ELECTROSTATICS

Keywords

• Electrical field
• Electrical potential
• Electrostatics
• Magnetic field

By the end of this chapter, you will be able to:

(a) Understand everyday effects of static electricity and explain them in terms of the build – up and transfer of electric charge.

 (b) Apply the knowledge of electrostatic charge to explain the operation of devices like lightning conductors.

(c) State the laws of electric charges.

(d) Describe methods of charging

Introduction

When one object is rubbed against another, both the objects will become charged. The charges remain at one place, Le, the changes are static. This branch of Physics which deals with the study of electric charges at rest is called electrostatics.

In this chapter, you will understand electrostatics and use it to explain lightning and other phenomena.

7.1 Charging a Body

 In their normal state, all the atoms in an object contain an equal number of electrons and protons. However, when one object is rubbed against other objects, a few electrons will move from one object to the other object. The object that received extra electrons has a greater number of electrons than protons. So, it becomes negatively charged. Similarly, the object that donates electrons loses a few electrons and has a smaller number of electrons than protons. So, it becomes positively charged.

A body can be given a charge (charged) by using a variety of methods and some of them are discussed below.

Charging by friction (rubbing)

Activity 7.1 charging a body

Key Question: How can one charge a body by rubbing?

 What you need

• A pen, paper,

What to do

1. Rub a pen in your hair (head) for about 5 minutes.
2. When you feel it is heated, stop rubbing and bring it closer to the small pieces of paper. Note down what you observe.
3. Discuss your findings to your friends and, finally, to the whole class.

The electrification of a body is the procedure by which a neutral body can become electrically charged. Charging a rod by rubbing does not create or make electricity. The rubbing simply transfers, for example, a negative charge from the rubber to the rod. This leaves the rubber with an unequal amount of positive charge.

Electrification by friction is the process of giving an object a net electric charge when it is rubbed on another object.

After we have charged both plastic rods in Figure 7.1 by rubbing them with the piece of fur, we find that the rods repel each other. When we rub glass rods with silk, the glass rods also become charged and repel each other. But a charged plastic rod attracts a charged glass rod, furthermore, the plastic rod and the fur attract each other, and the glass rod and the silk attract each other.

These experiments and many others like them have shown that there are exactly two kinds of electric charge: negative and positive charges

Figure 7.1 Experiments in electrostatics (a) Negatively charged objects repeat each other (b) Positively charged objects repel each other, (c) Positively charged objects and negatively charged objects attract each other

LAW OF ELECTROSTATICS

 Like in magnetic poles, there is always an interaction between electric charges. This interaction defines the law of electrostatics, which states that:

Two positive charges or two negative charges repel each other.

 A positive charge and a negative charge attract each other.

Or. Like charges repel, and opposite charges attract.

The interaction between electric charges is illustrated in Figure 7.2.

 Figure 7.2: Charging a spherical metal surface by induction

 Charging by contact

 Exercise 7.1

In groups, brainstorm the statement below and answer the questions.

1. Two neutral conducting pop cans are touching each other. A positively charged glass rod is brought near can X as shown below in Figure 7.3.
2. What happens when the cans are separated while the charged rod is still in contact with can X?
3. Describe what happens when the charged rod is made to touch can X
4. If the rod is removed and the cans are placed in contact again, what will happen?

Figure 7.3: Charging by contact

Figure 7.4: Interaction of charged bodies

Charging negatively an object by contact

 Charging negatively by contact involves touching a negatively charged object to a neutral object. Upon contact, electrons move from the negatively charged object onto the neutral object. When finished, both objects are negatively charged, as shown in Figure 7.5.

Figure 7.5: Charging negatively a neutral object by conduction

Charging positively object by conduction

What happens if a positively charged object is made to touch a neutral object? Study Figure 7.6 to provide an accurate response,

Figure 7.6: Charging positively a neutral object by conduction

When a body is charged by contact methods, it acquires charges that are similar to the ones on the charging rod.

7.2 Charging by Induction

Activity 7.2 Charging by induction

Key Question: How do you charge a body by induction?

What to do

In pairs, analyses case (a) – (c) in Figure 7.4 and answer the questions that follow.

 As shown in the figure above, the metal sphere was charged positively as indicated in part (c).

(i) Describe how the metal sphere was charged positively.

(ii) Why is it necessary to earth the sphere in the presence of the charged rod?

(iii) Share your findings with your friends and then finally with the whole class.

 Electrification by induction is the process of giving an object a net electric charge without touching the object to a second charged object.

Consider a negatively charged rubber rod and a neutral uncharged conducting sphere, as shown in Figure 7.7 (A). The negatively charged object is far away from the conducting sphere and does not affect it

Figure 7.7: Charging by induction

 As the two objects are brought close to each other, the negatively charged rod will attract positive charges and repel negative charges. That is, the region close to the rod will have an excess of positive charges, whereas the region farthest away will have an excess of negative charges. The resulting charge distribution is shown in Figure B.

1. What do you think will happen when the sphere is earthed?
2. If the earthing is removed in the presence of the rod, describe the electrical properties of the sphere.
3. Suppose the rod was removed while the sphere remained earthed, what would be the electrical property of the sphere?

This process of influencing a conducting material to acquire electric charges as described above is known as electric induction.

THE GOLD LEAF ELECTROSCOPE

An electroscope is often used in electrostatic experiments and demonstrations in order to test for charge and to deduce the type of charge present on an object. The electros typically consists of a conducting plate or knob, a conducting base and either a pair of conducting leaves or a conducting needle, as shown in Figure 7.8.

 Since the operating parts of an electroscope are all conducting, electrons are capable of moving from the plate or knob on the top of the electroscope to the needle or leaves in the bottom of the electroscope.

Figure 7.8 Electroscope

Charging a gold leaf electroscope (GLE)

Charging an electroscope by using a negatively charged rod

Suppose a needle electroscope is used to demonstrate induction charging. An aluminium plate is first charged positively by the process of induction. The aluminium plate is then held just above the plate of the electroscope.

Figure 7.9 charging an electroscope by Induction

Describe what is observed in diagrams (i ) , ( ii) , ( iii) , ( iv ) and ( v ) in Figure 7.9 .

The electroscope can be charged positively in the same way by using a negatively charged rod.

Assignment 7.1

This assignment can be done in pairs!

What you need

• Internet, Manila paper, markers, or you can use PowerPoint to make your presentation!

What to do

 (a) Using the materials provided, prepare a presentation on the following:

1. How a gold leaf electroscope (GLE) is charged.
2. How the build – up of charges on a GLE is transferred.
3. How opposite charges attract and like charges repel.
4. How to test the sign of the change of the body.

 Test for the sign of the charge on a body

 Objects are typically touched to or held near the plate or knob, thus inducing the movement of electrons into the needle or the leaves (or from the needle / leaves to the plate / knob). The gold leaves or needle of the electroscope are the only mobile parts. Once an excess of electrons (or a deficiency of electrons) is present in the needle or the gold leaves, there will be a repulsive effect between like charges, causing the leaves to repel each other or the needle to be repelled by the base that it rests upon. Whenever this movement of the leaves / needle is observed, one can deduce that an excess of charge ___either positive or negative is __present there. An increase in divergence occurs when the charge on the electroscope and the test charge are of the same kind.

It is important to note that the movement of the leaves and the needle never directly indicates the type of charge on the electroscope. It only indicates that the electroscope is detecting a charge.

 Exercise 7.2

1. Select the answers in their respective order.

 (a) A neutral metal sphere is touched by a negatively charged metal rod. As a result, the sphere will be___________ and the metal rod will be________

Charge distribution

 How does the charge distribution change if the surface of the conductor is NOT smooth and regular?

 In this case, there will be more charge where the surface curves more sharply, as shown in Figure 7.10 and a smaller build – up of charge at locations where the surface is less curved or flat.

As a result, the charge density at sharp points becomes large. Since like charges repel, the electrons are emitted rapidly from sharper points. This is called action at points.

Figure 7.10: Distribution of charges on a conductor

For smooth and regular surfaces, the charges are evenly distributed, as shown in the Figure 7.11.

Figure 7.11: Positive charges distributed evenly outside a circular conductor

APPLICATIONS OF ELECTROSTATICS

Electric discharge in air (lightning)

Figure 7.12: Electric sparks

 Lightning is a natural occurrence of electrostatic discharge during which two electrically charged regions in the atmosphere collide, causing electric sparks with the release of energy. It mostly occurs within the rain clouds during a storm. The random of the clouds during a storm enables the cloud particles to rub against each other. In this process, the rain particles get charged with the lighter hydrogen ions rising up into the sky, leaving the lower clouds negatively charged. When these negatively charged rain clouds collide during their random motions, they produce electric sparks as lightning with a resulting release of heat energy. This heat causes the surrounding air to rapidly expand and become unstable, which ends up exploding as thunder that we hear just after seeing the lightning flash.

Lightning conductors

As mentioned earlier, the rain clouds acquire negative charges by friction. During the lightning process, a lot of these negative charges are released into the atmosphere. The earth being a good receptor of the negative charges will attract the charges into itself through the shortest paths that can lead them there. Tall buildings and trees provide such short routes. This explains why lightning strikes tall trees. Therefore, it is extremely dangerous to shelter from the rain under tall trees. Also, do not take shelter near tall structures like telephone masks. Do not stand close to a window or talk on a telephone. Lightning can make windows shatter and can travel through wires. The best place to be during a thunderstorm is indoors. If you are outdoors, the safest place to be is inside a car.

To avoid the above from occurring, scientists protect tall structures by providing a route through which the negative charges can easily flow into the earth. This route is the lightning conductor.

 Assignment 7.2

 Figure 7.13 is a part of the roof of a certain house in Mayuge district. In groups, study the figure and answer the questions that follow.

Figure 7.13: Lightning conductor

1. What is the scientific name given to the spikes (pointed metal) installed on the roof of the house whose roof is indicated above?
2. State the implications of the spikes installed on the roof as shown in the figure above.
3. Briefly explain how the spikes work.
4. Discuss why in some regions of Uganda, people are affected by lightning more than in other regions.

A lightning conductor is a device which is intended to neutralize lightning. It is made of a thick copper strip fixed on a metallic plate placed in the ground , the other end , pointed ( sharp copper spikes ) , is placed above the roof of a building . During a storm, electric charges pass through the lightning conductor without damaging the building.

How to stay safe in a thunderstorm

 (i) If you are outdoors, avoid high ground and open spaces and all metal objects, including wires. Unsafe sites include trees, rain shelters or electricity pylons. Never touch the sides of a car.

(ii) If you are indoors, avoid water, and stay away from doors and windows. Do not use the telephone. Unplug and stay away from electrical appliances, computers and television sets.

 ELECTROSTATIC SPRAYING

 Metallic surfaces are painted using a spray gun. For example, cars are painted using this method. The car is usually earthed and the paint droplets coming out of the spray gun are given a positive charge. The car attracts these charged droplets of paint uniformly.

No paint is wasted as the negatively charged car attracts the positively charged paint, as shown in Figure 7.14

Figure 7.14 spraying a car

Research 7.1

 What you need

• Internet and computers.

 What to do

In pairs, carry out research about the following:

 (a) (i) the occurrence of lightning in some districts in Uganda.

(ii) Causes of lightning.

 (iii) How one can stay safe during a thunderstorm.

 (b) Write a comprehensive report about your findings and present it to your friends and finally to the whole class.

 7.3 Electric Field

An electric charge sets up an electric field in the space surrounding it and an electric force is exerted on any charged body placed in the field.

Did you know that an electric field is a region around an electric charge where an electric force is experienced?

 An electric field is shown by lines of force with an arrow on it. For a positive charge, the arrow is away from the centre of the charge and for a negative charge, the arrow is into (moves to the centre of charge) the charge, as shown in Figures 7.15 and 7.16.

A neutral point is a region in an electric field where the resultant electrostatic force is zero.

Activity 7.3 Drawing field lines

In groups, try out these tasks:

1. Draw positive and negative charges.
2. With your group members, discuss how field lines should be put on the charges, and hence draw them on the charges.
3. From lines of field drawn on to charges, can you define an electric field?
4. Share your answers with the rest of the class members.

Uniform field

A uniform field is one which has the same magnitude and direction at all points. This is the case for the electric field between two parallel – plate carrying charges which have opposite signs, as shown in Figure 7.17.

 Figure 7.17: Field lines in a uniform field

Chapter Summary

In this chapter, you have learnt that:

1. Electrostatics is the study of electric charges as they build up in various locations and interact with each other.
2. Static electricity is a build – up of electric charge in a particular location. It is more appropriately called electrostatic charge
3. Conduction is the process of charging an object by direct contact with a charged object.
4. Induction is the process of charging an object by bringing a charged object near.
5. Friction is the process of charging an object by “scraping “electrons from one object onto another.
6. Electric field – is a force field that exists in the space surrounding every electric charge or group of charges.
7. Electroscope is a device used to indicate the presence of charged particles; it usually consists of two small strips of metal foil suspended from a conducting material.
8. Coulomb is the SI unit of charge. It is equivalent to the magnitude of charge resulting from 6.25 x 10¹⁸ electrons.
9. Insulator is a material or object that generally prevents the flow of energy, for example, wood, plastic, glass conductor.