charged balloon to the wall the positive charge on the wall attracted it

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					High School Science                                                         The Periodic Table
Chemistry                                                                    Student Resource
Name: __________________________

                            The Atom, Opposites Attract
What really makes atoms of one element “stick” to atoms of another?

What force holds atoms together? This lesson will help you explore that force. But before you
begin, write what you remember here:




Group Challenge
Your challenge is to find a way to stick a balloon to a wall. Which group will succeed first?
When you do, describe your procedure here:




Plastic Wrap Activity
Procedure:

1. Obtain the following materials: 500 ml of puffed rice cereal, one piece of paper towel, 60 –
   70 cm of plastic wrap, and a couple pinches of salt.

2. Lay the plastic wrap flat on a desk. One student should hold it flat while another student
   vigorously rubs the plastic wrap with a piece of paper towel. Do this for at least a minute.

3. Two students (one on each end) should carefully remove the plastic wrap from the desk by
   holding two corners with their hands and gently peeling the plastic up from the surface.

4. A third student should sprinkle the salt on a nearby desk.

5. The two students holding the plastic wrap should position the plastic wrap over the salt and
   gently lower it towards the table without letting go and without actually touching the salt.

Observations:
6. Raise and lower the plastic a few times and describe what happens.




February 16, 2004                                                SCoPE SC100207 Page 1 of 5
High School Science                                                          The Periodic Table
Chemistry                                                                     Student Resource
   Repeat steps 1-5 with the puffed rice and describe what happens (look closely).




7. Describe what was similar and what was different about how the salt and rice attached to the
   plastic.




8. What do you think caused these differences?




9. Was the balloon activity similar to the plastic wrap activity?
   Why do you think this?




February 16, 2004                                                   SCoPE SC100207 Page 2 of 5
High School Science                                                           The Periodic Table
Chemistry                                                                      Student Resource

                                     Opposites Attract
Have you ever noticed how clothes sometimes cling together when you remove them from a
dryer or how your hair sticks up when you pull a sweater over your head? Have you ever
dragged your feet across a carpeted floor and gotten a “shock” when you touched something
metal? Why did the balloon stick to the wall? Why were the grains of salt and puffed rice
attracted to the plastic wrap?

These are examples of static electricity. Electric charges on the plastic wrap and the rice made
them attract each other, just as magnets can attract each other. A static charge on a balloon
makes it “stick” to the wall, even though there is no glue or anything sticky to hold it there –
only electric charges attracting each other.

When you rub your feet on a carpet, you are creating an electric charge on your body. When you
approach a metal doorknob, for example, or another person, the charge jumps from you to the
other object as a small bolt of lightning through the air. The movement of electric charges is the
shock you feel (and hear, just like thunder. Lightning and thunder are other examples of static
electricity phenomena, on a huge scale.

Where does this static electric charge come from? What is it about the nature of some materials
that they can have electric charges?

In order to figure this out (as a science detective) think about what you already know. One thing
you know is that rubbing often produces static charges. You rubbed the balloon on your body,
you rubbed the plastic wrap with paper towel, you rub your feet along the carpet, etc. It seems
like you might be rubbing something off of one surface and onto the other surface, leaving more
of something on one object and less on the other. What could this “something” be?

You also know that each atom has a positively charged nucleus surrounded by a negatively
charged electron cloud. The nucleus is the space where protons and neutrons reside. Protons have
a positive charge, neutrons have no charge, and electrons have a negative charge.

Subatomic particles:

Name                  Charge




As you dragged your feet on the carpet, you rubbed some electrons (that were loosely held) off
your shoes and onto the carpet! Your shoes no longer had an equal number of protons and
electrons. They had fewer electrons and therefore you had an overall positive charge. This
imbalance in charge produces static electricity. When you touch a metal doorknob electrons in
the doorknob jump onto your body, and you feel a “shock.”


February 16, 2004                                                  SCoPE SC100207 Page 3 of 5
High School Science                                                            The Periodic Table
Chemistry                                                                       Student Resource


They jump onto your body because the negative electrons in the doorknob are attracted to the
positive charge on your body. Unlike charges attract (a negative and a positive). Like charge
repel (two positives or two negatives). This is just like magnets – two south poles or two north
poles of a magnet repel each other, but a south pole of one magnet attracts the north pole of
another magnet.

When clothes tumble around in the dryer they rub against each other. Some of the clothes gain
electrons from other clothes, and some lose electrons to other clothes. Therefore, some get a net
negative charge and some get a net positive charge. The opposite charges are attracted to one
another and the clothes “cling” together. When you pull them apart you often hear the sparking
as electrons jump from one piece to another.

When you rubbed the balloon against your body, you rubbed electrons off your body and onto
the balloon. It had more electrons than protons, so it was negatively charged.

So how would it cling to the wall? The wall would have to be positively charged for it to cling,
right? How could the wall have a positive charge?

The wall would not have a positive charge because electrons left it and went into the balloon,
right? The balloon is already negative, so electrons would be repelled from the balloon.

The picture might give you a hint. It shows the balloon and wall before they touch and cling
together – when the negatively charged balloon is brought close to the wall.

The balloon has a negative charge, and the wall – near the balloon – has to have a positive
charge. Where did that positive charge come from?
                                                                +–
                                                                –+
                                                                +–
                                                                –+
                                              –     –           +–
                                          –               –     +–
                                      –                    –
                                      –                    –    +–
                                      –                    –
                                      –                    –    +–
                                          –               –     +–
                                               –    –
                                                                +–
                                              Balloon           +–
                                              with              +–
                                              excess            –+
                                              nega tive         +–
                                              charge            –+
                                                                +–
                                                                –+
                                                                +–
                                                                –+
                                                               Wall
                                                                +–




February 16, 2004                                                     SCoPE SC100207 Page 4 of 5
High School Science                                                            The Periodic Table
Chemistry                                                                       Student Resource
When you brought the balloon near the wall (a neutral object) the negative charge of the balloon
repelled the negative charges in the wall (the electrons in surface atoms). They moved away from
the balloon, back into the wall and left the atoms near the surface of the wall with a net positive
charge. The now-positive wall attracted the negative balloon, and it stuck. The force of
electricity is the only thing that held it in place!

This is called charging by induction – giving the wall a positive charge by pushing away the
negative charges. When you touched the negatively charged balloon to the wall the positive
charge on the wall attracted it.

10. Using what you just learned explain how the plastic wrap was able to pick up the rice and
   cereal.




You know that protons have a positive charge and electrons have a negative charge. Think about
a picture of an atom – a nucleus surrounded by electron clouds. What you know about particles
should explain how electrons and protons are held together in an atom. The attractive force
between the positively charged nucleus and the negatively charged electrons hold the atom
together.

But from this deduction, another question quickly follows. How can the nucleus stay together if
the positively charged protons are all packed together in it and they repel each other? Another
force, even stronger than the force between electric charges, binds the protons and neutrons
together. That nuclear force will be considered later, when you study nuclear reactions. For now,
we can see that the electrons stay attached to an atom because their negative charge is attracted
to the positive charge of the protons. You will use this idea to see how two different atoms
combine to form a compound.




February 16, 2004                                                   SCoPE SC100207 Page 5 of 5

				
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