Thursday, December 2, 2010

Some things to consider in preparing for your final.

Here is a preview summary of what you could choose to focus on for the final. (I will add more later.) These will be the things that I will emphasize in composing the final. The purpose of the final is to help you solidify your understanding of the most important aspects of this class in a way that will enable you to remember them for longer than usual.

First, it is useful to be able to quickly sketch electric fields due to two or three charges. This sort of skill helps you visualize electric fields, for example, especially those emanating from an oscillating electric dipole which plays an important role in our understanding of electromagnetic radiation generation.

Second, you would wish to be able to analyze the currents and voltages (voltage differences) in simple circuits with a few resistors. In my opinion, the key to understanding circuits is to understand current. In a simple loop circuit, with no bifurcations, the current is and must be the same everywhere along the circuit path. Additionally, current divides and recombines at simple bifurcating junctions in a simple way: if the there is a bifurcation (fork) where in the upper half the resistance is 1 Ohm and in the lower half resistance is 2 Ohms, then the current through the upper half will be twice as large as the current through the lower half. This implies that two thirds of the total current goes through the upper half and one third through the lower half. If you understand this intuitively you'll be able to do every problem. You did not need any formula other than V=IR. V=IR tells you the voltage drop across any particular resistor in terms of I. Tto find the current, I, however, you need to analyze the entire circuit as a system. You cannot look at just one part and tell what I is. That is what it means to say that the current is a property of the entire system not of an individual part.

Third, think about circuits which include capacitors and inductors --can you confidently articulate the behavior of circuits with capacitors and inductors. We describe the behavior of circuits in three ways: with equations, with graphs, and with words. All three are important. I hope you will be comfortable with all 3 modes of communicating and understanding circuits.

Circuits with capacitors and inductors exhibit time dependence. Why is that? Think about that for a while. That is a good practice problem. Why do circuits with capacitors or inductors exhibit time-dependent?  (There are two different answers, one for capacitors on for inductors. Feel free to post your thoughts on that here.)

Understanding LRC circuits is also a point of emphasis for us. Our focus, is on LRC circuits that are similar to LC circuits --the current oscillates, the charge on the capacitor oscillates -- however, they lose a little energy in each cycle. What part of the circuit causes that energy loss? How and when does it happen?

In general for LRC circuits, the key things to understand are: charge (in the capacitor), as function of time, current as a function of time, what their graphs look like, how to calculate them, and everything having to do with energy. Energy can be stored in some parts of the circuit --and you want to understand that and its time dependence. Energy can also be lost in one part of the circuit. Losing energy is a bit different from storing energy. Understand the rate of energy loss and how that relates to other things and how it adds up over time is a good thing.

Other things that are important to understand and remember include the essential nature of E&M radiation, how photosynthesis gets started,  how an oscillating electric dipole can generate an electric field wave...
anything i have left out?

Your input. comments and questions are welcome.

10 comments:

  1. Thanks for the helpful info! Do you mind posting which specific homework problems to focus on (like you did last time)?

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  2. Here are some:
    HW#5: The RL circuit, responding to changes

    Decay of current:

    Growth of current:

    27.26
    55
    24
    56 and 63: sort of; but don't spend too much time on them


    HW#6
    28.26
    46
    47


    Oscillations in an LC circuit.: (mostly the solutions and behavior)

    Energy within an LC circuit

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  3. anon student:
    Hi. I was wondering if the final will consist primarily of essay type questions. Or will there be much calculating? I feel a bit confused about how to study for this. For the midterm I was able to study from masteringphysics, but I feel much less confident about this final.

    me:
    I will answer your question, but first, with the caveat, that I do not think that the preparation is so different. If you understand LR and RC circuits, for example, you can solve problems or write essays about them to express that understanding. If you do not understand them, then you cannot really do either. The essays generally involve equations and graphs, as well as sentences and paragraphs.

    The answer to your question is: about a 50/50 ratio is what i am thinking.

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  4. do we get a notecard like we did for the midterm? Also, approximately how long will the final be?

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  5. Thank you.. how big can the notecard be? & I meant about how many questions you plan on putting on the final.

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  6. I this true: Energy can be stored in the capacitor and lost in the resistor. rate of energy loss, is the same as power lost in the resistor? so do we use power equations for calculations?

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  7. True.

    Yes, true also, the rate of loss is R*I^2.
    I is a function of time.

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  8. Is the reason that circuits with capacitors and inductors have time dependence because in those circuits, energy can be stored as either electrical or magnetic energy, and the current through each item is not instantaneous? Meaning that the current through either the capacitor and the inductor changes with time and is not constant?

    ReplyDelete