1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electrostatic homework questions

  1. Aug 2, 2010 #1
    1. The problem statement, all variables and given/known data
    1. a. When a negatively charged object brought close to negatively charged electroscope, the leaf rises more
    b. When a positive object is brought close to a positively charged electroscope, the leaf becomes closer.

    Explain the above phenomena

    2. Which of the following can attract a stream of water?
    (i) a positively charged conductor
    (ii) a positively charged insulator
    (iii) a neutral conductor

    a. 1 and 2
    b. 1 and 3
    c. 2 and 3
    d. 1,2, and 3

    3. I read there are 3 ways to charge an object. I know induction and conduction. What is the last one? Electromagnetic induction will do but I haven't covered that yet. Is there another method which is simpler?

    4. Which of the following figures shows a current of electrons?

    2. Relevant equations

    3. The attempt at a solution
    1. I understand part (a), the electrons repel each other more. But what about (b)? In my opinion, the leaf also rises more because the protons on outside object repel protons on electroscope. Why can the leaf become closer?

    2. I am guessing the answer is (d). A positive conductor or isolator can attract the negative charge of water (maybe the negative ion) and neutral conductor can also do it. Not sure though

    3. Do not know

    4. I guess there is no right answer in the choices. Electrons will flow from more negative to more positive object. (a) and (c) have the same amount of charge so they are neutral compared to each other. for (b) the electrons should flow from left to right because left one is more negative. for (d) electrons should flow from right to left because the left one is more positive.
    Right or wrong?

    Thank you very much
  2. jcsd
  3. Aug 9, 2010 #2
    Re: electrostatic

    can someone help me?
  4. Aug 10, 2010 #3
    Re: electrostatic

    have not gotten help yet
  5. Aug 11, 2010 #4


    User Avatar
    Homework Helper

    Re: electrostatic

    Are you sure that electroscope was not still negatively charged in the second case?

    Your answer is correct, but the explanation is a bit foggy. The phenomenon is connected to electrostatic induction.
    The rod of the electroscope and the leaf are of metal, a conductor, in which the negative electrons can move freely.

    If the electroscope is negatively charged, it has excess electrons. If a negatively charged object is brought close to the terminal of the electroscope, it will repel the electrons in the terminal, so they move inside. As a result, the leaf will have more electrons as before, so it opens. A positively charged object would attract electrons, so they would move towards the terminal, leaving the leaf less negative, so it closes. But a positive object will open the leaf of a positively charded electroscope.

    A neutral body has equal amount of positive and negative charges. A charge d body has some excess charge.
    You can pick up light pieces op paper with a rubbed comb. The excess charge of the comb attracts the opposite charges in the neutral body and repels the like ones. The opposite charges get closer, where the attractive force is higher. The opposite charges got farther so they experience less force. So the attractive forces are higher than the repulsive ones, the neutral body will move towards the charged one.
    But nothing happens if the comb has no excess charge.

    The stream of water is neutral,has equal amount of positive and negative charges Its molecules are dipoles - the hydrogen atoms carry positive charge, the oxygen atom carries negative charge. The positively charged object brought close to the stream of water will make the molecules turn in such way that the oxygen atoms get closer to the object,and the hydrogen atoms will be farther. So the oxygen atoms experience higher attractive force than the repulsive force on the hydrogen atoms, all molecules experience a net attraction.

    So what is the correct answer?

    It is electrostatic induction. The charges in a neutral object rearrange inside it by the effect of a charged object, so as the opposite charges move towards the object, the like ones away from it.

    Conduction means a flow of charges from one place to other.

    It is right, as the flow of electrons was the question.
  6. Aug 11, 2010 #5
    Re: electrostatic

    I just checked the question again and it is true the electroscope is positively charged in the 2nd case. So I think we are agree that the leaf will open more in 2nd case and there is something wrong with the question :smile:

    The correct answer is (a) 1 and 2 because neutral conductor will not cause attraction or repulsion to the water molecule.

    Sorry for not stating it clearly. I know 2 of the 3 ways, which are conduction and induction. By 'induction' I mean electrostatic induction, just as you explained above.

    What is the last way to charge an object (besides conduction, electrostatic induction, and electromagnetic induction)?

    I am so happy to have a reply. Thank you very much for the reply
  7. Aug 11, 2010 #6


    User Avatar
    Homework Helper

    Re: electrostatic

    You can not charge an object by electrostatic induction, because it means only rearranging the charges. BUT: if you have two pieces in contact, and put a charged body close, and take the pieces apart, both of them become charged.
    If you mean the induced voltage or current on the effect of changing magnetic flux, this will not result in excess charge.
    But you can make a body charged by rubbing it. Or you can put a metal into a dilute acid or salt solution and it gets excess charge. That is as batteries work. When two different metals are brought into contact, some electrons will go from one to the other. Or you can produce excess charge with light in solar cells (photoelectric effect).... and there can be some more ways.

  8. Aug 11, 2010 #7
    Re: electrostatic

    Is rubbing can be said the same as conduction? Or are they different because conduction is just touching and rubbing involves more physical contact than just touching the objects?

    thank you for the reply
  9. Aug 11, 2010 #8


    User Avatar
    Homework Helper

    Re: electrostatic

    No, rubbing is not the same as conduction. Electric conduction is a flow of charge through a transmitting medium (conductor) under the effect of an electric field. Charge can flow from one piece of material to an other one if they are in electric contact. Rubbing and other contact phenomena between two pieces of different media means charge transfer confined basically at the interface.

  10. Aug 11, 2010 #9
    Re: electrostatic

    OK. Thank you very much for your time, passion, and explanation.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook