Chapter One: static electricity

Introduction

Clothes containing nylon often crackle when they are taken off the body. Brushing a latex ballon against one’s face results to the balloon trying to stick to the face. This is due to static electricity which is a result of an imbalance of electric charges within or on the surface of a material. The charge remains until an electric discharge happens, hence the name static electricity. In this chapter, you will learn the concept of static electricity, origin of charges, fundamental law of static electricity and detection of charges. You will also learn about conductors and insulators, capacitors, charge distribution along the surface of a conductor and lightning conductor. The competencies developed in this chapter will enable you to protect buildings from lightning and use electrostatic precipitators to easily remove unwanted particles from air.

Concept of static electricity

Have you ever noticed that a nylon garment produces a crackling sound as it is taken off the body? Sometimes, tiny sparks are seen when undressing in the dark. The crackling sound is caused by small electric sparks as a result of charge discharge. The charge is caused by friction between the nylon and your skin, giving rise to static electricity. Pens and combs made up of certain plastics could also attract tiny pieces of paper after being rubbed on the hair or on synthetic clothing material, as shown in Figure 1.1.

Figure 1.1: Charged comb attracts pieces of paper

Static electricity is the electricity resulting from the accumulation of stationary electric charges on a material that does not conduct electricity.

The effects of static electricity

The effects of static electricity are familiar to most people because you see, feel and even hear sparks as excess charge is neutralised by being brought close to a region with an excess of the opposite charge. For example, when a dry cloth is used to clean a glass window, tiny pieces of cloth and dust adhere to the glass. When you rub feet on a carpet and then touch a metallic object, you experience a slight electrical shock. An aircraft in flight may accumulate charges on its outer surface because it rubs against the air. These effects of static electricity are pronounced under dry conditions because water conducts charges away.

All these examples and many others demonstrate static electricity. In each case, objects are electrically charged and therefore exert electrostatic force on each other. The charge acquired remains on the surface of the body, i.e. does not move, and so, it is called electrostatic charge. Electrostatic charges are stationary in nature. The rubbing only transfer, for example, negative charges from fur to an ebonite rod, leaving the fur with an equal amount of excess positive charge. Note that the rod initially has an equal number of positive and negative charges.

Electrostatics is the study of stationary electric charges.

Electrostatic force

The interaction between electric charges that are at rest (or nearly at rest) result to a force known as an electrostatic force. After charging plastic rods by rubbing them with a piece of fur, it is observed that the rods repel each other. Similarly, when glass rods are rubbed with silk, they become charged and repel each other. Furthermore, the plastic rods and the fur attract each other, and the glass rods and the silk attract each other.

A French physicist Charles Coulomb measured the force generated between two charged objects using a torsional balance. His work resulted in the development of a unit of electrical charge, named in his honour, the coulomb.

Activity 1.1

Aim: To demonstrate the existence of electrostatic force.

Materials: Plastic pen, plastic comb, tissue paper, human hair

Procedure

1. Tear a sheet of tissue paper into several small pieces and lay them on a table.
2. Rub a plastic pen or comb through your hair.
3. Bring the pen or comb near the pieces of paper but not touching them as shown in Figure 1.2.
4. Hold the pen or comb for 15 seconds.
5. Record you observations.

Questions

(a) What happens when the comb is brought near the pieces of paper?

(b) What happens to the pieces of paper on the comb or pen after a few seconds?

(c) Discuss your observations.

Figure 1.2: Charged plastic pen attracts pieces of paper

The comb picked up pieces of paper. The papers then dropped off after some time. The pen behaved in a similar manner. This shows that a plastic pen or comb rubbed with human hair acquires a charge that is able to attract other substances such as pieces of paper. After a few seconds pieces of paper fall off because they acquire similar charges as the comb or pen.

Origin of charges

When a glass rod is charged by rubbing it with fur or silk, no visible change in the appearance of the rod is observed. What happens to the rod when charging it? The answer to this question is obtained by understanding the structure of the atom, which is the building block of all matter.

All materials are made up of tiny particles of matter called atoms. The structure of the atom can be described in terms of the negatively charged particle (electron), the positively charged particle (proton), and the uncharged particle (neutron). The protons and neutrons in an atom make up a small, very dense core called the nucleus. Electrons surround the nucleus in shells. Figure 1.3 illustrates the structure of an atom.

Figure 1.3: Structure of a two-electron atom

An atom has an equal number of electrons and protons. Such an atom is said to be electrically neutral. An atom with more protons than electrons is positively charged, whereas the atom with more electrons than protons is negatively charged. Figure 1.4 illustrates the neutral, positively charged and negatively charged atoms.

Figure 1.4: Neutral atom, positively charged atom and negatively charged atom.

The number of protons in the nucleus of an atom is called the atomic number of an atom element. If one or more electrons are removed from a neutral atom, what remains is called a positive ion. On the other hand, a negative ion is an atom that has gained one or more electrons. The gain or loss of electrons is called ionisation. When the total number of number of protons in a body equals the total number of electrons, the total charge is zero, and the body is electrically neutral. To give a body an excess negative charge, you may add negative charge to a neutral body. Similarly, An excess positive charge can be created by removing negative charge. In most cases, because of their high mobility, negatively charged electrons are added or removed from an object. Thus, a positively charged body is one that has lost some of its electrons while a negatively charged body is the one that has gained excess electrons. When speaking about the charge of a body, it always means its net charge which is always a very small fraction of the total positive charge or negative charge in the body.

Electric charge is a property of matter that governs how matter behaves in an electric or magnetic field.

The SI unit of electric charge is the coulomb (C). The electron has the smallest amount of electric charge. The charge on the electron equals to -1.6 x 10^-19 C. The proton has a charge of the same magnitude as that of the electron. Therefore, the charge on the proton is +1.6 x 10^-19 C. Any other amount of charge is a multiple of 1.6 x 10^-19 C.

A coulomb is a much larger quantity of charge than that normally produced by rubbing. Hence, it is more convenient to use micro-coulombs in measuring charge.

I µC = 10^-6 C

Charging objects

There are two types of charge, namely negative and positive charges. The total number of positive and negative charges within a neutral material is the same. Thus, the net charge on the material is zero. When two materials are rubbed against each other, electrons may be transferred from one material to the other. This upsets the balance between the opposite charges within each material, leaving each with a net negative or positive charge. for example, when rubbing an ebonite rod with fur, the electrons from the fur are transferred to the ebonite rod. This results to less electrons in the fur, which becomes positively charged. On the other hand, the ebonite rod gains excess electrons and hence becomes negatively charged. Similarly, when a glass rod is rubbed with silk cloth, glass loses electrons and becomes positively charged, and the silk cloth gains excess electrons to become negatively charged. Figure 1.5 shows charges on a glass rod and silk cloth before and after rubbing.

Figure 1.5: Rubbing glass rod with silk

Another example is when one rubs a ballon with a woollen cloth. Electrons are transferred from the woollen cloth to the ballon. This makes the ballon to have excess electrons and the woolen cloth to have less electrons. Thus, the ballon will have net negative charge while the woollen cloth will have net positive charge. Figure 1.6 shows charges on the balloon and woollen cloth before and after rubbing.

Figure 1.6: Rubbing balloon with woollen cloth

The summary of acquisition of charges by rubbing for some materials is given in Table 1.1

Table 1.1: Acquisition of charges by different materials

In ancient times, experiments on static electricity involved the rubbing of a glass rod with silk to obtain a positive charge. Similarly, an ebonite rod was rubbed with fur so as to obtain a negative charge. Nowadays, scientists prefer cellulose and polythene for positive and negative charges respectively. This is because the materials are less affected by damp conditions.

Activity 1.2

Aim: To show the existence of opposite charge.

Materials: Water, plastic pen, plastic comb, human hair

Procedure

1. Take the pen or comb and rub it on your hair.
2. Bring it near a small slow stream of water coming from a tap as shown in Figure 1.7 (a).

Questions

(a) What did you observe?

(b) What types of charges were in water?

Figure 1.7: Slow stream of water is attracted to the comb

Electrostatic attraction between water and a charged comb causes water stream to bend as shown in Figure 1.7 (b). The water particles are attracted to the comb, causing the steady stream of water to bend. This means that the comb has charge that is opposite to that in the water.

Fundamental law of static electricity

The fundamental law of static electricity, also referred to as the first law of electrostatics, states that:

Like charges repel, unlike charges attract.

Activity 1.3

Aim: To verify the fundamental law of static electricity.

Materials: 2 dry glass rods, 2 ebonite rods, thread, stand, silk cloth, fur (cotton cloth), stirrup wire (metal ring)

Procedure

1. Rub a dry glass rod with silk cloth and place it in a metal ring suspended by a piece of thread.
2. Bring a second charged glass rod close to the suspended one as shown in Figure 1.8.
3. Record your observations.
4. Repeat steps 1 and 2 but using ebonite rod rubbed with fur.
5. Now, repeat the activity by bringing a charged glass rod close to the suspended charged ebonite rod and the charged ebonite rod close to the suspended charged glass rod.
6. Record your observations.

Questions

(a) What did you observe?

(b) What conclusion can you make from the observation?

Figure 1.8: Verification of the fundamental law of electrostatics

If two negatively charged materials or two positively charged materials are brought near each other, they repel. However, if a positively charged object is placed near a negatively charged object, the objects attract each other. This suggests that unlike charges exert an attractive force on each other and like charges exert a repulsive force on each other.

Activity 1.4

Aim: To show charge separation in materials.

Materials: Woollen cloth, glass rod, cotton fabric, plastic pen or comb, string, retort stand

Procedure

1. Take a pen or comb and hang it from a string tied to a retort stand.
2. Rub the comb with a piece of cotton fabric, then hang it.
3. Rub a glass rod with a piece of woollen fabric, then hang it.
4. Bring the charged glass rod near but not touching the hanging charged comb.
5. Bring the piece of woollen cloth that you used to rub the glass rod near but not touching the hanging comb.

Questions

(a) Which material will become positively or negatively charged in steps 2 and 3? (Refer to Table 1.1.)

(b) What do you observe in steps 4 and 5?

(c) Discuss your results.

The comb becomes negatively charged while the glass rod becomes positively charged. The comb and the glass rod attract each other. The woollen cloth and the comb repel each other. If objects are far apart, there is no interaction, or if any, very little.

It is worth noting that the presence of an excess charge induces a charge separation in nearby objects. Therefore, the strength of the attractive and repulsive forces decreases quite rapidly with increase in distance. This suggests that the electrostatic force weakens as the charges get far apart and grows strong as they get close to each other.

Methods of charging a body

A body consisting of an equal number of positive and negative charge is said to be electrically neutral. Such a body can become electrically charged by friction, contact and induction.

Charging by friction

This is also known as electrification by rubbing as illustrated by examples in page 4 and 5.

The examples show that, electrons can be exchanged between materials through friction as a result of rubbing. Materials whose electrons are weakly bound tend to lose them while those with sparsely filled outer shells tend to gain the electrons.

Charging by contact

Considering two metal plates X and Y, plate X is positively charged while plate Y is uncharged. Both plates are then placed on insulating blocks brought in contact and then separated as shown in Figure 1.9.

Figure 1.9: Charging by contant

Testing the metal plates after separation shows that, when a charged body is brought in contact with an uncharged body, the same electric charges distribute among the two bodies. As a result, the initially uncharged body acquires charges of the same sign as the charges of the initially charged body.

Charging by induction

Charging by induction is a method used to charge an object without actually touching the object with any other charged object. In order to obtain a charge of a given sign, the inducing charge must be of an opposite sign. Induction induces opposite charge to the uncharged body by bringing a charged body nearby an uncharged body. See Figure 1.10. Notice that in this example the negative charge in the body is repelled to one side. Connecting the body to the ground by a conducting wire allows the negative charge to drain to the ground. On removing the charging rod and the conducting wire, the body is left with net positive charge.

Figure 1.10: Charging