ELECTRIC CHARGE

Historically the credit of discovery of the fact that amber rubbed with wool or silk cloth attracts light objects goes to Thales of Miletus, Greece, around 600 BC. The name electricity is coined from the Greek word elektron meaning amber. Many such pairs of materials were known which on rubbing could attract light objects like straw, pith balls and bits of papers. You can perform the following activity at home to experience such an effect. Cut out long thin strips of white paper and lightly iron them. Take them near a TV screen or computer monitor. You will see that the strips get attracted to the screen. In fact they remain stuck to the screen for a while. 

FIGURE 1. Rods and pith balls: like charges repel and unlike charges attract each other.

It was observed that if two glass rods rubbed with wool or silk cloth are brought close to each other, they repel each other [Fig. 1(a)]. The two strands of wool or two pieces of silk cloth, with which the rods were rubbed, also repel each other. However, the glass rod and wool attracted each other. Similarly, two plastic rods rubbed with cat’s fur repelled each other [Fig. 1(b)] but attracted the fur. On the other hand, the plastic rod attracts the glass rod [Fig. 1(c)] and repel the silk or wool with which the glass rod is rubbed. The glass rod repels the fur.

If a plastic rod rubbed with fur is made to touch two small pith balls (now-a-days we can use polystyrene balls) suspended by silk or nylon thread, then the balls repel each other [Fig. 1(d)] and are also repelled by the rod. A similar effect is found if the pith balls are touched with a glass rod rubbed with silk [Fig. 1(e)]. A dramatic observation is that a pith ball touched with glass rod attracts another pith ball touched with plastic rod [Fig. 1(f)].

These seemingly simple facts were established from years of efforts and careful experiments and their analyses. It was concluded, after many careful studies by different scientists, that there were only two kinds of an entity which is called the electric charge. We say that the bodies like glass or plastic rods, silk, fur and pith balls are electrified. They acquire an electric charge on rubbing. The experiments on pith balls suggested that there are two kinds of electrification and we find that (i) like charges repel and (ii) unlike charges attract each other. The experiments also demonstrated that the charges are transferred from the rods to the pith balls on contact. It is said that the pith balls are electrified or are charged by contact. The property which differentiates the two kinds of charges is called the polarity of charge.

When a glass rod is rubbed with silk, the rod acquires one kind of charge and the silk acquires the second kind of charge. This is true for any pair of objects that are rubbed to be electrified. Now if the electrified glass rod is brought in contact with silk, with which it was rubbed, they no longer attract each other. They also do not attract or repel other light objects as they did on being electrified.

Thus, the charges acquired after rubbing are lost when the charged bodies are brought in contact. What can you conclude from these observations? It just tells us that unlike charges acquired by the objects neutralise or nullify each other’s effect. Therefore, the charges were named as positive and negative by the American scientist Benjamin Franklin. We know that when we add a positive number to a negative number of the same magnitude, the sum is zero. This might have been the philosophy in naming the charges as positive and negative. By convention, the charge on glass rod or cat’s fur is called positive and that on plastic rod or silk is termed negative. If an object possesses an electric charge, it is said to be electrified or charged. When it has no charge it is said to be electrically neutral.

FIGURE 2. Electroscopes: (a) The gold leaf electroscope, (b) Schematics of a simple electroscope.

A simple apparatus to detect charge on a body is the gold-leaf electroscope [Fig. 2(a)]. It consists of a vertical metal rod housed in a box, with two thin gold leaves attached to its bottom end. When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and they diverge. The degree of divergance is an indicator of the amount of charge.

Students can make a simple electroscope as follows [Fig. 2(b)]: Take a thin aluminium curtain rod with ball ends fitted for hanging the curtain. Cut out a piece of length about 20 cm with the ball at one end and flatten the cut end. Take a large bottle that can hold this rod and a cork which will fit in the opening of the bottle. Make a hole in the cork sufficient to hold the curtain rod snugly. Slide the rod through the hole in the cork with the cut end on the lower side and ball end projecting above the cork. Fold a small, thin aluminium foil (about 6 cm in length) in the middle and attach it to the flattened end of the rod by cellulose tape. This forms the leaves of your electroscope. Fit the cork in the bottle with about 5 cm of the ball end projecting above the cork. A paper scale may be put inside the bottle in advance to measure the separation of leaves. The separation is a rough measure of the amount of charge on the electroscope.

FIGURE 3. Paper strip experiment.

To understand how the electroscope works, use the white paper strips we used for seeing the attraction of charged bodies. Fold the strips into half so that you make a mark of fold. Open the strip and iron it lightly with the mountain fold up, as shown in Fig. 3. Hold the strip by pinching it at the fold. You would notice that the two halves move apart. This shows that the strip has acquired charge on ironing. When you fold it into half, both the halves have the same charge. Hence they repel each other. The same effect is seen in the leaf electroscope. On charging the curtain rod by touching the ball end with an electrified body, charge is transferred to the curtain rod and the attached aluminium foil. Both the halves of the foil get similar charge and therefore repel each other. The divergence in the leaves depends on the amount of charge on them. Let us first try to understand why material bodies acquire charge.

You know that all matter is made up of atoms and/or molecules. Although normally the materials are electrically neutral, they do contain charges; but their charges are exactly balanced. Forces that hold the molecules together, forces that hold atoms together in a solid, the adhesive force of glue, forces associated with surface tension, all are basically electrical in nature, arising from the forces between charged particles. Thus the electric force is all pervasive and it encompasses almost each and every field associated with our life. It is therefore essential that we learn more about such a force.

To electrify a neutral body, we need to add or remove one kind of charge. When we say that a body is charged, we always refer to this excess charge or deficit of charge. In solids, some of the electrons, being less tightly bound in the atom, are the charges which are transferred from one body to the other. A body can thus be charged positively by losing some of its electrons. Similarly, a body can be charged negatively by gaining electrons. When we rub a glass rod with silk, some of the electrons from the rod are transferred to the silk cloth. Thus the rod gets positively charged and the silk gets negatively charged. No new charge is created in the process of rubbing. Also the number of electrons, that are transferred, is a very small fraction of the total number of electrons in the material body. Also only the less tightly bound electrons in a material body can be transferred from it to another by rubbing. Therefore, when a body is rubbed with another, the bodies get charged and that is why we have to stick to certain pairs of materials to notice charging on rubbing the bodies.

Example 1:

As shown in Figure (a), an electroscope consists of a steel ball connected to a steel rod, with very thin gold foil leaves attached to the bottom of the rod (in good electrical contact with the rod). The bottom of the electroscope is enclosed in a glass jar and held in place by a rubber stopper.

(a) The electroscope is brought near to but not touching a positively charged glass rod as shown in Figure (b). The foil leaves are observed to spread apart. Explain in detail, using as many diagrams as necessary.

(b) The electroscope is moved far away from the glass rod, and the steel ball is touched momentarily to a metal block. The foil leaves spread apart and stay spread apart when the electroscope is moved away from the block. As the electroscope is moved close to but not touching the positively charged glass rod, the foil leaves are observed to move closer together. Is the metal block positive, negative, or neutral? How do you know? Explain.

Answer:

(a) When the electroscope is brought close to, but not touching, the positively charged glass rod (as shown in Figure (b)), the following occurs:

1. Electrostatic Induction:

   • The positively charged glass rod induces a redistribution of charges in the electroscope.
   • Electrons within the electroscope are attracted toward the steel ball at the top (closer to the glass rod), leaving the lower part (the rod and the gold leaves) with a deficit of electrons, making them positively charged.

2. Gold Leaf Divergence:

   • Both gold leaves acquire the same positive charge, as they are in electrical contact with the positively charged rod.
   • Like charges repel each other, causing the gold leaves to spread apart.

(b) When the electroscope is moved away from the glass rod, and the steel ball is briefly touched to a metal block, the following happens:

1. Charge Transfer:

   • Upon contact with the metal block (a conductor), some of the electroscope's charge is transferred to the block.
   • If the gold leaves remain spread apart after the contact, it indicates that the electroscope retains a net charge, suggesting that it has either lost or gained electrons during contact with the block.

2. Behavior Near the Glass Rod Again:

   • When the electroscope is brought close to the positively charged glass rod again, the gold leaves are observed to move closer together.
   • This indicates that the electroscope now carries

Example 2:

When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Answer:

To explain how the observation of charges appearing on both a glass rod and a silk cloth when rubbed together is consistent with the law of conservation of charge, we can break down the explanation into several steps:

1. Initial State of Charges:

   • Before rubbing, both the glass rod and the silk cloth are neutral. This means that the total charge on both objects is zero.
   • Mathematically, we can express this as: $$Q_{\text{glass}} + Q_{\text{silk}} = 0$$

2. Rubbing the Rod and Cloth:

   • When the glass rod is rubbed with the silk cloth, electrons (which carry a negative charge) are transferred from the glass rod to the silk cloth.
   • This transfer of electrons results in a change in the charge of both objects.

3. Charge Transfer:

   • Let’s assume that 2 electrons are transferred from the glass rod to the silk cloth.
   • The glass rod loses these electrons, leading to a deficiency of negative charge, thus becoming positively charged.
   • The silk cloth gains these electrons, resulting in an excess of negative charge, thus becoming negatively charged.

4. New Charges:

   • After rubbing, the new charge on the glass rod can be represented as: $$Q'_{\text{glass}} = +2e$$
   • The new charge on the silk cloth can be represented as: $$Q'_{\text{silk}} = -2e$$

5. Final State of Charges:

   • Now, we can express the total charge after rubbing: $$Q'_{\text{glass}} + Q'_{\text{silk}} = +2e + (-2e) = 0$$
   • This shows that the total charge remains zero, which is consistent with the initial state.

6. Conservation of Charge:

   • According to the law of conservation of charge, charge cannot be created or destroyed; it can only be transferred from one body to another.
   • Since the total charge before and after rubbing remains the same (zero), this observation is consistent with the law of conservation of charge.

Conclusion: Thus, the process of rubbing the glass rod with the silk cloth demonstrates that while charges appear on both objects, the total charge in the system remains constant, affirming the principle of conservation of charge.

Share :