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Homework Help: Please help someone! Charge and current questions nightmare!

  1. Sep 19, 2011 #1
    Please help someone! Charge and current questions!! nightmare!

    Hi! I'm new here and joined this forum for any future help :) So i just started Physics A level and i have the worst teacher ever who is so unhelpful..... :( I don't know what to do.... He set us 2 questions for homework and it looks nothing like what we did in class... I even asked for help in the end of the lesson and he didn't even get it himself!!

    Someone please help me... I have tried but i end up with funny answers that don't look very convincing :(

    Help would be appreciated... Could someone please write down the method and why? THANK-YOU SO MUCH!!! really...couldn't be anymore grateful Emilia x

    The beam current in a television tube is 32mA.The electrons travel at a speed of 4.2x10₁₇ through distance of 21cm. (sorry about the powers....they seem to be below..)
    1. Number of electrons striking the screen in 1 hour is : ( p stands for power..by the way )
    a)2.0x10 p 17
    b)7.2x10 p 20
    c)1.2x10 p22
    d)7.2x10 p23

    1b) The number of electrons on the beam at any instant
    a)1.0x10 p9
    b)1.0x10 p11
    c)1.0x10 p12
    d)1.0x10 p14

    2. State what physical quantity is represented by each term in the equation:


    b) A semiconducting strip 6.0mm wide and 0.5mm thick carries a current of 10mA.

    If the value of n for the semiconducting material is 7.0x10 p22m₋₃, show that the drift speed of the charge carriers, which carry a charge of 1.6x10 p -19 C, is about 0.3ms₋₁.

    c) The drift speed for electrons in a copper strip of the same dimensions and carrying the same current would be about 10 p -7₇ ms₋₁
    . Use the equation to explain why this value is very different from that of the semiconductor.

    Thank you so much everyone. I really don’t want anyone to get the wrong impression of me being lazy and not doing it. Because I do find it very hard.... and I have tried as much of it as i could.... Also just...this Physics teacher...he’s urgh!!! I wouldn’t be asking if have i known what to do.... maybe just to see if i did it right... but... I really would like some help. Thank-you again!! Emilia x
  2. jcsd
  3. Sep 19, 2011 #2


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    Homework Helper

    Re: Please help someone! Charge and current questions!! nightmare!

    You will need to clarify this part for a start. The speed of light is 3.0 · 1010 cm/sec, so the electrons cannot be moving faster than that. Is the numerical value you've shown typed correctly and is it in cm/sec?
  4. Sep 19, 2011 #3
    Re: Please help someone! Charge and current questions!! nightmare!

    Hi there :) I know right! It says 4.2x10 to the power of 7 ms-1 in the text book. i don't get it ...:(
  5. Sep 19, 2011 #4


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    Re: Please help someone! Charge and current questions!! nightmare!

    Let's start with this problem. So the speed of the electron's is v = 4.2 · 107 m/sec or 4.2 · 109 cm/sec . How long does it take a single electron to travel the 21 cm. from the emitter to the screen?

    Also, the current of electrons in the beam is given as I = 32 mA . How much charge is flowing in the beam past a specific location each second? (Remember that Amperes are Coulombs per second.) How many electrons does this mean are passing that point in one second?
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