Experiment 1b
Class:
Name:
(
) Date:
1b Fun with electric charges
Objective
To study several phenomena involving electric charges using a Van
de Graaff generator.
Background information
4
1
There are two kinds of charges: positive and negative. Like charges
repel and unlike charges attract. Charges can be transferred from one
object to another.
2
Induced charges appear on an uncharged object when it is placed
near a charged object. This results in a net attractive force between
them.
3
When a charged conductor touches a neutral conductor, charges will
be distributed between them.
4
When an object is connected to the earth through a conducting wire
or other conductors, it will become neutral in the absence of other
charged objects. This process is called earthing.
5
A Van de Graaff generator (Fig 1b-1) produces a large amount of
electric charges accumulated on its dome. The working principle of a
Van de Graaff generator is shown in Figure 1b-2:
(a) A rubber belt is driven by a motor.
(b) The belt passes through a voltage source maintained at a high
negative voltage. Electrons are discharged by the sharp points of
the voltage source and jump to the belt.
(c) The electrons are carried by the belt to a metal dome where they
accumulate, building up a large amount of negative charge on
the dome.
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Class:
Name:
(
Fig 1b-1
Experiment 1b
) Date:
Van de Graaff generator
charge
collector
metal dome
(supported on
insulating pillars)
polythene
roller
moving rubber belt
voltage source
maintained at a
high negative voltage
motor
Fig 1b-2
Apparatus
❏ 1 Van de Graaff generator and accessories
❏ 1 metal-coated polystyrene ball suspended from an insulating rod
❏ 1 ‘head of hair’
❏ 1 plastic stool
❏ 1 metal sphere
❏ 1 light-beam galvanometer
❏ 1 hair-dryer
❏ 1 retort stand and clamp
❏ several connecting leads
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5
Experiment 1b
Class:
Name:
Note
Electric forces
1
(a) Switch on a Van de Graaff generator.
(b)Bring near the dome of the generator a neutral metal-coated
polystyrene ball suspended from an insulating rod (Fig 1b-3).
Observe the ball.
(c)Alternatively, fix the insulating rod on the dome of the generator
(Fig 1b-4). Observe the ball.
The metal-coated
polystyrene ball is a
conductor.
✐ Van de Graaff
) Date:
Procedure
Precaution
Dry all the apparatus
thoroughly with a
hair-dryer before the
experiment.
(
generator experiments
are always delightful
to watch. If necessary,
extend the experiment,
but ensure that Ss
understand as well as
enjoy what they observe.
insulating rod
metal-coated
polystyrene ball
metal-coated
polystyrene ball
insulating rod
Van de Graaff generator
Van de Graaff generator
✐ Usually the
polystyrene ball is
attracted and repelled
repeatedly. This is
because the charged
polystyrene ball is
discharged in air. This
point is not included in
the question, but Ts may
choose to elaborate.
Safety precaution
1Students with heart
problem should
not perform this
experiment.
2After the experiment,
switch off the
generator. Keep
touching the dome
for about a minute for
discharging through
air before stepping on
the ground.
6
Fig 1b-3
✎
Fig 1b-4
Describe and explain your observation.
Being charged, the ball is first attracted to the dome due to the induced charges on
the ball. On touching the dome, the ball acquires the same charge as that on the
dome and is immediately repelled.
2
(a) Switch off the generator.
(b) Put a ‘head of hair’ on the dome of the generator (Fig 1b-5).
(c) Alternatively, stand on a plastic stool and touch the dome of the
generator (Fig 1b-6).
(d) Switch on the generator and observe the hair.
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Class:
Name:
(
✐ First get the
) Date:
Experiment 1b
‘head of hair’
student to stand on
a plastic stool and
touch the dome.
Then turn on the
generator. The
student should not
feel any electric
shock.
Van de Graaff
generator
✐ After the demonstration,
don’t try to touch the dome
with the discharging sphere
Van de Graaff generator
plastic stool
to pass all the charges to the
earth, otherwise the student
Fig 1b-5
Fig 1b-6
will get an electric shock
because suddenly there is a
path (student’s body-dome- ✎ Describe and explain your observation.
discharging sphere-earth) for
The hairs stand on end. Carrying the same kind of charge (as that on the dome), the
the charges on the student’s
body to flow. Instead, after
hairs repel each other and so stand on end.
switching off the generator,
teacher should ask the
student to wait (still touching
the dome) for around half
a minute for discharging
through air. The student
can then step down from
the stool without getting an
3 Connect a metal sphere to the earth and bring it near the dome of
electric shock on stepping on
the generator (Fig 1b-7). Observe what happens.
the ground.
Electric sparks
✐ The strong electric field
earthed metal sphere
set up between the dome
and the metal sphere is so
high that air molecules are
ionized. When the electrons
and the positively-charged air
particles fly through the air
towards the metal sphere and
negatively-charged dome,
intense heat is generated,
resulting in the sparks
Van de Graaff generator
✐ For demonstration, Ts
should place the discharging
sphere directly above the
dome. The sparks produced
can thus be seen from all
angles.
Fig 1b-7
✎
earth socket
Describe and explain your observation.
There are sparks between the dome and the metal sphere. They are produced as
electric charges jump from the dome through the air to the metal sphere and then to
the earth.
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© Oxford University Press 2007
7
Experiment 1b
Class:
✐ This experiment
serves to establish the
link between electrostatics
and current electricity. An
earth socket is usually
found at the base of
the generator. This is
connected to the earth
via the earth pin of the
3-pin power plug of the
generator.
Name:
(
) Date:
Moving charges
4
(a) Switch off the generator.
(b) Connect the dome of the generator to the earth via a light-beam
galvanometer (Fig 1b-8).
(c) Switch on the generator and observe any reading on the
galvanometer.
Van de Graaff generator
Fig 1b-8
✎
light-beam
galvanometer
earth socket
Describe and explain your observation.
An electric current passes through the light-beam galvanometer. This is due to the
flow of charges from the dome to the earth via the light-beam galvanometer.
Discussion
✎
Will a student get an electric shock if he touches the dome of the
generator in step 4? Explain your answer.
He/she will not get an electric shock. This is because the charges can no longer
accumulate on the dome as there is a path for them to flow away.
8
New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
Experiment 1b
) Date:
Van de Graaff generator
1A ____________________________________
produces a large amount
of electric charges accumulated on its dome.
Induced charges
2________________________
appear on a neutral object when it is
placed near the dome of the generator. This results in a net
attractive
________________________
force between them.
3When a neutral object touches the dome of the generator, it
the same
will carry ________________________
kind of charges as the dome.
repulsive
This results in a net ________________________
force between
them.
Sparks
4________________________
are produced when electric charges
pass through air.
electric current
5An ______________________________
is formed when electric
charges flow.
Further thinking
✎
In step 2, what will happen if the student is touched by (a) another
student standing on a plastic stool; (b) another student standing on
the ground in bare feet? Explain your answer.
In case (a), the second student’s hairs will also stand on end.
In case (b), both students will receive an electric shock as electric charges pass from
the first student to the second and then to the ground.
✎
In step 2, what will happen if another student standing on the ground
places his finger near the finger of the student touching the dome of
the Van de Graaff generator? Explain your answer.
Sparking occurs between their fingers as charges pass from the first student to the
second student through the air.
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© Oxford University Press 2007
9