Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
Electric charge and its types
In Class VII we have studied about winds, storms and cyclones. We learnt that cyclones are a destructive natural phenomenon which can cause a lot of damage to human life and property. We also learnt that we can protect ourselves from cyclones to some extent by taking timely action. In this Chapter we will discuss two other destructive natural phenomena called lightning and earthquakes. We will also learn about the various steps which can be taken to minimise the destruction caused by lightning and earthquakes. Before we discuss lightning, we should first learn about electric charges’. This is because lightning is an electric spark on a huge scale which is caused by the accumulation of electric charges in the clouds. Electric charge is the property of matter which is responsible for electrical phenomena. Electric charge exists in two forms: positive electric charge and negative electric charge. Let us discuss electric charge in detail.
About 2500 years ago, a Greek scientist called Thales observed that when a material known as ‘amber’ was rubbed with a silk cloth, it started attracting tiny feathers. Thales said that amber acquires electric charge (or electricity) on rubbing with silk. And the force of this electric charge attracts the tiny feathers. We can also show the existence of electric charges (or electricity) by performing some simple activities as follows.
Activity 1
We rub a plastic comb with our dry hair and bring it near tiny pieces of paper. We will find that the comb attracts the tiny pieces of paper towards itself. Due to this attraction, the tiny pieces of paper move towards the comb and stick to it (see Figure). Actually, when we rub the plastic comb with our dry hair, the plastic comb gets electric charge due to friction. The electrically charged comb then exerts an electric force on the tiny pieces of paper and attracts them. We can also take a plastic scale (in place of plastic comb). We will find that a plastic scale rubbed in
dry hair attracts tiny pieces of paper. Similarly, if we rub one end of a pen made of plastic with a sheet of paper (or even with our dry hair) for about one minute and then bring it near tiny pieces of paper, it attracts the tiny pieces of paper towards itself. Due to this attraction, the tiny pieces of paper stick to the pen. Here also the plastic pen gets electric charges on rubbing with a sheet of paper (or dry hair). The electrically charged pen then exerts an electric force on the tiny pieces of paper and attracts them.
A yet another example is that of a balloon. If we take an inflated rubber balloon, rub it carefully with a piece of woollen cloth and then touch it with a wall, the balloon sticks to the wall. In this case, when we rub the rubber balloon with a woollen cloth, the rubber balloon gets electric charges due to friction. This electrically charged balloon then exerts an electric force of attraction on the wall due to which it sticks on the wall. The electric charges which appear on a plastic comb or plastic scale (rubbed with dry hair), on plastic pen (rubbed with a sheet of paper) and on a rubber balloon (rubbed with a piece of woollen cloth) are the static electric charges (or stationary electric charges) which remain bound to the surface of an object and do not move. The static electric charges are also known as charges at rest. In this Chapter we will be dealing with static electric charges. We will now discuss uncharged objects and charged objects. Please note that whether we use the word ‘object’ or ‘body’ in these discussions it will mean the same thing.
Uncharged and Charged Objects
An object having no electric charge on it is called an uncharged object. An uncharged object does not have any effect on other objects. An object having electric charge on it is called a charged object. A charged object attracts other uncharged objects. This point will become clear from the following example. If we take a glass rod and bring it near some tiny pieces of paper, it will not have any effect on them. If, however, the glass rod is first rubbed with a piece of silk cloth and then brought near the tiny pieces of paper, then the glass rod attracts the tiny pieces of paper towards itself.
These observations can be explained by saying that initially the glass rod is electrically neutral or uncharged (having no electric charge), so it has no effect on the tiny pieces of paper. But when the glass rod is rubbed with silk cloth, then it gets electric charge. The electrically charged glass rod exerts a force on the tiny pieces of paper and hence attracts them. From this example we find that a glass rod rubbed with silk acquires the ability to attract small, uncharged pieces of paper. The objects showing this effect (of attracting other objects) are said to be electrically charged or just charged. The process of giving electric charge to an object is called charging the object.
Charging an Object by Rubbing (or Friction)
The simplest method of charging an object is to rub it with another suitable object (such as silk cloth, woollen cloth, hair, paper or polythene, etc.). When an object is rubbed with another object, then there is friction between them. This friction charges the object. The charging of an object by rubbing it with another object is called charging by friction. The charging of a glass rod by rubbing it with a silk cloth is an example of charging by friction. Here are some more examples of charging by friction. When a plastic comb is rubbed with dry hair, the plastic comb acquires an electric charge due to friction. The plastic comb gets electrically charged.
The electrically charged comb then attracts tiny pieces of paper. Similarly, when a ballpoint pen refill is rubbed vigorously with a piece of polythene, the refill acquires electric charge by friction. The refill is said to be electrically charged. The charged refill can attract tiny pieces of paper kept near it. And when an inflated rubber balloon is rubbed with a piece of woollen cloth, it gets charged due to friction. The electric charges generated by rubbing (or friction) are static electric charges. These electric charges remain bound on the surface of the charged object. They do not move by themselves.
Please note that all the insulator objects (like glass rod, plastic comb, plastic scale, plastic straw, ballpen refill and rubber ballon, etc.) can be charged by rubbing while held in hand. This is because being insulators (non-conductors), they do not conduct electric charges produced on their surface through our hand and body into the earth. On the other hand, conducting objects made of metals (like a steel spoon) cannot be charged by rubbing while held in hand. This is because as soon as a conductor (like a steel spoon) gets charged by rubbing with another material, the electric charges produced on its surface flow through our hand and body into the earth.
And the conductor object (like a steel spoon) remains uncharged. A metal object (like a steel spoon) can be charged by rubbing only when held by an insulating material like polythene, etc. Copper, aluminium and iron are also metals. So, a copper rod, an aluminium rod or an iron rod also cannot be charged by friction, if held by hand. Please note that our skin is a conductor of electricity which allows electric charges to move through it. But since the electric charges on insulators remain bound to their surface, they cannot reach the skin of our hand.
Friction Charges Both the Objects Which are Rubbed Together
When two objects are rubbed together, then both the objects get charged by friction (but with opposite charges). For example, when a glass rod is rubbed with a silk cloth, then both, the glass rod as well as the silk cloth get charged. The charged glass rod can attract tiny pieces of paper and the charged silk cloth can also attract tiny pieces of paper. The electric charges acquired by glass rod and silk cloth are, however, opposite in nature. As a convention, the electric charge acquired by a glass rod (rubbed with silk) is called positive charge.
So, in this case, the glass rod acquires a positive electric charge whereas the silk cloth gets a negative electric charge. Similarly, when we rub a ballpoint pen refill with a piece of polythene, then both, the refill as well as polythene get charged but with opposite charges. The ballpoint pen refill acquires a positive charge whereas polythene gets the negative charge. Please note that all the objects made of clear plastic called acrylic plastic (such as ballpen refill and clear plastic scale, etc.) get positive charge on rubbing. On the other hand, all the objects made of polythene plastic (such as polythene sheet and polythene plastic comb, etc.) get negative charge on rubbing.
When we rub a plastic comb in dry hair, then both, the comb and hair get charged but with opposite charges. In this case, the plastic comb gets negative charge whereas the hair get positive charge. Again when we rub an inflated rubber balloon with a piece of woollen cloth, then both, the balloon as well as woollen cloth get electrically charged but with opposite charges.
The rubber balloon gets negative electric charge whereas the woollen cloth acquires positive electric charge. In fact, when any two objects are rubbed together, then both the objects get charged but with opposite charges. If one object gets positive charge then the other object will get negative charge. Now, which object will get positive charge and which object will get negative charge, depends on the nature of materials of which the two objects are made.
How Rubbing Charges Various Objects
All the objects (like glass rod, silk cloth, rubber balloon, woollen cloth, plastic comb, hair, ballpoint pen refill, polythene, etc.) are made up of tiny particles called atoms. All the atoms contain two types of electric charges inside them : positive electric charges called protons and negative electric charges called electrons. In an uncharged object, the number of positively charged particles (protons) and negatively charged particles (electrons) in the atoms are equal. The equal number of positive and negative electric charges balance each other and make the object electrically neutral (having no over-all charge).
The positively charged protons are held strongly inside the nucleus of atoms, so protons cannot be transferred from the atoms of one object to another object during rubbing. But the negatively charged electrons are held loosely in the atoms (away from the nucleus), therefore, some of the electrons can be transferred from the atoms of one object to another object by rubbing.
- The object which loses negatively charged electrons during rubbing, acquires a positive electric charge (because then the number of positive protons in it becomes more than the number of negative electrons).
- The object which gains negatively charged electrons during rubbing, acquires a negative electric charge (because then the number of negative electrons in it becomes more than the number of positive protons).
Whether an object will lose electrons or gain electrons during rubbing depends on the nature of material of the object. The materials like glass, woollen cloth, hair and ballpen refill lose electrons more easily and hence get positively charged on rubbing. On the other hand, the materials like silk, rubber balloon, plastic comb and polythene gain electrons more easily and hence get negatively charged during rubbing.
Types of Electric Charges and Their Interactions
It has been shown by experiments that there are two types of electric charges: positive charges and negative charges. A positive charge repels another positive charge, but a positive charge attracts a negative charge. Similarly, a negative charge repels another negative charge, but a negative charge attracts a positive charge. We will now describe some activities to show the existence of two types of electric charges and their interactions.
Activity 2
(i) Take two rubber balloons and inflate them. Hang the two inflated balloons with long threads in such a way that though they are closeby but they do not touch each other [see Figure (a)]. Rub both the balloons with a woollen cloth and release them.
We will see that the two balloons move apart as if they are pushing away each other (or repelling each other) [see Figure (b)]. In this activity, we have brought close together two balloons which are made of the same material (rubber) and rubbed them with the same material (woollen cloth), so the two balloons must have acquired the same type of electric charges or similar electric charges.
Since two similarly charged balloons repel each other, we conclude that similar charges repel each other.
The same type of electric charges or similar electric charges are also called like electric charges. So, we can also say that like charges repel. Actually, when the two balloons are rubbed with a woollen cloth, they acquire negative electric charges. The two negatively charged balloons hung near each other exert a force of repulsion on each other and hence move apart (away from each other) [see Figure (b)].
Activity 3
Let us now take two used ballpoint pen refills. Rub one refill with a piece of polythene to charge it. Place this charged refill in a glass beaker carefully (see Figure). Now rub the other ballpoint pen refill also with a piece of polythene to charge it. Bring this charged refill near the first charged refill placed in the beaker. We will see that the charged refill placed in the beaker moves away from the charged refill held in our hand as if they are repelling each other (see Figure).
In this case, both the ballpoint pen refills are made of the same material (acrylic), and both of them have been rubbed with the same material (polythene), so their electric charges should also be of the same type (or similar). Now, since two similarly charged ballpoint pen refills repel each other, we conclude that similar charges repel each other. In other words, like charges repel. Actually, when the two ballpen refills are rubbed with polythene, they acquire positive electric charges. The two positively charged ballpen refills brought near each other exert a force of repulsion on each other and hence move away from each other (see Figure).
Activity 4
Rub a ballpoint pen refill with polythene and place this charged refill in a beaker as before. Take a balloon and also charge it by rubbing with a piece of woollen cloth. Now, hold the charged balloon in your hand and bring its charged end near the charged refill placed in the beaker. We will see that the charged refill moves towards the charged balloon as if it is being attracted by the charged balloon (see Figure).
In this case the ballpen refill and balloon are made of two different materials (acrylic and rubber), and they have been rubbed with different materials (polythene and woollen cloth), so their electric charges must be different (or unlike). Now, since a charged balloon and a charged ballpen refill having unlike charges (or different charges) attract each other, we conclude that unlike charges attract each other.
Unlike charges are also called opposite charges, so we can also say that opposite charges attract each other. Actually, when a balloon is rubbed with a woollen cloth, it acquires a negative
charge and when a ballpen refill is rubbed with polythene, it gets a positive charge. So, in this case, the negatively charged balloon attracts a positively charged ballpen refill.
From the above discussion we conclude that depending on the nature of their electric charges, two charged objects may attract or repel each other.
- If one object has positive charge and the other object has negative charge, then the two objects attract each other. In other words, a positive charge and a negative charge attract each other.
- If the two objects have positive charges, they repel each other. In other words, two positive charges repel each other.
- If the two objects have negative charges, they repel each other. In other words, two negative charges repel each other.
Many times when we take off woollen or synthetic clothes (like polyester and nylon clothes), our body hair stand erect on their ends. This is because rubbing (or friction) while taking off these clothes charges the body hair with the same kind of electric charge. Due to their like charges, the body hair repel one another. This repulsion makes the body hair stand erect. We will now answer a question taken from the NCERT book.
Example Problem.
When a charged glass rod (rubbed with silk cloth) is brought near a charged plastic straw (rubbed with polythene), there is attraction between the two. What is the nature of charge on the plastic straw? (NCERT Book Question)
Answer.
By convention, the charge present on a glass rod (rubbed with silk cloth) is said to be positive (+). There can be attraction between a charged glass rod and a charged plastic straw only if their charges are opposite to each other. The opposite of positive charge is negative charge. So, the nature of charge on the plastic straw is negative (-).
Why a Charged Object Attracts an Uncharged Object
A charged object attracts an uncharged object by producing opposite charges in the nearer end of the uncharged object by electric induction. As an example, we will explain how a charged plastic comb attracts an uncharged piece of paper. Suppose a negatively charged plastic comb is held over a small piece of paper (see Figure). The negatively charged plastic comb produces opposite charges (positive charges) in the top end of paper (which is nearer to it) by electric induction (see Figure).
This is because negative charge of plastic comb repels the electrons from the top side of paper to its bottom side, leaving the top of paper positively charged. The plastic comb has negative charge and the top of paper has now positive charge. The attraction between opposite charges (negative charge on plastic comb and positive charge on top of paper) results in the paper being attracted by the plastic comb. Please note that even a positively charged glass rod will attract the uncharged piece of paper by inducing opposite charges (negative charges) in the top end of the paper.
So far we have discussed the charging of various objects. We will now describe how the electric charge on an object is detected (even if the electric charge is very weak). The electric charge on an object can be detected by using an instrument called electroscope. We will now discuss the electroscope.