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Electricity and Electric Field Questions

  1. Jan 24, 2013 #1
    I just need some help and verification on some of these questions from my physics textbook, this chapter also covers electroscope.

    1) If you charge a pocket comb with a silk, how can you determine if the comb is positively or negatively charged?
    The attempt at a solution: We can use an electroscope thats positively or negatively charged. To do this we can charge the electroscope conductively or inductively. After that say the electroscope is negative and we have it make contact with the comb, if it closes we know the comb is positive, if it opens we know the comb is negative.
    2) Why does a shirt taken from a cloth dryer sometimes cling to your body?
    The attempt at a solution: Because all that tumbling and rubbing inside the dryer builds up electrons on the shirt. Out body is neutral by default, and neutral object can attract either positive or negative object, thats why sometimes shirts taken from dryers cling to your body
    3) Why does a plastic ruler that that has been rubbed with a cloth have the ability to pick up small pieces of paper? why is it harder to do that on a humid day?
    The attempt at a solution: Because rubbing the cloth on the plastic builds up electrons on to the plastic ruler through fiction, The paper, which is neutral, will be attracted to the now charged plastic ruler. It's harder on humid days because all the water molecules in the air, which acted as insulators, made this process a lot harder.
    4) When an electroscope is charged the two leafs repel each other and remain at an angle. What balances the electric force of repulsion so that the leaves don't separate further?
    The attempt at a solution: the Force normal and Force gravity kept the 2 leaves at an angle, it's the even amount of protons and electrons on each leaf that kept them from separating further.
    5) The form of Coulomb's Law is very similar to Newton's Law of Universal Gravitation. What are the difference between these 2 laws? Compare also gravitational mass and electrical charge.
    The attempt at a solution: Newton's law deal with gravity and mass, Coulomb's law deal with electrical charge and electrical current. Newton's law uses universal constant and Coulomb's law uses coulomb's constant, both have different values.
    6) When a charged ruler attracts small pieces of paper, sometimes a paper jumps quickly away after touching the ruler. Explain.
    The attempt at a solution: Because even though the electrons on the neutral paper shifts to attract to the charged ruler, sometimes the opposite charge gets too close to each other, which repels, making the paper jump back down.
    7) When determining an electric field, must we use a positive test charge, or would a negative one do as well? explain.
    The attempt at a solution: I don't know if I got it right, but my guess is no, because a negative test charge, or electrons, have tendency to move, so even though the reaction will be the complete opposite of positive test charge, it will move if attracted or repeled, which makes it hard to determine an electric field.
    8) Why can line of force never cross?
    The attempt at a solution: Because every line repelled from or attracted to the test charges is always perpendicular to the test charge.

    thanks for helping!
  2. jcsd
  3. Jan 25, 2013 #2
    can someone tell me if I did any/all of these questions right?
  4. Jan 25, 2013 #3


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    Hello Mrclean214. Welcome to PF !

    (Please be patient when waiting for a reply. The rules of this Forum state that you should wait 24 hours before "bumping" your new thread.)
    That's a pretty long list to reply to.

    #1. Your answer is good. One question: How do you know the electroscope is negatively charged?

    In #2 & 3 you mention that a charged object will attract t a neutral object. You might want to explain why that is so. Otherwise #2 is good.

    #3. Why would the fact that water molecules are insulators cause it to be harder to charge a ruler? The ruler is also an insulator.

    #4. It's OK until you say that there are an "even amount of protons and electrons" on the leaves of the electroscope.

    #5. Coulomb's Law deals with electric charge and electric force not current. There's one other big difference. (Is gravitational force ever repulsive?)

    #6. Incorrect. What happens when the paper touches the comb? Does the paper stay neutral?

    #7. How is the electric field determined when using s test charge with a charge of q ?

    #8. What does a line of force represent? How would that work at a location where two lines of force were to cross ?
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