Charge It!
Cage: Shields the pail
from outside charges
(like nearby hands)
Pail: THIS IS KEY! A charged object inside but not
touching the pail induces the opposite charge on the
inner wall and the same charge on the outer wall;
which can be measured by the charge sensor.
To Charge
Sensor
Ground Lead: Keeps zero
charge on cage.
Ground Plane:
Serves as a zero
charge reference.
Base: Insulates
cage & pail from
ground plane
Charge It!
EXERCISE 1: Simple charge measurement
The “wands” with plastic disks at the end are called
“charge separators”. When rubbed together, one becomes
positive (white nylon) and the other negative (gray PVC).
Basic procedure:
Connect the pail, cage and ground plane as shown
previously. Don the wrist strap and connect it to the ground
plane. Press “Reset”, which discharges the sensor and the
pail.
Rub the separators together. Start data collection and
insert one disk into the pail (without touching) and hold for
a couple seconds. Remove the separator.
Charge It!
Basic procedure (cont.):
Repeat the sequence with the other charge separator, and
then with both separators at the same time.
Examine the data graph and record the following: (1)
charge of first separator, (2) charge with no separator, (3)
charge with second separator, and (4) charge with both
separators.
Comment on whether the measurements agree with your
expectations. You may need to practice a few times so you
can do all three parts in sequence smoothly!
Remove any remaining charge from the separators by
moistening the small cotton cloth and touching it to the
separator surfaces.
Charge It!
EXERCISE 2: Triboelectric Table
You will now try to figure out what materials are best and
worst at transferring charge by friction (triboelectricity). Use
the same procedure as in Exercise 1, except use the
different cloths and rods as charge separators.
4 insulating rods: clear acrylic, hollow polyvinyl chloride,
white nylon and maple wood
4 cloths: blue silk, gray wool, patterned cotton and
polyester felt
Record all your measurements. Then list the rods from
strongest negative to strongest positive. Do the same for
It should be possible to make a combined list of rods &
the cloths.
cloths — but I haven’t figured that one out yet.
Charge It!
EXERCISE 3: Induction
We’ll probably do this in class — but in case we don’t …
Rub the disk charge separators, and insert the white one
into the pail. Record the charge. (Should be positive.)
While the separator is still in the pail, ground the pail by
touching it. Record the charge. (Should be about zero.)
Remove your finger from the pail & record the charge. (Still
zero.)
Now, remove the charge separator, and record the charge.
(Should be negative and similar magnitude to original
reading.)
Charge It!
EXERCISE 3 (cont.): Induction
Here’s what happened: When you inserted the separator originally, you
induced a charge on the pail because the separator repelled like
charges from the inner surface and pushed them to the outer surface
where they could be measured by the sensor.
When you touched the pail, negative charge from the ground plane was
attracted to the positive charge on the pail, so the net charge on the pail
is zero. When you take your finger away, the negative charges are
trapped on the pail, and still cancel the positive charge on the separator.
When the separator is removed, the positive charge induced on the pail
also vanishes, leaving behind the stranded negative charge.
DRAW A CARTOON ILLUSTRATING THIS PROCESS.
Small print: The sensor is actually measuring the voltage difference
between the cage and the pail, which is determined only by the
charge on the outside of the pail.
Charge It!
EXERCISE 4: Conduction
Rub the disk charge separators, and insert the white one
into the pail — and rub it along the inner wall, and then
remove the separator. Record the charge.
Why did the charge remain after the separator was
removed from the pail?
Check to see if the charge ACTUALLY left the separator by
grounding and resetting the pail, and re-inserting the
separator. Record the charge. Does the result indicate that
charge leaves the separator when it contacts the pail?
Repeat the procedure with the other charge separator.