Static Electricity (pg. 23-25)

• A static charge is a charge which builds up in one place and is not free to move.
• Static charge is more common on insulators, where current cannot flow, rather than on conductors.
• A common cause of static electricity is friction.
• When two insulating materials are rubbed together, electrons will be scraped off one and dumped on the other. This will leave a positive electrostatic charge on one and a negative electrostatic charge on the other.
• Positive and negative electrostatic charges are only ever produced by the movement of electrons. The positive charge does not move.
• A positive static charge is always caused by electrons moving away elsewhere.
• Cars often get a static charge on the outside because they've gained or lost electrons from the air rushing past them as they travel at high speeds.
• A charged conductor can be discharged safely by connecting it to the earth with a metal strap. The electrons flow down the strap to the ground if the charge is negative and flow up the strap from the ground if the charge is positive.
• As electric charge builds on an isolated object, the voltage between the object and the earth (which is at zero volts) increases.
• If the voltage of the charged object gets large enough, electrons can jump across the gap between the object and the earth - this is the spark.
• If the voltage of the charged object gets large enough, electrons can jump to any earthed conductor that is nearby, causing static shocks. This usually happens when the gap is fairly small.
• Investigating static electricity:

  1. Rub polythene and acetate rods with a cloth duster.
  2. Electrons move from the duster to the rod.
  3. The duster becomes negatively charged and the rod is left with an equal positive charge.
• Gold-leaf electroscope:

  1. When a rod with a charge is brought near the disc of the electroscope, electrons will either be attracted to, or repelled from, the metal disc.
  2. This induces a charge in the metal disc, which induces a charge in the gold leaves. Both gold leaves will have the same charge, so they will repel each other, causing them to rise.
  3. When the rod is taken away, the gold leaves will discharge and fall again.
  4. If the foil does not rise when the rod is brought near the disc, the rod is not charged.
• Suspending a charged rod:

  1. Suspend a rod with a known charge on a thread and see if there is repulsion or attraction when the rod you're testing is brought close to it.
  2. If there in attraction, then the test rod has the opposite charge to the suspended rod.
  3. If there is a repulsion, the test rod has the same charge as the suspended rod.
• A Van de Graff generator is made up of a rubber belt moving round plastic rollers underneath a metal dome. An electrostatic charge is built up on the metal dome as the belt goes round.
• If you stand on an insulated chair and place your hands on the dome of a Van der Graff generator, electrons will move between your body and the dome, giving your body a charge. The human body conducts charge, and like charge repel, so the charge will spread out as much as possible throughout your body.
• Inkjet printer:

  1. Tiny droplets of ink are forced out of a fine nozzle, making them electrically charged.
  2. The droplets are deflected as they pass between two metal plates. A voltage is applied to the plates - one is negative and the other is positive.
  3. The droplets are attracted to the plate of the opposite charge and repelled from the plate with the same charge.
  4. The size and direction of the voltage across each plate changes so each droplet is deflected to hit a different place on the paper.