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Homework Help: Need help on Electric Field Questions

  1. Mar 6, 2008 #1
    These are a few of the harder questions from my homework that is due tomorrow. Please Help.

    1. The problem statement, all variables and given/known data

    A) How much energy is wasted when 35C of charge flows through a 120V toaster in 5 sec? How much power?

    B) An electron gun fires electrons from rest and smashes them into a screen that is .33m away. The accelerating voltage that causes this is 2850V.
    1. What is the kinetic energy?
    2. What is the velocity right before impact
    3. What is the magnitude and direction of the electric field accelerating the electron?
    4. What is the force experienced by the accelerating electron?
    5. how much time does in take for the electron to make this trip?

    C) Two opposite charged students bring their fingers towards each other and a spark jumps between their fingers at 7 cm apart. What is the electric field in the region between their fingers when the spark occurs. What is the potential difference between the two students (in voltage)?

    2. Relevant equations


    3. The attempt at a solution

    I am really confused with these questions.

    For the first one I have 840Watts and 4200J for the toaster

    I don't get the electron gun one.

    The spark one is equally confusing, we were given that the Emax in air is 3*10^6J, but I do not know if this is needed for this question.
  2. jcsd
  3. Mar 6, 2008 #2


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    Science Advisor
    Homework Helper

    You got the first one right. Though I wouldn't describe the energy as 'wasted'. Toasters are supposed to heat up. For the second one you have to try harder. Volts are joules/coulomb. How much energy does one coulomb gain passing through the potential difference? How many electrons in a coulomb? What's the energy in a single electron? Etc. For the last one I don't understand your given. What is Emax supposed to mean if it's an energy? If it's an electric field then the units are wrong.
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