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5A22.10-Braun Electroscope

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New Electroscope

Description: Demonstrates charging by induction and attraction and repulsion of charges.

Equipment:

  • Rods and Fur
  • Braun Electroscope
  • (Optional) Hair dryer to reduce humidity

Demonstration Procedure:

  1. Show that a neutral rod placed near the neutral plate does nothing.
  2. Charge a rubber rod with fur to give it a net negative charge.
  3. Bring the rod near the top plate and note the repulsion in the needle.
  4. Touch the top plate with your finger to induce a net positive charge on the electroscope. Note how the needle returns to its starting point.
  5. Remove your finger from the top plate and then move the rod away from the plate. Note how the needle returns to where it was when the rod was near the plate. Bring the rod back near the plate and note how the needle returns to its starting point.
  6. Charge an  acrylic rod with fur to give it a net positive charge.
  7. Bring the rod near the top plate and note that the needle moves even farther.

Tips:

This demo works best in low humidity. Be sure not to touch the rod to the top plate on the electroscope. When inducing the charge, make sure to remove your finger from the top plate BEFORE moving the rod away.

Concept:

When you bring the charged rubber rod near the top plate of the electroscope, polarization occurs. The negative charges in the electroscope are repelled by the negatively charged rod and move down the central metal bar of the electroscope. Since the needle is also metal and in direct contact with the central bar, all the negative charges repel one another and cause the needle to deflect. All the positive charges of the electroscope are attracted to the negative charges in the rod and so collect on the top plate. When you touch the electroscope with your finger, you give a path for the negative charges to flow to ground. This causes the entire electroscope to be positively charged now (Induction). Since the electroscope and rod are oppositely charged now, they attract one another and the positive charges collect on the top plate. Once you move the rod away from the top plate, there is no longer attraction and so the positive charges repel one another and spread out over the entire electroscope. This causes the needle to return to a deflect position. When you bring the acrylic rod near the top plate, there is repulsion since the rod and electroscope are both positively charged. So the needle will deflect even farther.