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					                       Serrata Electric Charges Experiments

                                                                        Many materials can be charged by contact in this way, including an
   Electric Charges Experiments                                         ebonite rod when rubbed with woollen cloth. Try this latter combi-
                                                                        nation using your paper detector to tell whether or not the ebonite
                                                                        and wool become charged.
  Apparatus Required:
                                                                        We have no evidence as yet that there is any difference between
  Thin plastic sheet                                                    the charges on any of these materials because they all act the same
  2 clear plastic rods - Cat No. 114001                                 way with the paper detector. We are accustomed, however, to the
  Ebonite rod - Cat No. 1010129                                         terms 'positive' and 'negative' charge. How do we know that there
  Woollen cloth or fur - Cat No. 1010142 or 1141001                     are two types and only two types of charge?
  Stirrup support, for suspending rod
  Insulating thread (e.g., silk)                                        Charge one of the clear plastic rods by rubbing it with the plastic
  Metal rod on glass beaker                                             sheet, and place the rod in the stirrup suspension. (The stirrup is
  Metal Vane Electroscope - Cat No. 1052029                             suspended by a special thread so that any charge on the rod does
  Van de Graaf Generator - Cat No. 1052222 or 1052220                   not readily leak away.) Charge the other rod similarly and bring it
  Insulating pad (on which to stand)                                    close to the charged suspended rod, as in the following figure.
                                                                        What do you notice?
Background:
How does an object become charged? How do you detect a charge
on an object? How many types of charge are there? These are some
of the questions we are attempting to answer in this experiment
with static electric charge. You probably know some of the an-
swers before you start, but see if your prior knowledge is sufficient
to adequately explain the results which follow.




                                                                        Other charged materials when brought close to the suspended rod
                                                                        have the same effect as the charged plastic rod, and therefore must
                                                                        have a similar type of charge. But is there an alternative type of
The Experiment                                                          charge which will have a different effect on the suspended rod?
Take a small piece of paper and tear it into several very small         Charge the ebonite rod with the woollen cloth and bring it close to
pieces, allowing them to fall randomly onto a table. Rub a plastic      a freshly charged and suspended plastic rod. What do you notice?
rod briskly with your jumper or a piece of woollen material and         Any material which, when brought close to the suspended rod, has
then carefully bring the rod slowly towards the pieces of paper.        the same effect as the charged ebonite rod, has a similar type of
What do you notice as the rod nears the pieces of paper? Observe        change to the ebonite rod. There are only two effects which occur
carefully and record accurately the behaviour of the pieces of pa-      with the suspended charged plastic rod and so we conclude that
per.                                                                    there are only two types of charge, one (on the plastic rod) which
                                                                        we arbitrarily call positive, the other (on the ebonite rod) which we
The action of the paper suggests that the plastic rod is charged.       call negative. To complete the possibilities, what do you expect
Now rub one of the clear plastic rods briskly with the plastic sheet-   will happen when a charged ebonite rod is brought near to a sus-
ing. Test whether or not the rod becomes charged by using the           pended similarly charged ebonite rod? Try it.
small pieces of paper as a detector. If the rod is charged, does the    So far we have charged objects by direct contact, but there is an-
plastic sheeting also become charged? Use the pieces of paper as        other very useful method which incorporates the action at a dis-
your detector again.                                                    tance properties of charges. This method is called charging by in-
                                                                        duction. Bring a charged plastic rod near to, but not touching, a



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Serrata Electric Charges Experiments

metal rod placed on top of a glass beaker with your finger (thereby     demonstrates clearly that the charge is of the same sign. If the nee-
providing an earthed connection). After removing your finger from       dle moves in the reverse direction, the body is either of opposite
the rod, remove also the charged rod. Test whether or not the metal     charge or uncharged.
rod has become charged, and determine whether or not the charge
is similar to that on the (positively) charged plastic rod which was    Questions:
held nearby.
                                                                        1. Assuming that only negative charges move, explain the action of
The Electroscope                                                           the small paper detectors in firstly being attracted towards a
                                                                           charged body and then being repelled.
So far our testing procedures for detecting a charge and determin-
ing its sign have been rather crude. An instrument which provides       2. When you rubbed the plastic rod with the plastic sheet both ma-
much more sensitive results is the electroscope, one type of which         terials became charged, as evidenced by the action of your paper
a Metal Vane (Braun) Electroscope is illustrated.                          detector. At that point, however, you were unable to test the type
                                                                           of charge on each material. If the charge on the rod was positive,
                                                                           what do you suspect was the sign of the charge on the plastic
                                                                           sheet? Explain your answer.
                                                                        3. Explain, in terms of the properties of charges, why the needle of
                                                                           the electroscope moves when the central metal rod is charged.
                                                                        4. The boy in the figure is standing on an insulating pad with his
                                                                           hands on a charged dome of a Van de Graaf. Explain why his
                                                                           hair reacts as shown.




The electroscope consists essentially of a central metallic rod to
which at one end is attached a metal plate and at the other a light-
weight pivoted needle. The pivoted needle is enclosed in a metal
case which is insulated from the central metal rod.
Charge a plastic rod by rubbing with the plastic sheet, and bring
the rod near to the metal plate of the electroscope. Note and record
what happens to the needle of the electroscope.
Remove the rod and record what happens to the needle as you do
so.
The movement of the pivoted light-weight needle of the electro-
scope clearly indicates the presence of charges near the metal plate.
The electroscope can be made to retain the charge by charging the
metal plate either by contact or by induction. Charge a plastic rod     If you have time, try it!
by rubbing with plastic sheet and use it to charge the electroscope
firstly by contact, and then separately by induction. Assume that
the charge on the plastic rod is positive and that only negative
charges move.
Was the final induced charge negative or positive?
The charged electroscope can now be used to detect the sign of a
charge on a body. Bring near to the charged electroscope a posi-
tively charged plastic rod, a negatively charged ebonite rod and an
uncharged conductor. Record what happens to the needle of the
electroscope in each case.
If an object with charge similar to that on the charged electroscope
is brought near to the metal plate, the movement of the needle



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