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Brightness of lamp connected to transformer

  1. Jan 29, 2015 #1
    1. The problem statement, all variables and given/known data
    A lamp is connected to the secondary part of an ideal transformer. What will happen to the brightness of the lamp if the number of turns in secondary coil is increased?

    2. Relevant equations
    Vs / Vp = Ns / Np = Ip / Is

    P = V.I = I2R = V2/R

    3. The attempt at a solution
    I have three different cases:
    a. Because this is an ideal transformer, the power at primary will be the same as at secondary so the power delivered to the lamp will stay constant and the brightness will stay the same

    b. If the number of turns in secondary coil is increased, the voltage at secondary will also increase. It means, the power delivered to the lamp will also increase (P = V2 / R) and the brightness will increase

    c. If the number of turns in secondary coil is increased, the current at secondary will decrease. It means, the power delivered to the lamp will also decrease (P = I2 R) and the brightness will decrease


    Which one is correct? And what factors affect the brightness of a lamp? Is it voltage only, or current only, or power only, or combination of them?

    Thanks
     
  2. jcsd
  3. Jan 29, 2015 #2

    phinds

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    This is extremely basic. Do you not have any idea? What is your level of education and you knowledge of electronics? Do you understand the relationship among current, voltage, and power?
     
  4. Jan 29, 2015 #3

    haruspex

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    You need to consider what is sure to remain constant and what is sure to change in a known way.
    Two of your three arguments assume constant something that will in fact change.
     
  5. Jan 29, 2015 #4
    I am in grade 11. I can't say for sure about my knowledge of electronics.

    I have written down the relation: P = V.I
    I don't have idea what the basic factor that affects the lamp brightness. I did several calculation problems and in those problems I encountered different cases; the brightness will increase when the value of emf is increased, the brightness will decrease when more loads are connected in series because the current drawn is less.

    But, for this case, there are several factors that change in a time, the voltage increases and current decreases. I don't know how to analyze it.

    Thanks
     
  6. Jan 29, 2015 #5

    haruspex

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    I'll make it a bit easier. Rank the following from most certain to least certain regarding what will happen when turns are added:
    1. The power will stay the same
    2. The lamp resistance will stay the same.
    3. The current on the primary side (not the lamp side) will stay the same
    4. The voltage on the primary side will stay the same
     
  7. Jan 29, 2015 #6
    By "the turns", it means secondary turns, right? If so, then:
    1
    4
    3
    2

    I am not so sure. I think they have same certainty to happen, but that's my best guess
     
  8. Jan 29, 2015 #7

    haruspex

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    You think the lamp resistance (2) is not that likely to stay the same?! Why should that change? Or have you ordered them from least certain to most certain?
     
  9. Jan 30, 2015 #8
    No. I ordered them from the most certain to least certain. Maybe it is more accurate if I stated that I ordered them from the factor I thought related stronger to my question.

    That's why I started from power, then to voltage (because transformer works based on induced emf), then current and the last is resistance. If I didn't have to order them, I would say that they would all happen with the same certainty. The factors that would change, in my opinion, are only the voltage and current of the lamp side (secondary coil).
     
  10. Jan 30, 2015 #9

    haruspex

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    In case this is what's confusing you, let's say we focus on the immediate consequences of the extra turns on the secondary. So there won't be time for the lamp to have changed temperature much. Do you now understand that you can assume the lamp resistance does not change, regardless of how the voltage and current change? It is an inherent property of the lamp.
     
  11. Jan 30, 2015 #10
    Yes, I know the resistance of the lamp will be constant. If we consider the heating effect, the resistance of the lamp will increase. But for this case, we neglect it so the resistance won't change.

    So, the most certain will be the resistance of lamp will stay the same. Then the voltage will stay the same because the number of turns only changed for the secondary part. The current will also stay the same and because power is the product of voltage and current, it will be constant.

    So the order maybe 2, 4, 3,1. But, I still don't get where your hint is leading to. I mean, the relation between the brightness of the lamp and all the factors you mention (voltage, current and power on primary coil)
     
  12. Jan 30, 2015 #11

    phinds

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    Clearly you do not even understand the basics of what transformers are FOR and what they do. You should study up at least a little bit on transformers before you go any further in trying to answer this problem.

    EDIT: I am not trying to give you a hard time on this. This forum is a great place to get help but I don't think that giving you an answer to this specific problem is going to be particularly helpful to you. What you should do first is figure out the answers to much more important fundamental questions

    What does a transformer DO. What do we even have them? What good are they? How do they work?

    Once you answer those then this particular problem becomes trivial and so will many other such problems with transformers.

    In other words, your question sounds to me like this "hey guys, I don't have a clue what transformers are or how they work or why they even exist, but I have this problem that I have to solve regarding transformers and I'd like your help solving it." Can you see how that sounds like you are addressing the wrong issue?
     
    Last edited: Jan 30, 2015
  13. Jan 30, 2015 #12
    I mean there is no change on the primary voltage because we only increased the number of turns on secondary coil, so only secondary voltage will increase. Are you implying my opinion is wrong? If I am wrong, then you are right, I really have no clue about transformer
     
  14. Jan 30, 2015 #13
    Haruspex:
    when you say I assume "something" is constant when that "something" will in fact change, are you referring to my first and third arguments? And that "something" is power, am I right?
     
  15. Jan 30, 2015 #14

    phinds

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    My bad. Sorry. I read it as change on the secondary voltage.
     
  16. Jan 30, 2015 #15

    haruspex

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    Good, we're getting somewhere.
    So take the first two: resistance is constant and primary side voltage is constant.
    If the secondary turns are increased, what will happen to secondary voltage?
    Given that the resistance is constant, what happens to the power?
     
  17. Jan 31, 2015 #16
    This is my newest thought process:
    1. Because the number of turns in secondary coil is increased, the emf produced will be bigger since higher magnetic flux linkage is produced.
    2. The resistance of the lamp is constant. Therefore, the current drawn by the lamp on secondary coil will be bigger since the lamp experiences higher potential difference on secondary coil
    3. This means there is increase in both apotential difference and current on secondary coil, resulting increase in power on secondary coil
    4. Because of ideal transformer, the power in primary coil should be equal to secondary coil. The primary coil will produced higher power
    5. Since the primary voltage is fixed at certain value, in order to produce higher power, the current on primary coil will increase.

    Am I correct?
     
  18. Jan 31, 2015 #17

    haruspex

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    Bingo.
     
  19. Jan 31, 2015 #18
    Thank you very much for all the help here
     
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