Activity
Day 1 (A possible introductory activity--a demonstration
of a Van de Graff generator)
1. Have students bring the end of the plastic tube rod
near the bits of paper. (The small bits of paper should
not attract to the rod).
2. Have students rub the plastic rod with the cloth and
bring it near the scraps of paper and observe what happens.
(The bits of paper should attract to the tube where it
was rubbed.) Introduce the term "neutral" and "charged".
We will say that something is neutral if it does not
attract tiny bits of paper. We will call something charged
if it attracts tiny bits of paper.
3. Have students bring the end of the plastic tube rod
they did not rub near the pieces of plastic. (The small
bits of paper should not attract to the rod where it
was not rubbed.) Note: This is because the plastic is
an insulator, which does not let the rubbed on charge
freely flow.
4. Have students try rubbing other objects around the
room (a comb or pen) to see which ones can hold a charge.
(Answers will vary)
5. Have students take about 15 cm (6 inches) of scotch
tape and fold the first few cm of each end sticky side
together. Have them place the sticky end down on a tabletop
and peel the tape up quickly. Bring the tape near the
small bits of paper. Is the tape charged? (It should
be.)
6. Have students make another tape just like before and
bring it near the other tape where both tapes freely
swing. What happens? (They should repel)
7. Have students make two new pieces of tape like before,
but this time, have them place the first one sticky side
down. Have them place the second tape on top of the first
and press down. Peel the two tapes up together and separate
them. Bring the two tapes near each other hanging freely.
What happens? (The tapes should attract)
8. Have students hang the two tapes freely from the side
of the desk. Re-rub the plastic tube and bring it near
each tape. What happens? (The tube should repel the tape
that was on the bottom and attract the tape that was
on top.)
NOTE: This could be an opportunity where student inquiry
is encouraged. Give students sometime to make other observations
on their own.Now is a good time to re-group and provide
the students with some content. There are two types of
charges, some attract and some repel. Benjamin Franklin
set the standard that when glass is rubbed with silk
the glass becomes "positive" and when amber
(a semi-precious stone) is rubbed with fur it becomes "negative." Ask
students to accept that the rod became "negative." The
rule for charges is opposites attract and likes repel
so positive and positive repel, negative and negative
repel and positive and negative attract. The idea of
the atom and positive protons and negative electrons
may be also introduced along with the idea of electrically
polarized. (That explains why neutral objects attract
to charged objects.)
9. Ask the students if the rod is negative and this tape repels, what must the
charge of the tape be? (Negative)
10. Ask the students if the rod is negative and this other tape attracts, what
must the charge of the tape be? (Positive)
11. Take the Styrofoam and rub it with the cloth. Place the pie-tin on the Styrofoam
while holding it by the insulated handle. Touch the metal (you should get a small
shock). Lift the pie-tin by the insulated handle. Now touch the metal. You should
again get a shock. This method of charging is called "induction." (When
the pie-tin was near the negatively charged Styrofoam and you touched the metal
with your hand, the negative charges were repelled leaving the pie-tin positive.
When you picked up the pie-tin by the handle, it was positively charged. When
you touched it, negatives from your hand were attracted and entered the pie-tin
making it neutral again.
Day 2 (Computer lab needed)
1. As students enter, have these questions posted for them to respond to on paper:
- What did you learn about attraction and repulsion of
charges yesterday?
- What is one way in which you “charged” an object during yesterday’s
class?
- What is another word for when these objects are pushed
together or pulled apart? In other words,
what must be present in order for motion
to occur? (FORCE!)
- If
motion is occurring, then we must be performing ______
on the object. (WORK!)
There are a series of applets that examine electric force and electric
fields on various point charges in different scenarios. The amount
of direction you
give students is up to you. They could “play” quite a bit, and although
they wouldn’t get a great quantitative grasp on the concept, it would still
be educational and fun.
You can direct the students to the following applets:
These are some great applets. Here is a suggestion
for more structured activities:
2. Now that the ideas of force and work have come up in the
discussion, we can begin to see exactly how strong the forces
are under different
circumstances.
The first applet that should be used is labeled Electric
Field. This shows the
electric field generated by moving charges around. While
you are moving the charges, their coordinate point and current
force are
shown in
the lower-left
hand corner.
Have your students move the two charges closer together in
measurable amounts (keep it on either axis so that the Pythagorean
Theorem
won’t be necessary
to calculate distance) and create a table of distance and force. They should
be able to see the force varies inversely with the square of the distance. Let
each group/set of partners discover this.
3. Have students go to the link labeled Points on an Axis.
This demonstration is for multiple charges, so before beginning,
the
values for Q1 and
Q3 should be set to zero. Move the green charge exactly one
unit away from
Q2, and just
like the last applet, while dragging the point, the coordinates
and force are listed in the lower-left hand corner. With
the green charge
exactly
one unit
away, the force should be one unit. Now have students experiment
with different values for Q2, keeping Q1 and Q3 zero. See
what effect this
has on the force.
Students should see this is a direct relationship. If you
double the charge, the force doubles.
4. Use the results from steps 2 and 3 to help student arrive
at Coulomb’s
Law which is:
k is a constant, q1 and q2 are charges, and r is the distance
Your student’s math experience with formulas will show how much leading
to give, but they should know what effect inverse and direct relationships will
have on the formula.
5. Explain briefly when there is more than two charges present,
the forces generated by multiple charges add together as
vectors (assuming
some
previous knowledge
here), and just let them play! All of the applets are cool
simulations, so you can change variables.
6. Use a summary discussion at your discretion. Monitor students’ experimentation;
ensuring that they’re on task, and see what questions or conclusions come
up. Present them to the class if you choose. Homework
Quantitative problems
on Coulomb’s Law could be provided, if desired.
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Embedded
Assessment
Day
1--Have the students make a negative charged tape by
following procedures in number 5. Rub the Styrofoam
with the cloth. Bring the Styrofoam near the tape that
is free to hang. The tape should repel. Ask the students, "What
can we say about the two charges?" (They are the
same. Some students may say they are both negative,
this is a great response if they can back the statement
with facts, not opinions.)
Day 2--Two of the applets are puzzles for learning
about unknown charges based on the force generated
on a known charge. Challenge students to solve these
puzzles.
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