Teacher’s Notes
This sequence of slides is designed to introduce, and explain
electrostatic charging by friction, as explained on page 246 in
New Physics for You.
On each slide the key points are revealed step by step, at the click
of your mouse (or the press of a key such as the space-bar).
Before making the next mouse-click you can ask questions of the
class or make statements about what is about to be revealed.
This should help students to become clearer about the ideas
involved.
Naturally it pays to have quick practice-run first.
To start the slide-show, press function-key F5
(or right-click->Full Screen)
(to return to ‘normal view’ press the <Esc> key).
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Charging by friction
Physics for You, page 246
Learning Objectives
You should learn :
• How insulated objects can be
charged by friction,
• That this is because some charges
can move.
Charging by friction
In this diagram:
The wool and polythene
are each ‘uncharged’.
What does this mean?
Charging by friction
The wool is ‘uncharged’,
because…
…it has equal amounts
of positive and negative
charge.
Equal numbers of
+ and −
On the wool
+
+
−
+
+ −
−+−
−
+ +
−−
+
On the polythene
−
+
−
−
+
Charging by friction
Equal numbers of +
and − on the wool:
(Count them!)
7+ 7−
Equal numbers of +
and − on the polythene:
(Count them!)
3+ 3−
+
+
−
+
+ −
−+−
−
+ +
−−
+
−
+
−
−
+
Charging by friction
If you rub the wool on
the polythene, some
electrons (−) move from
the wool to the polythene.
There are now more +
than − on the wool:
+−
+
−
+
−
+−
+
−+
−
−
+ + −−
−
+
+
(Count them!)
So now the wool is charged positively,
with a surplus of 3 +
Charging by friction
What has happened to the
polythene?
There are now more −
than + on the polythene:
(Count them!)
So now the polythene is
charged negatively,
with a surplus of 3 −
+−
+
−
+
−
+−
+
−+
−
−
+ + −−
−
+
+
Charging by friction
This is summed up
in the diagram on
page 246:
Both objects are now
equally charged,
with opposite charges
because electrons −
(only) have moved.
Learning Outcomes
You should now:
• Understand what it means when an object
is ‘charged’ or ‘uncharged’,
• Understand how an insulated object
becomes charged by rubbing,
• Understand why the objects have equal
but opposite charges,
• Know that only electrons (−) can move.
For more details, see:
 Physics for You, page 246
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