ELECTRIC CHARGE

SECOND TERM: WEEK 2

TOPIC: -                ELECTRIC CHARGE

INSTRUCTIONAL MATERIALS: -                  Clinical  and laboratory thermometer

REFERENCES BOOK: -                                       

1.       FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.       M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.  

3.       INTERNET

PREVIOUS KNOWLEDGE: - The Students have been familiar with thermometer.

INSTRUCTIONAL OBJECTIVES: -      At the end of the lessons students should be able to: -

I.                    State the types of charge

II.                  Explain the production of charges

III.                Explain ways of producing charges

IV.                Uses of electroscope

CONTENT: -

ELECTRIC CHARGE

Electrostatics is a branch of physics that deals with static electricity which is a non-moving electric charge on the surface of an object. All substances are made of atoms .Atoms have three sub- atomic particles namely:

Protons have a positive charge. Electrons have a negative charge. Neutrons do not have any charge (i.e. they are neutral).Proton, Neutron and Electron.

Types of Charges

Positive charge(s). These charge(s) are obtained when an atom loses an electron(s). The production of charge by rubbing a glass rod with silk.

Negative charge(s). These charge(s) are obtained when an atom gains an electron(s). The production of charges by rubbing an ebonite rod with fur.

The SI unit of charge is the coulomb, C.

A charged atom (positively or negatively) is called an ion. A body that has neither a negative charge nor a positive charge contains an equal number of protons and electrons (Neutral atom). When these charges are equal in number, a body is electrically neutral.

Electrostatic Charges of a Material

A hard rubber rod gains millions of electrons when it is rubbed with a wool cloth. The cloth loses electrons to the rod and becomes positively charged. The rubber becomes negatively charged. When glass rod is rubbed with fur, the fur loses electrons to the glass rod. Glass rod becomes negatively charged and fur becomes positively charged.

A body is said to be positively charged if it has an excess of positive charges or it has lost some electrons. A body is said to be negatively charged if it has an excess of electrons i.e. If it has gained some electrons.

The following materials becomes negatively charged by friction when rubbed:

Polythene, Most plastics, and Ebonite

The following on the other hand becomes positively charged when rubbed: Charging Methods

Acetate, Perspex, Glass and Fur.

The Basic Law of Electrostatics

It states that like charges repel, unlike charges attract.

a) Conductors and Insulators.

Substances which do not allow electrons to pass through them are called insulators. Substances which allow electrons to pass easily through them are called conductor. The reasons they (metal) conduct easily is because within any metal some electrons are not attached to specific atoms but are to wander about.

If you were to hold a metal rod and rub it with wool would it be charged? NO However if you fit polythene handle onto the metal rod it can be charged by rubbing it. We explain this by saying that the polythene is an insulator and will not allow electrons to move through it on along its surface. When an insulator is rubbed, the electrons which are transferred come from atoms on the surface. The metals become charged by electrons from the wool. But if the metal is directly held, these electrons immediately ‘leak’ away to the earth through the hand on the body whereas when the handle is insulated they remain on the metal because they cannot pass through the insulating handle.

WAYS OF PRODUCING CHARGES

 The following methods are used to charge materials:

(a) Induction    (b) Contact      (c) Separation

(a) Induction

Charging polystyrene ball negatively by induction

I.  Suspend the polystyrene ball coated with aluminium using dry silk thread. Bring a positively charged glass rod close to but not touching the suspended ball.

II.  Note: The positive charges are repelled while the negative charges are attracted to the rod.

Touch the side of the ball away from the glass rod with the finger.

III.  Note: Electrons flow from the earth to neutralize the positive charge on the ball. While holding the glass rod near the ball, withdraw the finger and then the glass rod.

                                                                           

Note: The remaining negative charges redistribute themselves uniformly on the ball.

When a body is charged by induction, it acquires the charges that are opposite to the inducing charge.

 (b) Contact method

I.  Suspend the polystyrene ball coated with aluminium using dry silk thread. Bring a charged glass rod close to but not touching the suspended ball.

II.   Bring a charged glass rod in contact with the ball, rolling it over the surface.

III.   Withdraw the charged rod.

I.                                    II.                            III. 

Notes

a) When a positive rod is rolled on the ball, some of the negative charges induced in the ball are

neutralized by some positive charges on the rod.

b) When the rod is withdrawn, the positive charges redistribute themselves all over the surface of the rod.

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

 (c) Separation Method

I.  Place two spheres A and B together so that they form a single conductor. Bring a charged polythene rod close to but not touching sphere A.

Move sphere B away so as to break the contact, while holding the charged rod in position.

II.   Withdraw the charged rod and test the two spheres using a negatively charged rod for the presence and type of charge in each sphere.

                                     

I.                                                              II.

 

The Gold Leaf electroscope

It consists essentially of a metal cap (brass) and rod (boast) at which is connected a piece of very thin metal foil called the leaf (sometimes gold foil, aluminum) supported with a piece of insulating material (could be cork, cellulose acetate) which forms part of a box with glass slides. The metal casing protects the leaf from the effect of the draught. The glass window is transparent for observation to be made. The rod is supported by a plug of good insulators which stops charges given to the cap from spreading to the case and hence leaking away. The cap is circular to ensure uniform distribution of charge on it.

Earthing-Process of losing charges to or gaining charges from the earth through a conductor. Represented by the symbol:

Charging a Gold Leaf Electroscope by Contact

An electroscope is charged negatively by bringing a charged polythene strip up to the electroscope and roll it over the electroscope cap. The negatively charged polythene rod repels the negative charge which spreads on the plate and the leaf, making them repel each other hence leaf divergence. If the process is repeated several times the leaf divergence will keeps increasing until it reaches the maximum point.

This way the electroscope is said to have been charged by contact.

The metal cap, leaf and rod will therefore remain diverged. In damp weather, this process may be difficult. It is helped if a razor blade is cello taped on to the cap so that it projects over the edge. The razor edge will then be found to gather the charge more readily than the cap alone.

To charge an electroscope positively, a charge cellulose acetate strip is rubbed along the edge of the cap. Electrons are accepted from the cap to the strip which means that the cap and leaf gain a net positive charge. The leaf therefore diverges again.

Charging a Gold Leaf Electroscope Positively by Induction

Touch the cap of the electroscope with your finger to ensure that it is fully discharged. Bring a charged rod (-) close to the cap of the uncharged electroscope. While the rod remains in its position, touch the cap.

 Observations

When the charged rod (-) is brought close to the cap, the leaf rises. When the cap is touched while the rod is in position, the leaf divergence decreases. When the finger is withdrawn and the then the rod, the leaf diverges.

Explanation

When the rod is brought close to the cap, electrons are repelled to the plate and the leaf making the leaf to diverge. When the cap is touched, electrons flow to the earth through the body. When the earth connection is broken by removing the finger and the rod, the positive charges redistributes onto the leaf and the plate. The leaf as a result diverges and the electroscope becomes positively charged.

Uses of Electroscope

1. Detecting the presence of charge on a body Charged bodies brought near the cap will cause the leaf to rise (diverge). If the leaf doesn’t diverge the body is uncharged.

2. Measuring the insulating properties of an object (material) Charge the electroscope and bring the object to the top of the electroscope and earth the electroscope. The rate at which the leaf fall is the measure of the conducting ability of the object.

3. To measure the quantity of charge on a charged body If a body being tested has little charge on it the leaf of the electroscope will diverge a little and if the body has much charge on it the leaf will diverge more.

4. testing the sign of charge on a charged body If a charged body is brought near the cap of a charged electroscope, the leaf of the electroscope will diverge more if the body and the electroscope have similar charges and the leaf divergence will decrease if the two have different charges.

Testing for charges

a) Charge an electroscope negatively. Bring a negatively charged polythene strip close to the cap. Notice that the leaf diverges further.

b) Bring up a positive charge cellulose acetate strip close to the same negatively charged electroscope. Notice that this time the leaf collapses (decrease in divergence.)

c) Place your hand on any uncharged metal close to the charged electroscope. Notice again that the leaf collapses. Note that the results in experiments ( b) and (c) were the same hence the collapsing of the leaf is not evidence for the presence of a charged body. We can therefore state the rule for testing a charge as follows :

1) If the leaf of charge electroscope diverges further when a body brought close to the cap then that body is charged.

2) The sign of the charge on the body is the same as that on the electroscope. Increase in divergence is the only sure way of testing the sign of charge on a body. Note that charging by contact is

Charge on  electroscope              Charge brought near the cap       Effect on leaf divergence

+                                                             +                                                            Increase

-                                                                  -                                                              Increase

+                                                             -                                                             Decrease

-                                                              +                                                             Decrease

+or-                                       Uncharged body                                              Decrease

Charges in Air

Air can be charged by:   Heating  and  Radiations

Applications of Electrostatic Charges:

   1.Electrostatics precipitators     2.Finger printing       3.Spray painting          4.Photocopying

Danger of Electrostatics

Rubbing liquid molecules can be charged. If the liquid is inflammable, it can spark and explode. It is advisable to store fuels in metal cans so that any charges generated continually leak out. This is the reason behind the presence of loose chain in tankers carrying inflammable liquids.

Distribution of charges

Experimental works have shown that charges are distributed where there is a sharp curve. The density of this charges are greater at the surface of sharp curve. The charge per unit area of charged surface is called surface density.

Surface density is greater at the corner or pointed edge than at the plain surface.

STORAGE OF CHARGES (THE ELECTROPHOROUS)

The electrophorous is a device for transferring and storing charges. It produces electric charges by electrostatic induction.

It consists simply of a circular metal disc or plate fitted with an insulated handle and another flat disc made of an insulating materials such as ebonite or polythene tile.

PRESENTATION

Step I: The teacher explains the electrostatic charges of material

Step I: The teacher explains the production of charges

Step I: The students mention the methods of charge production

Step I: The teacher explains Gold leaf electroscope

Step I: The teacher explains distribution and storage of charges.

EVALUATION

The teacher evaluates the lessons by asking the following questions:

I.                    State the types of charge

II.                  Explain the production of charges

III.                Explain ways of producing charges

IV.                Uses of electroscope

ASSIGNMENT

1. A nylon dress sticks on the body and crackles when removed. Explain.

2. You are given a positively charged electroscope and a charged body. Explain how you will determine the type of the charge on the charged body.

3. You are provided with the following: polythene rod, duster and a uncharged electroscope. Explain how you will charge the electroscope negatively.

4. Explain by use of diagrams how the above material can be charged positively by induction method.

5. Explain how a material is charged negatively by contact method

6. When a charged rod is held close to a metal sphere placed on an insulated stand, the charge distribution on the sphere is as shown below.

 

(a) What is the sign of charge on the rod?

(b) Describe a simple method to charge the rod.

(c) Explain why the far side of the metal sphere has a positive charge.

(d) What happens to the charges on the metal sphere, if the charged rod is moved away from the sphere?