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Homework Help: A few different questions

  1. Feb 27, 2007 #1
    Firstly, I was just wondering if anyone could tell me what 8 binary levels are. The question is 'State the number of bits per sample needed to code for the 8 binary levels' I thought that this would just be 8 bits as that covers 256 choices, but I doubt it's that simple.

    Secondly, you have a normal car mirror, and then you extend it slightly with a flatter bit of mirror. Now obviously this will give you a wider vision, but would a disadvantage be that it distorts the view of the normal mirror?

    If you have two photos, the second photo is just a part of the first photo blown up without the amount of pixels changing, but the actual photo is the same size as the first photo. I presume that the resolution does not change as it is still the same image and same amount of pixels, just the pixels have increased in size?

    Finally, the last question.The rear surface of a car mirror can be heated electrically to clear frost and demist the mirror. A current I is passed through the reflecting alloy at the back of the mirror. The dimensions of the mirror are 0.20m length, height 0.08m. The heater dissipates 50W from the 12V car battery and the current is approx. 4A
    Calculate the thickness of the reflecting alloy film used to heat the mirror.

    Conductivity of reflecting alloy = 3.1 x 105S m-1

    I actually haven't a clue how to approach this as when I tried, I got a ridiculous figure of 4.975 metres!
  2. jcsd
  3. Feb 27, 2007 #2


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    Staff: Mentor

    -1- Since you understand that if you have 8 bits, that can encode 256 choices or levels, then how many bits does it take to encode just 8 levels?

    -2- Sorry, I don't understand the mirror question.

    -3- Resolution is usually defined in units of pixels per cm or inch or some distance unit. So when you blow up a 3x5 picture to 8x10 with the same number of pixels, the resolution drops. Does that make sense?

    -4- Check to be sure that you are using units correctly in your calculation. Remember that S = Siemon (sp?) = 1/Ohms. Can you show us how you calculate the resistance needed for that power level, and then how you calculate the resistance of the volume of the reflecting alloy. You have to show us your work in order for us to help you on this one.
  4. Feb 27, 2007 #3
    P=Vsquared/R, so R=Vsquared/P. R=2.88 Ohms.

    What purpose does working out the resistance of the volume of the reflecting alloy have when I am trying to work out the thickness? I don't understand why the dimensions are given as I haven't learnt to use dimensions with conductance and resistance etc.

    Thanks for 1 and 3 btw, your rephrasing suprisingly helped.

    Actually, we can ignore q.4, apparently that's an example that'll be walked through in class. Thanks for trying to help though!

    If you want to help with another question though,

    b) Whentheequation V = t(emf) - I r is multiplied throughout by the current I, an equation for the power, IV, delivered to the load resistance is I V = I t (emf) - Isquared r .
    Complete the gaps in the following sentence to explain the physical meaning of the second equation.
    The electrical power delivered to the external load resistance is equal to
    the power ... minus the power .......
    I think the two gaps are 'the total power' minus the power delivered to the 'internal load resistance'.

    I appreciate that that may not make much sense, apologies if it doesn't.
  5. Feb 28, 2007 #4


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    Staff: Mentor

    Without more background, it's hard to know what is being asked about in your new (b) question. My guess is it's something like the total power delivered to the load is the input power minus the I^2R losses in the delivery wires?
  6. Feb 28, 2007 #5
    Yeah that was correct, never thought about using that particular power equation, just thought about circuitary components!

    We're now on to transmission systems. These are our questions:

    12 In this question, you are asked to choose and discuss a practical example of a transmission
    system in which signals carry information.
    (a) (i) Name the kind of information to be transmitted by the system that you have chosen, and state your example of signal transmission.

    kind of information transmitted-
    example of signal transmission system-

    (ii) The frequency of the signal is related to the rate at which information is transferred. Give an estimate of a transmission frequency in your system. State what physical quantity the frequency represents.

    For part a.) I presume kind of information transmitted can be something like audio and the transmission system is just a radio transmitter?

    For part ii) though, I am unsure as to what it's asking. I know radio waves are around 90MHz, but when it asks for physical quantity, what is it asking for?

    I know I keep asking questions, which may be annoying, but it is very useful for revision purposes!
  7. Feb 28, 2007 #6


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    Staff: Mentor

    I'll answer your questions with some directed questions for you to think about and maybe look up on wikipedia.org or some of your other learning resources:

    -- What is the difference between baseband transmission of information versus modulated transmission?

    -- For modulated transmissions, what is the relationship between the carrier signal and the modulated waveform? Given a carrier frequency, what is the maximum data transmission rate for various modulation schemes (AM,
    FM, QPSK, CDMA, etc.)?
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