Electrons and Electricity
Relevant KS2 Links: SC1 1a, 1b, 2a, 2c, 2d, 2e, 2j, 2k, 2l ; SC3 1c;
SC4 2a. 2b, 2e;
Plus basic foundation for electricity and the nature of forces
Base Concepts Conveyed: Atoms and molecules are held together by
“negative and positive charges,” which act like N and S on magnets: like
charges repel, unlike charges attract. Electricity is due to the movement of
charges, which can exert a “force” on other charges. Conductors are materials
in which charges can move easily. Lightning is a form of electricity.
EQUIPMENT
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Two small balloons;
A 1 metre long piece of string;
A paperclip;
The accompanying picture of a hydrogen atom.
For Each Student or Pair of Students:
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An empty fizzy drinks can;
A small polystyrene plate (or inverted polystyrene bowl);
A large paperclip;
A long, thin sliver of kitchen foil (see “preparation” below);
One or more of the following:
o a plastic comb;
o a small balloon;
o another piece of polystyrene
o a wool or leather cloth with a plastic rod or ruler
(a piece of chamois leather cleaning cloth together with a small
length of thin PVC pipe from a DIY store works particularly well)
The accompanying work sheets.
INTRODUCTION:
This lesson links the idea of atoms to electricity by introducing the important concepts of
electrons and charges. Testing of these concepts is emphasised through prediction and
demonstrations of how charges can be separated, transferred and studied in terms of their
interactions on each other. The idea of electrical “conductors” as materials where charge
can easily move between atoms is introduced and tested. The term “force” is repeatedly
used to describe the pushing and pulling action of charges upon each other and students
learn to draw arrows that represent these forces. The movement of charges is also linked
to an understanding of lightening and a discussion of safety during thunderstorms.
PREPARATION
From a sheet of kitchen foil, cut a number of very thin strips, each about 1-2mm wide
and about 8cm in length. There should be one for each student plus extras in case some
get torn. So, for a class of 30, you may want to cut about 40 such strips. These should be
appropriately distributed to each table, along with the other materials listed above.
Suggested
Year Group:
4-6
Assumes
Intro to Atoms
TEACHING:
Briefly review the idea of atoms as constituents of matter and how atoms make up
molecules like H2O. But what sticks these atoms together and what do atoms look like?
Show them the picture of the hydrogen atom, explaining that this is the simplest atom.
It’s made up of two much smaller particles: at the centre is a proton and, moving around
it, a bit like the earth goes around the sun, is an electron. We sometimes draw simple
pictures like this to represent what an atom is made from, but it really doesn’t show the
different sizes involved properly. If you imagined that the proton were the size of a pea,
then the electron would be about 500 metres away! Indicate how far this is by referring to
a known landmark that is about this distance from the classroom. When we picture the
whole atom as a fuzzy ball, like when we draw a water molecule or think about how a
crystal is stacked, the edge of that fuzzy ball represents where the electrons are moving
way out here (indicate on your picture).
But what keeps the electron moving around the proton? Well, what keeps the earth
moving around the sun? Right: gravity. Gravity is a force that pulls on the earth and
keeps it from flying off (and keeps us from flying off as well)! Could gravity be holding
electrons and protons together? Let’s have a think: if gravity were strong enough for a
pea to hold onto an electron that’s half a kilometre away, you’d probably end up pulling
everything together to make on, big, enormous ball (which would subsequently get
squashed into an even smaller ball)! So there must be a different type of force that pulls
on the electron to keep it from flying away from the proton. This is called the electric
force. Protons and electrons have something called charge, which works a bit like
magnets: charges that are the same, push each other away and charges that are different
attract each other… just like north and south poles of magnets. Instead of “north” and
“south,” the proton charge is called positive and the electron charge is called negative,
just to tell them apart.
Hang on, how do we know this is true and isn’t just fairy stories? A scientific theory has
to make good predictions. What predictions can this theory make? Well, among other
things, we should be able to pull some electrons off of atoms and see if they attract and
repel like we said. Shall we try that? Blow up a balloon (it should be fairly well inflated),
tie it off and ask for a volunteer to come up and help... but pick one with long and stringy
hair. Rub the balloon back and forth on their head and then slowly raise the balloon to
show that some of their hair is now sticking up towards the balloon! What’s happening?
Some may know this as “static electricity,” which just means electric charges that are not
moving around much. Where do these electric charges come from? The atoms in some
materials, like balloon rubber, tend to grab electrons more than those in other materials,
like hair. So the balloon has now taken some of the electrons from the hair and has more
negative charges on it. Having had some of its electrons stolen, the hair now has more
positive charges (from the protons) left over, so that the two attract each other.
Do this a couple more times. Notice that, even when you take the balloon away, some
of your volunteer’s hair still sticks up in a funny way. Why? What are the positive
charges left on their hair doing to each other? Right, pushing away!
What should happen if we put negative charges on two balloons and bring them together?
Yes, they should also repel each other. Let’s try: blow up another balloon and tie each to
opposite ends of the string. Rub each balloon on your hair or a wool jumper, being careful
not to touch too much of the balloons with you hands and making sure they do not bump
into anything else after they are charged (which can be a bit tricky since they will want to
attract things – it’s probably worth practising this once or twice to get the hang of it).
Holding the string at its midpoint, allow the balloons to dangle and come towards each
other. You should notice that they repel each other and stay several centimetres apart.
The electrons are exerting a force on each other that pushes them apart.
What does
an atom look
like?
What is
“charge?”
How can
you separate
electrons to
show they
are real?
Can you see the
effect of positively
charged protons?
How can you
show that
electrons repel
each other?
There are some materials that allow electrons to move fairly easily from one atom to
another. Does anybody know what these are called? Right: conductors. Does anybody
know of any good conductors? Yes, metals like you might use in wires… anything else?
How about water? Water can conduct electricity too. How about people? Actually, yes,
skin can conduct electricity a bit, even if it’s not as good as for metals. So, if you were to
hold a hand near the balloon, the extra electrons on the balloon should push away the
electrons in your hand that move somewhere else, leaving more positive charges nearest
the balloon. What do you predict should happen then? Right, they should attract!
Carefully move your hand closer to one of the balloons without touching it and, indeed,
the balloon will be attracted. In fact, you can just about make it float upwards. The
negative and positive charges exert a force on each other that pulls them together.
What do you predict would happen to the electrons on the balloon if you move a very
good conductor near it? Bring a paperclip close to one of the balloons. Everyone should
be very quiet and listen: when you get within about 10 centimetres, there will be a very
faint “crack” sound. Do it a few more times… does everybody hear it? What is happening
to the balloons? They are coming back together, which must mean they have lost their
charge. Charge must be moving between the balloon and the paper clip. Rub one of the
balloons and bring it near the paperclip again, taking this around the room so everyone
can hear. Why does that make a “crack?” The charges are moving so fast that they rapidly
heat up some of the air. This makes the air expand very quickly, like a mini explosion! If
you did this at home in a very dark room, what do you think you’d see? Yes, a long spark
coming from the heated molecules in the air. Imagine you did this on a very, very large
scale, with lots and lots of charge moving over very long distances. What would it look
and sound like? Yes, thunder and lightning!
So lightning must have something to do with charges moving between clouds and the
ground. But how do the clouds get charged up? There’s not a giant head rubbing a balloon
up there! Can you think of anything else that could be rubbing electrons from the clouds?
Is there anything moving inside rain clouds? Yes, raindrops and, if it’s cold enough up
there, ice crystals. Thunderclouds tend to have large winds that churn the ice crystals up
through the cloud and back down again, round and round, rubbing off electrons from the
cloud in the process and moving them around such that the bottom of the cloud becomes
more and more negative.
What does all this suggest you should do to be safe if you find yourself in the middle of a
thunderstorm? It’s safest to be inside buildings, though you should still stay away from
conductors to the outside such as phone lines, power lines and water pipes. Charges tend
to move between clouds and the closest point on the ground so, if you are stuck outdoors,
avoid high places, isolated trees or towers, or open fields (where you look like the highest
place around!).
ACTIVITY:
Have students do the accompanying worksheets in which they make an “electroscope”
and use it to study how charges interact. Go to each table in turn to demonstrate pages 1
and 2. Then, after a bit, go around to demonstrate page 3. Then, finally, page 4.
PLENARY:
Discuss the results of the worksheet. Why do you need the polystyrene plate? (even table
tops conduct electricity a little bit, so it insulates the can to stop charge from “leaking” off).
Watch the NOVA Science Now web programme on lightning from October, 2005:
www.pbs.org/wgbh/nova/sciencenow/3214/02.html
What are
electrical
conductors?
What is an
electric
spark?
How is
lightening
made?
How can you
protect yourself
during
thunderstorms?
Quick Reference
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Briefly review the idea of atoms. What do they look like?
Show picture of hydrogen atom with proton and electron.
Describe the scale (if proton were size of pea, electron would be 500m away).
What “force” holds them together? (not gravity!)
Introduce idea of positive and negative “charges” (like N and S of magnets).
Rub balloon to separate electrons. Attracts positive charges left on hair.
Charges on hair push each other away (so positive charges repel each other).
Two charged balloons exert force to push each other away (negative charges repel each other).
Conductors allow electrons to move between atoms. Name some (including skin).
Charged balloon pushes away electrons in hand and attracts positive charges. (Pulling force)
Move paperclip near repelling balloons. Listen for sound, see balloons move together.
Charge must have moved from balloons.
Charge one balloon and move towards paperclip so others can hear “crack.”
Sound from rapidly heated air… mini explosion!
In dark would see long spark.
On large scale: thunder and lightening!
Ice crystals circulating in thunderclouds rub off and move electrons to bottom of cloud.
Protection in thunderstorms: Best indoors, but stay away from conductors to outside.
If outdoors, avoid high points closer to clouds (or open fields where you are high point).
Hydrogen Atom
Electron
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+
Proton
Making an Electroscope
Page 1
An electroscope is an instrument for detecting the presence of static
electricity. Here’s how to make a simple one using:
a fizzy drinks can,
a polystyrene plate,
a paperclip and
a long, thin sliver of kitchen foil:
Place the can on the plate and then
insert one end of the paperclip into
the pull-tab so that part of the clip
hangs over the edge of the can.
That’s it!
Carefully straighten the kitchen foil
and bend the top into a hook. Hang it
on the paperclip as shown. It should
hang close to the can but not touch it.
Now, to test it out, you need to make something charged by
transferring some electrons from one material to another. There are
lots of ways to do this:
• run a plastic comb through your hair;
• rub a balloon on your hair;
• rub another piece of polystyrene on your hair
• stroke a plastic ruler or rod with a wool or leather cloth.
Page 2
After charging up one of these objects, hold it near the can on the side
opposite the sliver of foil (without touching it!).
What happens?
What happens when the object is removed again?
The can, paperclip and kitchen foil are all conductors, which means that
electrons can easily move from one part to another. If you bring negative
charges near them, the electric force will push on the electrons in the
conductor to make them move away.
Draw an arrow on the picture below from the c harged objec t to the
can to show this force pushing on the electrons. Colour in the side of the
can to show where you think the electrons might move to.
Charged object
(like a comb)
Some electrons can get pushed into the foil as well. Does this explain why the
kitchen foil might be repelled from the side of the can? Draw an arrow on the
picture that shows the electric force on the foil.
Page 3
Charge up your object again and see if you can actually transfer electrons to
the electroscope (instead of just pushing them) by touching it to either the
top of the can or to the kitchen foil. Make sure you do not touch the can with
your hands while you do this! If you are successful, there will be extra
negative charges on the can that will push against the extra negative charges
in the foil, which should stick up.
Charge up your object once more and, this time, slowly move it towards the
foil. What happens? Can you explain this? Draw an arrow on the picture above
to show the direction of the force between your charged object and the foil.
Now touch the can with your hand. What happens to the foil?
Where do the electrons go?
Extra Credit!
Charge your object again and bring it close to the can on the opposite side
from the foil, as before. After the foil rises, touch the can with your hand,
then let go and move the charged object away again. What happens?
Electrons can move off the electroscope and onto you when you touch it!
If you have taken some electrons away, what type of charge is left over on
the electroscope?
What do you predict will happen if you charge up your object with negative
electrons and move it slowly towards the raised piece of foil now? Why?
Try this and draw an arrow on the picture below to show the direction of the
force acting on the piece of foil.