MCQ related to electric current and ohms law

Discussion in 'Homework Help' started by Muhammad Kifl, Jan 14, 2016.

  1. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    hi,
    a carbon resistor has to meet the following requirements
    IR drop : 5V
    Current : 100mA
    Safety factor for power dissipation : 2
    Which one of the following resistors will be suitable
    (A)5 ohm, 10 W
    (B)0.5ohm, 100W
    (C)50ohm, 1W
    please explain the answer
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,517
    785
    This smells like homework. Moved to Homework Forum.

    Read this.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    Please show an attempt to come up with an answer.
     
  4. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    i thought the correct option is B because
    I=(100/0.5)^0.5
    I=14.14Amp
    so it satisfies the current and power rating thats why i think the correct option is B but it is not.. :(
    instead the answer given in answer sheet is C.
    i dont know how C can be the correct option as it does not satisfies the power rating.
     
    Last edited: Jan 15, 2016
  5. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    How did you get 100/0.5 and I=14.4Amp? :confused:
    The problem stated the voltage was 5V and the current was 100mA.
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,398
    497
    Power dissipated by the resistor == Voltage across resistor * Current through resistor
    P=5*0.1=0.5 Watt
    Safety factor of 2 means: 2*P=2*0.5=1 Watt
    Therefore the answer is C because all you need is the resistor that will dissipate 1 Watt, you don't need 10 Watt, you don't need 100 Watt.

    And lol for trying to use the answers to figure out the question.
     
    Muhammad Kifl likes this.
  7. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    thanks a lot.but i still cant figure out what is meant by safety factor??
     
  8. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    again go through the calculations and question bro.. :(
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    If you had bothered to track your units you would have immediately seen that your answer is wrong. Instead, you threw pure numbers at some formula and then tacked on the units that you wanted the answer to have.

    What you SHOULD have done:

    I = (V/R)^0.5 = (100 V / 0.5 Ω)^0.5 = (200 A)^0.5 = 14.14 √A

    At this point you could have noticed that √A is not amperes and therefore known that your answer had to be wrong.
     
  10. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    I=(V/R)^0.5 :eek:
    plz recheck your formula.
     
  11. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    Looking (guessing, really) again I suspect that you were trying to use

    P = I²R => I = √(P/R)

    and then using the power rating given in the answer.

    Okay, so what you found is that a 0.5 Ω resistor dissipates 100 W at a current of 14.14 A.

    So what?

    The question neither asks nor cares about that.

    The question is asking for a resistor that drops 5V at a current of 100 mA. What voltage will a 0.5 Ω resistor drop at 100 mA?
     
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  12. WBahn

    Moderator

    Mar 31, 2012
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    You didn't GIVE a formula! I'm having to GUESS what your starting point was because you couldn't be bothered to furnish it nor could you be bothered to properly supply the units of the quantities in your expression. I am NOT a mind reader and engineering is NOT about guessing!
     
  13. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    So you are picking the resistor based ONLY on the power rating matching exactly what you need? The resistance doesn't matter at all?

    What if the answers had been:

    (A) 5 ohm, 10 W
    (B) 50 ohm, 100W
    (C) 0.5 ohm, 1W
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    Safety factor means that you choose the component so that it can handle more than you expect it will actually have to.

    If you weigh 200 lb and you buy a chair that is rated for 400 lb, you have a safety factor of 2.
     
  15. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    sorry my bad.
     
  16. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    thanks a lot... :) now i can grasp it.
     
  17. Muhammad Kifl

    Thread Starter New Member

    Jan 14, 2016
    11
    0
    Sir, i used the formula P=(V^2)/R but i have now figured out that what i was doing wrong. Thanks a lot again for your help. :)
     
  18. WBahn

    Moderator

    Mar 31, 2012
    17,777
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    You're welcome. Good luck.
     
    Muhammad Kifl likes this.
  19. EM Fields

    Member

    Jun 8, 2016
    181
    30
    Since the drop across the resistor is 5 volts when there's 100 milliamperes through it then, from Ohm's law, its resistance must be R = E/I =5V/0.1A = 50 ohms.

    The power it must dissipate is then P = E I = 5V X 0.1A = 0.5 watts.

    Safety factor refers to giving the load a break and not running it balls - to - the - wall, (its maximum rating) so the only answer that fits is (C).

    Looking at it another way, if you trusted the test, (which you never should) you could blow off everything but the 50 ohm choice and buy some time time for the rest of the test.
     
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