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Earth wire position and charging by induction

  1. Nov 26, 2011 #1
    Not sure if this should go here, but it's from a physics textbook.
    I was reading about charge by induction and the use of an earth to achieve this.
    Here's a wonderfully detailed and exquisite image that took many laborious hours to construct. The red and black earths are not intended to be present at the same time, but represent to different to scenarios:
    So the book says that the earth is both a source and a sink of electrons, and can be used to charge the object on the left. It says that the object will acquire a negative charge exactly opposite to that of the other charged object. It will acquire this charge from electrons in the earth. The electrons flow in to the object and the earth wire is removed.
    However, since the earth is supposed to act as both a source and a sink, how does it know to input electrons, and not take them away? If the eart wire was where the positive charge has been induced (the black eart wire), then okay. If the earth wire is placed where the electrons have migrated to (the red earth wire), to be in close proximity to the positively charged object, do they not then migrate into the earth - with the earth acting as a sink?
    I am guessing not because the eart does not ACTIVELY act as an electron acceptor or donor, and only responds to demands placed on it: it's not like a charged object that promotes electrons to move. As such, the accumulation of negative charge near to the positively charged object remains there and is not taken into the ground through the red earth wire because the positive charge holds them in place, and the earth does not 'compete' for the electrons. Or can a negative charge or a positive charge be introduced based on the location of the earth wire, with a positive charge being induced in the object through the red earth wire allowing electrons to be transferred to the earth?
    I don't think any of this makes sense...
    Many thanks.
  2. jcsd
  3. Nov 26, 2011 #2
    I have read your whole post. I think what I mean is correct yet precise.
    Your text book is correct. Earth can be regarded of sea of electrons and protons and hence is potentially neutral. It is sink of electrons as well as source of electrons.

    Now let's move to scenario regarding concept of induction. You bring a positive charged body near a neutral body (both bodies must be good conductor of static electricity). Now this positive charged body attracts electrons from extreme end of neutral body. Thus electrons are attracted and the rightmost end of neutral body having excess of electrons acquires negative charge. Thus the opposite leftmost part acquires positive charge.

    Connect both the ends to the earth wire. Since the electrons in right end are held by strong force they cannot go to earth , thus only electrons can flow from earth to the left end and equalize the positive charge.

    And yes , you say how can earth only give electrons and not take them in this case ? Unless you apply any "electromotive force" , electrons will flow from more to less due to net electrostatic attraction.

    First attachment is a sequential image of yours and second is its consecutive one when you remove earth wires.

    Attached Files:

    Last edited: Nov 26, 2011
  4. Nov 26, 2011 #3
    I agree with Sankalpmittal explanation
  5. Nov 28, 2011 #4
    Thanks Sankalpmittal and technician.
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