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Physics questions watch

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    17 (a) A student uses a computer for an average of 5 hours every day. The battery supplies a current of 3.5 A to the computer. Calculate how many electrons flow through the computer’s battery in 5 hours.

    b) The computer’s screen emits visible light photons with an average frequency of
    5.5* 10^14 Hz. The power of the light emitted is 10 W.
    Calculate the number of photons emitted per second from the computer screen.


    Please can some explain their workign as well
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    (Original post by jsmith6131)
    17 (a) A student uses a computer for an average of 5 hours every day. The battery supplies a current of 3.5 A to the computer. Calculate how many electrons flow through the computer’s battery in 5 hours.
    What is the definition of current?


    b) The computer’s screen emits visible light photons with an average frequency of
    5.5* 10^14 Hz. The power of the light emitted is 10 W.
    Calculate the number of photons emitted per second from the computer screen.
    Each photon has some energy - how does it depend on frequency? Power is energy per second. How much energy is emitted each second? How does it depend on the number of photons emitted each second?
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    a) Current = rate of flow of charge (NOT NUMBER OF ELECTRONS)

    b)
    energy of each photon = 3.6 * 10^-19 J
    therefore power per photon (per second) = 3.6*10^-19

    10 Watts per second

    Total watt / watt per photon
    10 / 3.6*10^-19 = 2.78 * 10^-19

    ??
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    (Original post by jsmith6131)
    a) Current = rate of flow of charge (NOT NUMBER OF ELECTRONS)

    b)
    energy of each photon = 3.6 * 10^-19 J
    therefore power per photon (per second) = 3.6*10^-19

    10 Watts per second

    Total watt / watt per photon
    10 / 3.6*10^-19 = 2.78 * 10^-19

    ??
    If you have rate of flow of charge you can calculate the amount of charge passing in a certain amount of time, and if know the charge of an electron you can figure out how many times the charge of an electron goes into the total charge 'used'
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    Q = It
    Q = 3.5 * 5 * 60 * 60
    Q = 63000 C
    Q/1.6*10^-19 = 3.9 * 10^23
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    (Original post by jsmith6131)
    Q = It
    Q = 3.5 * 5 * 60 * 60
    Q = 63000 C
    Q/1.6*10^-19 = 3.9 * 10^23
    Correct


    (Original post by jsmith6131)
    b)
    energy of each photon = 3.6 * 10^-19 J
    therefore power per photon (per second) = 3.6*10^-19

    10 Watts per second

    Total watt / watt per photon
    10 / 3.6*10^-19 = 2.78 * 10^-19
    There is a miscalculation, but generally correct. One reservation would be that it doesn't make sense to speak about 'watts per second' (in this context). 1 watt is 1 joule per second.
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    I dont get part b at all.

    P = E/t <<< so how come we are dividing power by energy?

    somebody explain please.
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    (Original post by ANTI-GADDAFI)
    I dont get part b at all.

    P = E/t <<< so how come we are dividing power by energy?

    somebody explain please.
    The energy is that of a single photon. The power is total energy output per second. So the number of photons is power / energy.

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    The screen emits P = 10 joules per second.

    On average, you have the following:
    Energy is carried by photons of frequency f = 5.5 * 10^14 Hz.
    The energy of a single photon is hf joules.
    So photons are emitted at a rate of P / hf = 2.7 x 10^19 Hz.
 
 
 
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