Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Physics parallel circut question ( Test Exam)

  1. May 17, 2010 #1
    Physics parallel circut question !!! ( Test Exam) "URGENT"


    Here is the image
    http://img168.imageshack.us/img168/606/physexam.jpg" [Broken]

    The image is slightly blury, however still readable!

    IF YOU ARE UNSURE OF WHAT THE IMAGE READS...

    the following information relates to question 4 and 5

    Richard is conducting an experiment with a number on light emmiting diods (LEDs).
    The I-V characteristics are shown in figure 3.






    Question 4
    Richard sets up a circut involving 6 identical LEDs as shown in figure 4




    What is the value of the current through R2









    I ALSO NEED ANY OTHER ANSWERS THIS QUESTION YIELDS




    many THANKS :-)
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 17, 2010 #2
    Re: Physics parallel circut question !!! ( Test Exam) "URGENT"

    You know you can't have any potential drop across the LEDs that's more than 3V each. You won't be able to get the exact current without knowing the IV function, just an estimate.
     
  4. May 17, 2010 #3
    Re: Physics parallel circut question !!! ( Test Exam) "URGENT"

    Yeh i just need an estimate and any other answers this question has
     
  5. May 17, 2010 #4
    Re: Physics parallel circut question !!! ( Test Exam) "URGENT"

    the iv characteristics are shown in figure 3 the graph
     
  6. May 17, 2010 #5

    collinsmark

    User Avatar
    Homework Helper
    Gold Member

    Re: Physics parallel circut question !!! ( Test Exam) "URGENT"

    As Mindscrape points out, finding the current in this problem is just an estimate. But as it turns out, you can get a really, pretty good estimate, just by keeping things simple.

    Problems such as this one have real world applications. If you're ever going to breadboard LEDs to light up in a simple circuit as part of a (simple) fun project, simple calculations like these are generally all that is necessary.

    Look at the IV curve that you have attached. It's acceptable to make the following assumptions:
    (I) There's current going through each LED -- enough current such that the LED is in its normal operating range. In other words, there is at least enough current going through the LED such that it does what it was designed to do (i.e. light up).
    (II) There is not too much current going through the LED such that it overheats, blows-up, etc.

    With the above assumptions in mind, look at your attached IV curve and ask yourself,

    (1) What is the voltage drop across each LED?

    After that, analyze the circuit and ask yourself,

    (2) What is the voltage drop across R2?

    With that, you should be able to calculate the current through R2.

    As a final step, it is usually a good idea to perform a sanity check. From your above results, calculate the current flowing in any given LED. Eye-ball your attached IV curve once again, and see if the resulting voltage is fairly close to (ball-park figure) your estimate in (1).
     
  7. May 18, 2010 #6
    Re: Physics parallel circut question !!! ( Test Exam) "URGENT"

    As I said before, you know the most voltage that the LED's can drop will be 3V each for a total of 9V. Do a KVL and you will know how much voltage is dropped across the resistor, and you can then get an upper limit on the current going through the resistor. Seeing as how this was "urgent," you're probably past this question by now, but if you come back and look this will tell you how it's done. In the future, post some thoughts and work. ;)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook