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Homework Help: Charge, Current & Power

  1. Apr 2, 2015 #1
    1. The problem statement, all variables and given/known data
    Can someone please confirm if I answered correctly?

    Question: According to rumours the next generation smart phone will be powered by a 3.82V battery that can deliver 1.81A of current for one hour. The phone automatically turns off when the battery has less than 2% of charge remaining.

    i) Calculate the amount of charge that can be delivered by the battery.
    ii) How much charge has passed before the phone loses power?
    iii) Show that if a charge Q is transferred across a p.d V, the energy consumed is equal to QV.
    iv) In an idle state the phone uses a constant 70mW of power. How many days will pass before the phone loses power completely?
    v) The majority of components inside a smart phone are made up of semiconducting material. Briefly explain how the electrical conductivity of an intrinsic semiconductor increases as the temperature rises.
    vi) Explain the origin of holes in intrinsic semiconducting materials and explain the process by which holes participate in current flow.

    2. Relevant equations

    3. The attempt at a solution
    i) Solution: I=Q/T Q=IT Q=(1.81*60*60)= 6516C

    ii) 100%-2% = 98%
    6516*0.98= 6385.68C

    iii) P=IV = (1.81*3.82) = 6.9142W
    Calculating Charge at 1 second:
    I=Q/T Q=IT (1.81*1) = 1.81C
    Therefore P=QV = (1.81*3.82) = 6.9142W

    iv) P=IV I=P/V I=70mW/3.82V = 0.0183A
    Therefore charge every second = 0.0183C
    Total time = 6516C/0.0183C = 356065.6s
    356065.6/3600 = 98.9 hours.

    v) Upon temperature rise, valence electrons gain energy and break free becoming free electrons. The more free electrons the greater the conductivity. The semiconductor is known as negative temperature coefficient. Temperature rises and resistance decreases. Since resistance decreases so does the resistivity. Conductivity is the reciprocal of resistivity therefore conductivity will increase.

    vi) When the atom has absence of electrons it means it has a hole. Therefore the atom would lack any free electrons, this would will result in no current flow making it an insulator.
  2. jcsd
  3. Apr 2, 2015 #2


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    Homework Helper
    Gold Member

    (iii) the energy consumed is E=VIt assuming that the current is constant. By replacing I=Q/t you have E=QV.

    (vi) I think you understood it wrong, holes behave as current carriers like the free electrons. A P-type semiconductor has holes as the current carriers while a N-type has free electrons .
  4. Apr 2, 2015 #3
    Oh my bad, yes you are right. Holes travel the opposite direction from a higher potential to a lower potential. They too help with conductivity. Delta thank you for correcting my mistakes this helps me a lot. :)
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