Pre-­‐ Lab for Faraday Ice Pail and Electric Charge An electron has a charge or -1.6 x 10-19 C while a proton has a charge of +1.6 x 10-19 C.
Since these charges are equal in magnitude but opposite in sign, the net charge of an atom
is zero. The positive charge of the atom is inside the nucleus and is fixed so objects
develop a positive charge by removing electrons from the object while objects develop a
negative charge by adding electrons to the object.
Questions:
1. A hard rubber rod is charged negative by rubbing it with a cloth. Then the rubber rod
is brought close to by not touching two conducting spheres that are touching each other
as shown below. The two conducting spheres are separated while the rubber rod is still
present. What will be the type charge on each sphere?
2. An electron is traveling with velocity v between two plates that are oppositely charged
as shown in the diagram below. In which direction will the electron deflect? A proton?
+ + + + + + + + + + + + + + + + ++ e -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ 16 3. An uncharged metal sphere hangs from a nylon thread. When a positive charged glass
rod is brought close to the metal sphere, the sphere is drawn toward the rod. But if the
sphere touches the rod, it suddenly flies away from the rod. Explain why the sphere is
fist attracted and then repelled.
4. You have a pure gold ring with a mass of 17.7 g. Gold has an atomic mass of 197
g/mole and atomic number of 79. How many protons are in the ring, and what is their
total positive charge?
17 Faraday Ice Pail and Electric Charge Theory
The Faraday Ice Pail works on the principle that any charge placed inside a conducting
surface will induce an equal charge on the outside of the surface. It is an excellent
method for sampling charges and charge distributions. The version used in this lab
consists of an outer mesh cylinder that serves as a shield and an inside mesh cylinder that
serves as the actual pail. The shield when grounded helps to eliminate stray charges and
AC fields. When a charged object is placed inside the pail, but without touching it, a
charge of the same magnitude is induced on the outside of the pail. An electrometer
connected between the pail and the shield will detect a potential difference. The greater
the charge, the greater the potential difference will be. So even though the electrometer
will give readings of voltage, it is possible to use those values as relative charge
measurements.
Fig. 1 Procedure:
1. Connect the electrometer input lead (red alligator clip) to the ice pail to create a
system for measuring charge. The electrometer ground lead (black alligator clip)
attaches to the shield. Connect a third wire from the ground connection of the
electrometer to a close-by ground to provide an extra ground. The electrometer has
several settings that adjust the full-scale readings. Adjust the setting so that most
readings appear in the upper 2/3 of the scale. To zero the scale there is a button on the
electrometer. However, you may need to ground the system by touching the inner pail
and the outer pail at the same time with the finger of one hand. Tip the finger so it
loses contact with the inner pail before if loses contact with the outer shield. Make
18 sure the electrometer is reading zero before proceeding.
2. The charge producers consist of two wands, one with a blue and one with white
material attached to a conductive disk. Briskly rub the blue and white surfaces of the
two-charge producer together. The disk with the white surface will acquire a positive
charge; the disk with the blue surface will acquire a negative charge. Insert one of the
wands into the ice pail but do not touch the pail. Record which wand is positive or
negative?
3. Try rubbing other materials together and determine the nature of the charge. For
example, try rubbing fur and a rubber rod. Place the rod in the Faraday’s ice pail to
determine the nature of the charge. Repeat for silk cloth and a glass rod. Record the
sign of each in the data table.
4. There are some additional materials provided. There is clear acrylic tube and tan PVC
tube and three types of material (fur, felt, and silk) to rub the tube. Record the nature
of the charge for all the possible cases.
5. Tear up some paper into a few tiny bits. Rub the rubber rod with fur and bring the rod
close to the paper bits. Record what happens in the data table. Repeat with a glass rod
rubbed with the silk cloth. Explain what happens.
6. Tear up some aluminum foil into tiny bits and repeat the observations done in
procedure 4.
7. A small stand that has a needlepoint and a small stand balance on the needlepoint.
Rub the black rubber rod with the fur cloth and balance the rod on the stand. Now rub
the glass rod with silk and bring it close to the end of the glass rod but not touching. Is
the force attractive or repulsive? Record your observations on the data sheet. Repeat
for other rods and materials.
19 Data
1. Observation of different charged object on the Faraday Ice Pail. (Procedure 2 , 3 and
4)
Material
Nature of charge (Positive or Negative)
White Wand
Blue Wand
Black Rubber rod with fur
Solid Glass rod with silk cloth
Tan PVC tube with fur
Tan PVC tube with felt
Tan PVC tube with silk
Clear Acrylic tube with fur
Clear Acrylic tube with felt
Clear Acrylic tube with silk
2. Observations of a charged rod and paper bits. (Procedure 5)
Material
Rubber Rod with Fur
Observation of Paper Bits
Glass Rod with Silk
3. Observations of charged rod and aluminum foil bits. (Procedure 6)
Material
Rubber Rod with Fur
Observation of Aluminum Bits
Glass Rod with Silk
20 4. Observations of charged black rubber rod balanced on the stand. (Procedure 6)
Material
Glass Rod with silk
Observations
Clear Acrylic tube with fur
Clear Acrylic tube with felt
Clear Acrylic tube with silk
Tan PVC tube with fur
Tan PVC tube with felt
Tan PVC tube with silk
21