1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    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!

Homework Help: Van De Graff Generator: Explanations to why object repel generator

  1. Apr 20, 2009 #1
    1. The problem statement, all variables and given/known data
    a) Why do pie pans fly off the Van de Graff Generator when turned on?

    b) When foam peanut are placed inside a pie pan with the Generator turned on, what causes the foam peanuts to fly out?

    c) Why does a person's hair stand up when they touch the generator?

    2. Relevant equations
    No equations necessary

    3. The attempt at a solution
    I'm not sure if my answers are correct or if I'm elaborating enough. Like should I talk about conductors/insulators...charging by conduction/induction.

    a) Since the van de graff generator's dome is positively charged and the the pie pans tend to be positively charged, then the pans fly off because like charges repel.

    b) The foam peanuts are insulators. The van de graff generator transfers charge to the pan, leaving it positively charged. This is charging by conduction. The pan transfers the charge to the foam peanuts. The foam peanuts fly off the pan because they are both positively charged and so the peanuts repel the pan.

    c) Hair stands up when a person touches the van de graff generator because the body is a conductor. Postive charge moves freely through the body into the hair making it stand up.
  2. jcsd
  3. Nov 29, 2010 #2
    Yuo are quite right ...
    van de graff Generator works on the principle: " when some insulating material is seperated from others leave those surfaces electrically charged, each with the opposite sign of charge and with a high potential difference (p.d.).

    So, at the bottom, the comb takes a High voltage negative charge viz converted by conduction into positive charge on the belt..

    This belt carries the positive charge on the dome(sphere) and hence having inherent electrical capacity (proportional to its radius) will collect and store the charge on its outer surface until discharged either by breakdown of the surrounding air as a spark, or by conduction to an adjacent earthed object.

    And as your experimental examples: Faraday’s cylinder to show electric charge resides on the outer surface of a charged hollow conductor.

    Bouncing ball Suspend a conducting ball a non-conducting thread. When the ball touches the charging sphere, it will become charged and be repelled away from the sphere. If the ball is then allowed to discharge (touching an earthed surface, or leaking charge to the air) it will be attracted once more to the sphere, to be recharged ... and so the process continues.

    The head of hair is another demonstration of repulsion. Real hair or shredded paper strips bunched at one end are used and provide a sensitive means of detecting charge.

    The electric wind is produced by release of ions at the end of a pointed conductor and is enough to deflect a candle’s flame.

    Hamilton’s mill utilizes the electric wind at the pointed ends of four arms to cause rotation about a pivot. This is similar to the action of a lightning conductor, which allows charge transfer at sharp points.

    Kinetic theory model You can show random motion of metallic balls continuously affected by repulsion and loss of charge within a transparent vessel.

    Neon indicator shows luminous discharge from the gaseous excitation by the high electric fields near the generator.
    i hope this helps ..:-)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook