Measuring Electric Charge

We can use the electroscope to measure the amount and type of charge on various objects.

For this experiment, we will use the following equipment: an electroscope, a Faraday Ice Pail, rubber, plexiglass and glass rods and silk and fur.

Name:

Lab Partners:

When entering numeric data, use exponentials: ie., 1.6 * 10-19 = 1.6E-19.

Procedure

  1. Between each reading, be sure to ground yourself and all of the equipment, and zero the electroscope.

    For each entry in the table below, charge the rod by rubbing it with the silk or fur (as indicated), and record the number of strokes used to charge it.

    Place the charged object into the center of the Ice Pail without touching the sides. The charged object will induce a charge on the pail, which will be registered on the electroscope as a voltage. Since the pail will leak charge, record the highest reading immediately after inserting the object. Include the sign of the reading.

    rubber with fur
    strokes: V = V
    rubber with silk
    strokes: V = V
    plexiglass with fur
    strokes: V = V
    plexiglass with silk
    strokes: V = V
    glass with fur
    strokes: V = V
    glass with silk
    strokes: V = V

  2. After grounding and re-zeroing, scuff your feet 3 times on the floor and insert your finger into the Ice Pail. Record the reading as
    Vfinger = V.

Analysis

  1. For each reading, compute the induced charge in Coulombs (fill in the blank with the number printed on the bottom of the Ice Pail)
    Q = * 10-12 * V
    and the number of positive charge carriers per stroke as
    N = Q / (1.6 * 10-19 * number of strokes)
    (this number can be negative: a deficit of positive charge carriers indicates that the charge carriers were negative):
    rubber with fur
    Q = C, N =
    rubber with silk
    Q = C, N =
    plexiglass with fur
    Q = C, N =
    plexiglass with silk
    Q = C, N =
    glass with fur
    Q = C, N =
    glass with silk
    Q = C, N =

    2. What determines the sign of the charge: the type of rod or the rubbing material?

    What determines the number of charge carriers per stroke?
  2. Compute
    Qfinger = C and

    Nfinger = .

    3. How was your body charged by the scuffing?

©2004, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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