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Thread: Electricity Questions

  1. #1
    Member looi76's Avatar
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    Electricity Questions

    Question:
    (a) The current in a Led is $\displaystyle 20mA$ when it has a p.d of $\displaystyle 2V$ across it. Calculate the resistance.
    (b) A $\displaystyle 240V$ heater takes a current of $\displaystyle 4.2A$. Calculate:
    (i) The charge that passes through the heater in 3 minutes
    (ii) The heat energy produced per second by the heater
    (iii) The resistance of the heater.


    Attempt:

    (a) $\displaystyle I = 20 \times 10^{-3} A \ , \ V = 2V$
    $\displaystyle R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega$

    (b) $\displaystyle V = 240V \ , \ I = 4.2A \ , \ t = 3 \times 60 = 180s$

    (i) $\displaystyle Q = It$
    $\displaystyle Q = 4.2 \times 180$
    $\displaystyle Q = 756 C$

    (ii) $\displaystyle W = QV$
    $\displaystyle W = 756 \times 240$
    $\displaystyle W = 3.15J$

    (iii) $\displaystyle R = \frac{V}{I} = \frac{240}{4.2} = 57\Omega$

    Can you please correct my mistakes?
    Thnx in advance
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by looi76 View Post
    Question:
    (a) The current in a Led is $\displaystyle 20mA$ when it has a p.d of $\displaystyle 2V$ across it. Calculate the resistance.
    (b) A $\displaystyle 240V$ heater takes a current of $\displaystyle 4.2A$. Calculate:
    (i) The charge that passes through the heater in 3 minutes
    (ii) The heat energy produced per second by the heater
    (iii) The resistance of the heater.


    Attempt:

    (a) $\displaystyle I = 20 \times 10^{-3} A \ , \ V = 2V$
    $\displaystyle R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega$

    (b) $\displaystyle V = 240V \ , \ I = 4.2A \ , \ t = 3 \times 60 = 180s$

    (i) $\displaystyle Q = It$
    $\displaystyle Q = 4.2 \times 180$
    $\displaystyle Q = 756 C$

    (ii) $\displaystyle W = QV$
    $\displaystyle W = 756 \times 240$
    $\displaystyle W = 3.15J$

    (iii) $\displaystyle R = \frac{V}{I} = \frac{240}{4.2} = 57\Omega$

    Can you please correct my mistakes?
    Thnx in advance
    I'm not seeing any mistakes. You are saying there's something wrong somewhere?

    -Dan
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  3. #3
    Member looi76's Avatar
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    Quote Originally Posted by topsquark View Post
    I'm not seeing any mistakes. You are saying there's something wrong somewhere?

    -Dan
    I think part (b)(ii) is wrong
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  4. #4
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    Isomorphism's Avatar
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    Quote Originally Posted by looi76 View Post
    I think part (b)(ii) is wrong
    Yes it is wrong:

    Since you want heat energy per second,

    Energy = $\displaystyle W = QV$

    But Energy per second =$\displaystyle \frac{W}{t} = \frac{Q}{t}\cdot V = IV$ = ?
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  5. #5
    Forum Admin topsquark's Avatar
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    I'll just say "Oops!" about part b) and move on. Thanks for the catch Isomorphism.

    -Dan
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