Physics 7B FAQ

• What topics are covered in this class?
  • fluid flow
  • electrical circuits (especially resistors and capacitors)
  • exponential change
  • vectors
  • mechanics of translational and rotational motion
  • conservation of momentum
  • Newton's laws of motion
  • force and torque


• What math skills will I need?
  • using algebra to manipulate and solve an equation
  • solving systems of equations
  • using exponents and logarithms
  • setting up and evaluating simple derivatives and integrals
  • trigonometric functions
  • solving right triangles
  • radian measure of angles
  • derivatives as applied to graphs
  • 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.


• In a section of pipe, how can I tell what types of energy density are changing?
  • Is there some reason to think that the start and end MUST be at the same pressure (for instance, maybe they're both exposed to the atmosphere)? If so, then ignore pressure. If not, you should include pressure; it's often what changes to make up for something else changing.
  • If the start and end of the pipe are at the same height, you can ignore potential energy density. (This often involves a straight horizontal pipe, but doesn't have to--the endpoints are all that matters.)
  • Speed can only change if the pipe gets wider or narrower, so if the start and end of the pipe are the same cross-sectional area, you can ignore kinetic energy density. (This often involves a pipe of uniform cross-section, but doesn't have to--the endpoints are all that matters.) Also, if the fluid is not moving at all, you can ignore kinetic energy density no matter what shape the container is--from zero to zero is no change.
  • The "pump" term should be included if there is a pump in the section of pipe that you're analyzing, and should be ignored if there isn't one. Note that we only care about that particular section of pipe! If there's a pump somewhere else in the system, we don't include it.
  • The dissipation term (-IR) can be ignored if
    • the fluid is not moving, or is moving very slowly (little or no I)
      and/or
      
    • the pipe is very smooth or very short or very wide (little or no R)


• In an electrical circuit, what's the difference between current, voltage, resistance, and power?
  • Current is a measure of flow, in terms of how much charge flows past that point per second.
    • It's measured in amperes (A), or "amps" for short; one amp means one coulomb of charge per second (A=C/s).
    • This is very similar to current in fluid flow, except that fluid current is volume over time whereas electrical current is charge over time--they're both measuring how much stuff flows past per second.
    • Much like fluid flow, electrical current must be the same at any two points on a given "branch" of the circuit; for instance, if the current on the left side of a resistor is 3A, then the current on the right side will also be 3A.
    • The only situation where the current wouldn't be the same everywhere is if there's a junction where the wire splits into several paths, or several paths merge into one. It still must follow the "junction rule": total current in equals total current out. For instance, a 9A current flowing into a junction could split into, say, 5A and 4A. The exact split depends on the resistances of the paths.
  • Voltage is a measure of energy density, describing how concentrated the energy is per unit of charge.
    • It's measured in volts (V); one volt means one joule of energy per coulomb of charge (V=J/C).
    • The formal name for voltage is "electrical potential," a term you'll see again in 7C. It's also often called "electromotive force," but it's really more like a pressure; in fact, it basically serves the same role for electrical flow that pressure does for fluid flow: a difference in voltage causes current to flow through a wire, much like a difference in pressure causes current to flow through a pipe.
    • Much like pressure, voltage is a property of each point in the circuit, and each circuit element is associated with a change in voltage. A battery, for instance, provides an increase in voltage (+ε) from one side to the other; a resistor provides a decrease in voltage (-IR) from one side to the other.
    • It is often useful to label the voltage at every location in a circuit, starting by choosing any convenient location to call 0V as a reference point. Note that any two points connected by just plain wire (with no batteries, resistors, etc. in-between) MUST be at the same voltage!
  • Resistance is a measure of how difficult it is for current to flow through an object.
    • A resistor causes a voltage drop proportional to the current flowing through it, and the resistance is simply the constant of proportionality (ΔV = -IR).
    • It's measured in ohms (Ω); a one ohm resistor will allow one amp of current when attached to a one volt battery (Ω=V/A to get the units to work out right).
    • Each resistor (which includes light bulbs, buzzers, electric heaters, and most other appliances) has its own resistance which can usually be assumed to be constant (e.g., if you remove a 5Ω light bulb from one circuit and attach it to a different circuit, it's still a 5Ω light bulb, even though its other properties--current, voltage drop, and power--might change).
    • In many cases, a good way to start analyzing a circuit is to "simplify" all the resistors by combining them into one big equivalent resistor.
  • Power, whether electrical or not, is a measure of how fast energy is being transferred (or gained, or lost, or converted to a different form, etc.).
    • It's measured in watts; one watt means one joule of energy per second (W=J/s).
    • A battery provides positive power to a circuit (adding energy as time passes); a resistor provides negative power (removing energy as time passes by converting it to another form, such as light, heat, sound, motion, etc.), but we often just write the amount of power without a negative sign.
    • The usual formula is P = I·ΔV (write out the definitions and see what cancels out!), which works for batteries and resistors alike, but other formulas for resistors can be created by substituting ΔV = IR (try it!).


• What's the difference between series and parallel?
  • Series
  • Parallel
  • (nothing here yet; still writing)



• 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.