CHARGE RESIDE ON THE SURFACE OF

On a charged insulator the charges are localized at places where they are supplied. But, in case of a conductor the charges given to any point immediately spread themselves over the surface. Extra charges do not reside inside the volume of a conductor. Three famous experiments show this.

(i)                         BIOT’S EXPERIMENTS

Take a metal sphere mounted on an insulated stand. Charge it with +ve or –ve charge. Then take two metallic hemispheres which exactly fit onto a sphere and are provided with insulating handles as shown in the figure 32.11. Now fit them over the sphere enclosing it completely. Bring this set near the electroscope. The leaves will diverge. When the hemispheres are separated and tested they are found to possess charge. But the sphere does not show any charge on it. This clearly shows that charges spread out over the outer surface and don’t reside in the interior of the conductor.

(ii)                     FARADAY’S BUTTERFLY- NET EXPERIMENTS

A brass ring is mounted on an insulating stand and conical muslin net is attached to the ring. Two silk threads are tried to the cone so that it can be turned in or out, as desired (fig, 32,12). Give some charges to the net and test for the charge inside and outside. It is found that the charge is entirely on the outer side and no charges are found inside.

Now turn the net inside out by pulling the outer thread, and test for the charge. It is found that the charges are present on the outer side.

(iii)                  FARADAY CASE

Faraday built metal cage supported on an insulating base. He entered the cage and sat on the seat which was insulated from its base. It was charged to very high potential. But he could not detect any charge inside the cage even though the outer surface was sparking into air.

These experiments confirm that charges don’t reside into a hollow charged conductor. This assigns can be tested by taking a hollow charged spherical shell with a hole on it and inserting a proof plane inside it. 

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INSULATOR

It is the clear that the inducing charge of the body (rod) produces opposite type of induced charges. It should be noticed that the inducing charge on the glass rod does not change. We can charge many conductors by the same rod i.e. we can obtain limitless supply of electrical charge this way. By connecting positively charged and negatively charged bodies to a bulb we can desire electrical energy. Does this violate the principal of conservation of energy? Definitely not. In removing the inducing rod we must do some work against the attractive forces between the opposite charges on the rod and conductor.

The phenomenon of electrostatic induction allows us to understand the attraction between a charged body and a neutral body. Suppose a positively charged rod A is brought near a small metal sphere B suspended by a thread (fig.32.4). Instantly, free electrons in the sphere will move towards the side, which is near the rod A. The near side be negatively charged and the far side will be positively charged by induction (fig. 32.4) The induced negative charge will be attracted towards and the induced positive charge will be repelled away from the positive charge on A. But, since the induced negative charges on B are relatively closer than the induced positive charge on it, the force of attraction is greater than the repulsive force. So the net force on B will be attractive and the ball will be pulled towards the rod. Note that induction takes place first which result the attraction.

If we bring the a positively charged rod near a conductor ( insulator) the net force will be attractive but the force will be smaller than in the case of conductor. It is because there are no free electrons inside the insulator- all the electrons are bound to their respective atoms. Thus, the negatively charged particles (electrons) are an atom in C is drawn closer ton A than the nucleus. This type of displacement of negatively charged electrons and positively charged nucleus in an atom is called ‘electric polarization’. Insulation is also called dielectrics in this sense. First there is electric polarization and then there is attractive. We shall talk about it in detail later in chapter 35.

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CHARGING A CON DUCTOR BY INDUCTION

Take a metal rod (A.  conductor) AB mounted on an insulating stand as shown in figure 32.3. Bring a positively charged rod towards one end, say A, of the conductor without touching it (fig 32.3(a)).  The end A will develop negative charge and the end B will develop positive charge due to the electrostatic induction. The reason that is the free electrons in the conductor will be attracted towards the end A by positively charged rod and the same number of electrons will be deficient on the side B. End A will be negatively charged and end B will be positively charged.

Now if we connect the end B of the conductor to ground using the metal wire the positive charges will move to the ground. (fig.32.3 (B) (Actually, the electrons come from the ground to the conductor and neutralized those positive charge). But the induced negative charges near A remains there, because these charges are free to move.

Because the electrons at the end B have flown from earth, they are called free charge. The electrons near end A are not free to move. They are held by the inducing charges. Therefore, they are called bound charge.

Again, disconnect end B from the earth. The –ve charges will remain near the end A as before under the influence of the inducing charge (fig. 3©.

Finally if we remove the glass rod as well, the negative charge will spread over the surface over the surface of the conductor evenly and the conductor will become negatively charged (fig.3 (d)).

These are the steps to be followed to charge a conductor by the method of induction. In this activity we described methods to charge a conductor negatively. Exactly the same steps should be carried out to charge the conductor positively, with the difference that we should take a negatively charged rod in place of the glass rod above or connect end A to the ground in the previous activities.

It is a clear that the inducing charges of the body (rod) produce opposite type of induced charges. It should be noted that the inducing charge on the glass rod does not change. We can charge many conductors by the same rod i.e. we can obtain limitless supply of the electrical charge this way. By connecting positively charged and negatively charged bodies to the bulb we can desire electrical energy. Does this violate the principal of conservation of energy? Definitely not. In removing the inducing rod we must do same work against the attractive forces between the opposite charges on the rod and conductor.

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TESTING THE CHARGES

Bring a body under test near the disc of an uncharged electroscope. If the leaves remain parallel the body is neutral. The leaves will be diverge if the body has got charge on it. This shows the presence of charges on the body.

To examine the nature of charge, we have to put a known type of charges on the electroscope. It can be done by conduction or by induction. To charge by conduction, the disk is touched by hand so that the leaves collapse completely (if there were any charges). Then the disk is touched by an ebonite rod rubbed with fur. The charges on the rod will be transferred to leaves of electroscope and they diverge. The rod is removed and electroscope is negatively charged. If similar process is carried out with glass rod rubbed with fur. The charges on the rod will be transferred to the leaves of electroscope and they diverge. The rod is removed and electroscope is negatively charged. If similar process is carried out with glass rod rubbed with silk the electroscope will be positively charged.

Charge it by induction one should follow the procedures described in section 5 of this chapter. First touch the disk of the electroscope by hand and the leaves will collapse. Remove the hand. Bring a glass rod rubbed with silk near the disc. The leaves will be diverging. Keeping the rod there ground the disc on the opposite sides. The leaves will collapse. Disconnect the ground first and then remove the glass rod. Again the leaves will diverge and the electroscope will be negatively charged. The same process is carried out with an ebonite rob rubbed by fur to charge the electroscope positively.

After charging the electroscope the body under the test is brought near the disc of the electroscope. If the electroscope was negatively charged and leaves diverge more then the body is negatively charged. But if the divergence diminishes, the body is either positively charged or simply uncharged i.e. neutral. To make sure the disc is touched by hand to make the leaves collapse. Then the body is brought near the disc. If the leaves diverge the body possesses positively charged and if the leaves are unaffected the body is electrically neutral.

It should be clear that attraction does not always confirm the presence of charge, but repulsion does.

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PROPERTIES OF ELECTRIC CHARGE

Let us look at the properties was the of electrical charges.

i)                 The “positive” and “negative” adjectives added to the charges are not just for convenience. In fact, the positive and negative are to be treated algebraically, so that net amount of charge produced in any process is zero. This is summarized as law of conversation of charges produced in a process is zero”.

In our previous discussions we saw that the glass rod is positively charged when rubbed with silk. Simultaneously, the silk also develops negative charge on it. The amount of positive charge developed on the glass rod is exactly equal to the amount of negative charge developed on the silk. Thus total charge on the rod and silk is again zero, which was the case before rubbing. This shows that charges are conserved.

ii)             Like charges repel and unlike charges attract each

Other. A charge is a scaler quantity. Since there are two types of charges the addition must be carried with proper sign. For example if there are various charge distributed over a body, then the total charge on the body will be the sum of the individual charge taken with their proper sign. If +4C, -2C, -5C, +6C (C stands for coulomb, unit of charge on SI) are added the total or net charge is +3C. Addition of charges is similar to the addition of positive and negative numerals.

iii)          Charges are quantized. Scientist like R. A. Millikan

Have shown experimentally that the smallest amount of electric charge is found on an electrons and protons. All other electric charge is simply integral multiples of this smallest amount. This fact is called quantization of electrical charge. The charge of the proton is given by symbol ‘e’ and its value in SI units is +1.6×10ˉ19 C. So, charges on an electrons would be -1.6×10ˉ19 C. (More about it in Modern physics part in class XII). Charges, which are fraction of charge on an electrons or a protons gas not been ordinarily observed in nature.    

    

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ELECTRIC CHARGE OF ITS NATURE

It is a common experience that a comb, after running through dry hair a few times, can attracts small dust particles or small pieces of paper. Similarly if we rub our hands on silk vest it would also attract small pieces of paper of dust particles, feather etc. When we take off acrylic sweater in a dark room we can see the sparks flying. All these observation demonstrate charging of friction.

To study the nature of charge closely let us take a glass rod, rub it with silk clothes and hang it freely by a silk thread. Let us take another glass rod and rub it with silk cloth as in the first case and take it towards the first rod. The suspended rod will swing away. This means that the two charged ends repel one another.

In the next experiment, take a glass rod, rub it with silk and suspended it as before. Take a hard rubber rod, rub it with woolen clothes and bring it near to the suspended glass rod. It is observed that they attract each other.

Here the charges on the glass rod are same in nature because both the glass rod were rubbed with the same material-silk. It is seen that, when brought closer, they repel. So, we conclude that similar charges repel. Charges developed on the rubber rod behave differently- they attracts the glass rod. Therefore, we conclude that charges on the glass rod are different in nature that on the rubber. Also unlike charges attract.

To investigate it further, take a rubber rod, rub it with the woolen cloth and hang it by a silk thread. Bring another rubber rod, rubbed with woolen clothes, near to the suspended rod will be repelled.

This type of experimental has been done with different set of materials. They revel that two different kinds of electrical charges are produced. Two bodies carrying the same kind of electrical charge repel one another. On the other hand two bodies carrying different type of electrical charge attract one another. Thus we conclude that charges repel; unlike charge attract.

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ELECTIC CHARGE OF ITS NATURE

INTRODUCTION

Today, the word electricity is very mush familier to all of us and everybody knows about it use of electrical appliances like motors, fans, heater, radio, television etc. These function on aspects of electricity and magnetism. In this chapter we will study the nature of electrical charges.

Historically study of electricity goes back to 6’th century B.C. Thales, a famous Greek philosopher, was first to notice the attraction caused when different substance were rubbed against each other. He noticed that when the piece of amber, a yellow resinous substance found on the shores of Baltic sea and used for decoration, was rubbed against silk or woolen cloth is attracted light dust particles, lint, light feathers, pieces of leaves etc. This was the modest beginning of science of electricity. However, a concerned interest was shown to this phenomenon only in the 16’th century. DR. Gilbert, a British scientist, reported that the property of attracting light things was acquired due to friction. Bodies having this property were said to be’ electrified’. When the substance is electrified it is said to possess electric charge.

ELECTRIC CHARGE

Just as mass does a property of all matter and causes gravitational attraction, matter is composed of particles possess electric charges that have electric and magnetic interactions.

Electric charge is a physical quantity and it can be measured. However, effect of electrical charges in an object is not observed normally. This is because there are two different kinds of charges, which generally balance each other in any matter. Therefore, matter is electrically neutral and electrical effects and absent. This balance is disturbed when as excess of one or other kinds of charges is created and only then we observe electrical effects.

  

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