scientific calculator will be enough for most purposes; if you want to do anything with graphing, matrices, statistics, programming, etc., I'd recommend a TI-82/83/84.
  • Your T.A. will occasionally give you stuff to take home and experiment with (light bulbs, lenses, magnets, etc.), so you might want a pouch or small box to keep those organized and make sure they don't get lost.


• What exactly is energy?
  • That's difficult to define precisely without getting into a lot of calculus, but a good conceptual definition is "energy is the ability to do work." Since work is a transfer of energy, that may seem circular, but work can be defined as "a force acting on a moving object." If an object or situation has the ability to apply a force to some other moving thing, then that object or situation has energy. For example, a compressed spring has the ability (by re-expanding) to push on a moving block, so the spring has (potential) energy. A thrown rock has the ability to push whatever it crashes into, so the rock has (kinetic) energy. A hot gas in a container has the ability to push a piston outward, so the hot gas has (thermal) energy.


• How can I tell what type of energy is changing?
  • Each type of energy is associated with an observable "indicator":
    • Is an object's speed changing? That's kinetic energy.
    • Is an object's height changing? That's gravitational potential energy.
    • Is a spring's length changing? That's spring potential energy.
    • Are charged particles getting closer together or further apart? That's electrical potential energy.
    • Is an object's temperature changing? That's thermal energy.
    • Is an object's phase changing? That's bond energy.
    • Are molecules splitting and/or re-forming? That's chemical energy.
    • Are atomic nuclei splitting and/or re-forming? That's nuclear energy.


• What's the difference between heat and work?
  • Heat and work are both transfers of energy, but they happen in different ways.
    • Heat is energy "flowing" due to a temperature difference between one object and another or between one location and another. The energy is transferred from the high temperature region to the low temperature region. Note that this does not have to involve temperature changing as time passes; it just has to involve different temperatures at different locations.
    • Work is energy transferred due to a force acting on a moving object; specifically, Work = Force · distance. If a moving object is experiencing a force (and the force isn't at right angles to the motion), that counts as work.


• What's the difference between a closed system and an open system?
  • A "system" in this case could be thought of as a collection of objects or a list of types of energy associated with those objects.
    • "Closed" means that no energy can enter or leave the system. It's fine for energy to transfer WITHIN the system (from one object to another or from one type of energy to another).
    • "Open" means there IS some transfer (heat or work or both) of energy between something in the system and something else that's not included in the system.
    • It is often possible to rethink an open system as a closed system by including at least one more object or one more type of energy. For example, consider heating a pot of water over a campfire. If you think of the water's thermal energy alone as your system, it's open, because it's gaining heat from something else. But if you include the increase of the water's thermal energy AND the decrease of the firewood's chemical energy as it burns, you can treat it as a closed system. (That isn't entirely realistic either, though, because not all of the heat from the fire goes to the water. What else would you have to include to make it a truly closed system?)


• When should I use an energy interaction diagram, and when should I use an energy vs position graph?
  • An energy interaction diagram is all about comparing initial and final values, which can be both a strength and a weakness. It's a strength because you don't have to worry about what happens in-between at all; you just need to know initial and final values... but it can also be a weakness because it doesn't tell you anything about what happens in-between.
  • The energy vs position graph, on the other hand, shows you what happens to every relevant type of energy at all locations--initial, final, and everywhere in-between. (The downside: it doesn't work well for open systems, it can get cluttered if there are several types of potential energy involved, and any numerical results will only be as accurate as your graph is drawn... so use graph paper!)
  • In general:
    • If you're comparing a time you know everything about and time you want to know something specific about, an energy interaction diagram would probably be the most useful way to do it.
    • If you want a more general idea of what happens in-between, or over a period of time, or a "big picture" view of the entire system's overall behavior, try drawing an energy vs position graph.
    • And it may be useful to draw both! Comparing two or more different models for the same situation can often help you understand what's going on better than either model individually.



• This FAQ is a work in progress. If you have any questions that are not addressed here, please ask.
• Don't forget to check my general FAQ as well.