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Math Help - Electricity Questions

  1. #1
    Member looi76's Avatar
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    Electricity Questions

    Question:
    (a) The current in a Led is 20mA when it has a p.d of 2V across it. Calculate the resistance.
    (b) A 240V heater takes a current of 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) I = 20 \times 10^{-3} A \ , \ V = 2V
    R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega

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

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

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

    (iii) 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 20mA when it has a p.d of 2V across it. Calculate the resistance.
    (b) A 240V heater takes a current of 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) I = 20 \times 10^{-3} A \ , \ V = 2V
    R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega

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

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

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

    (iii) 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
    Lord of certain Rings
    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 = W =  QV

    But Energy per second =  \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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