Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Physics 30 - Static Electricity

  1. Apr 24, 2008 #1
    [SOLVED] Physics 30 - Static Electricity

    Imagine that you have an object with an unknown electrostatic charge. If you are given a rubber rod, fur, and a neutral electroscope, describe how you could find the type of charge on the object.
    My answer: By rubbing the rod on the fur, the fur will deposit some of its electrons onto the rod. The rod would then be negatively charged. If the object is attracted to the negatively charged rod, we can infer it is positive. If the object is repelled, it can be assumed the object is also negatively charged. So what I'm wondering is, where does the neutral electroscope come into hand? Did I do this question entirely wrong? Miss a step? Or did they just throw that in to mess me up?

    2. Two objects are separated by a distance of 2.00 m where q1= -3.00 C and q2 = 4.00 C.
    A) Calculate the magnitude of the electrostatic force between the two charges.

    I used F=((Kq1q2)/r2) and got an answer of -2.70 x 10^10. Is that the right way to do this question/does that seem like the right answer? And how do I know if it's attracting or repelling?

    B)Imagine that the two identical objects are brought to the middle, touched to each other, and then returned to their original positions. Determine the new charge on each object. Concisely explain your answer. Use your answer to calculate the new electrostatic force between the objects.

    I'm going to be honest here, I'm totally lost...

    3. A charge of -4.27 x 10^-8 C experiences an electrostatic force of repulsion of 7.91 x 10^-4 N, north.
    A) Calculate the magnitude of the electric field at the point in space occupied by the charge.

    I used E=F/q and got -1.85 x 10^4 N/C. Does that seem right?
    B) Determine the direction of the electric field. Explain your answer.
    Again, I don't know how to figure this out.

    A lightning bolt can deliver up to 20 C of charge to Earth.
    A) Calculate the number of electrons transferred from the cloud to the ground in this discharge.

    I went 20.0 C/1.60 x 10^-19 C and got 1.25 x 10 ^20 electrons. I'm not sure if that's the correct procedure though.
    B)Explain why a bolt of lightning looks jagged.
    I wrote: A bolt of lightning will take a jagged path because the negative charges are attracted to the pockets of positive charge which are randomly dispursed throughout the atmosphere.

    Thanks in advance for any help. It's greatly appreciated.
  2. jcsd
  3. Apr 24, 2008 #2
    1. the object and the rod are likely too heavy and the charge too small to directly measure the attraction.

    2a The value of F is OK. you know opposite charges attract?
    2b if object 1 has 3 coulombs of electrons extra and object 2 lacks 4 coulomb of electrons, what will happen if you bring them together? (I assume the objects are conducting)

    3a for the magnitude, you want a positive number.
    3b the direction is the magnetic field is the direction of the force on a positive charge

    4a Ok
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook