﻿ Electrostatics I

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Electrostatics I

Objectives

By the end of this topic you should be able to:

1. Describe electrostatic charging of objects by rubbing
2. Explain the source of electrostatic charges
3. State the two types of charge
4. State the basic law of charges
5. State the unit of charge

Electrostatic Charging by Rubbing

Explanation

The force pulling the pieces of paper towards the pen is called electrostatic force.The word electrostatic is developed from two words electro and static. Electro is derived from electron while static means stationary (not moving).

Summary

The pieces of paper were attracted to the pen because the pen had acquired some charge. Rubbing causes some objects to be electrically charged.

The Origin of Charge

Activity: Click on the word Neutrons, Protons or Electrons to view their positions in the atom.

An atom with equal number of protons and electrons is said to be neutral.

During rubbing, an electron may move from an atom of one material to an atom of another material. This causes each of the atoms to become electrically charged.

Transfer of Charge

Activity: The objects below represent atoms A and B. Click on the button for an electron to move from A to B.

When an atom loses an electron, the atom remains with fewer negative charges than positive charges; so it becomes positively charged.

After losing an electron, A has excess protons hence it is positively charged. We represent this state by " +". B acquires excess electrons at the same time; so it becomes negatively charged, a state represented by " -".

Transfer of Charge (cntd)

As seen earlier, rubbing of some materials causes transfer of electrons.

Explanation

When the glass rod is rubbed with fur or tissue paper, it loses some electrons to become positively charged. At the same time the polythene rubbed with tissue paper gains electrons and becomes negatively charged.

Summary

The rubbing material always acquires an opposite charge to that of the rubbed rod.

Types of Charge

Drag the glass rod A onto the tissue paper and move it so that it gets charged. Drag the second glass rod B onto the tissue paper so that it gets charged. Drag rod B to a position near rod A. What happens?

Drag a polythene rod C onto the tissue paper and move it so that it gets charged. Drag a second polythene rod D onto the tissue paper so that it gets charged. Drag rod C to a position near rod D.

Drag the charged rod A towards the charged rod C and observe what happens.

Basic law of electrostatics

Like cmm Like charges repel and unlike charges attract each other. This is the basic law of electrostatics.
There are two types charge, namely: positive charges and negative charges The unit of charge is called the coulomb, symbolized as C.

Q uizzes

Q uizzes

Leaf Electroscope

An electroscope is an instrument used for identifying charge as either positive or negative. It is also used to investigate the effect of charges. A leaf electroscope is made up of various parts as illustrated on the next page.

Features of Electroscope

Charging leaf electroscope negatively by contact

Charging Leaf Electroscope by Contact

1. Drag the glass rod onto the tissue paper
2. Move the rod on the tissue paper several times
3. Drag the charged glass rod until it touches the brass cap of the electroscope and roll it on the cap. What happens to charge and leaf?
4. Drag away the charging rod from the brass cap. What happens to the leaf of the electroscope?

Observation

Charging leaf electroscope by induction

Apparatus

Leaf electroscope, glass rod, tissue paper.

Procedure

Step I: Charge a glass rod positively by rubbing it with tissue paper.
Step 2: Move the charged glass rod and hold it near the cap of the leaf electroscope but without touching the cap. Observe what happens to the leaf.
Step 3: While keeping the charged rod near the cap, touch the cap with the finger. Observe what happens.
Step 4: With the charged rod still held near the cap, remove the finger from the cap and observe what happens.
The leaf remains collapsed.
Now remove the charged glass rod and observe what happens to the leaf.

Summary

When a positively charged rod is used to charge an electroscope by induction, the electroscope becomes negatively charged during the process. The electrons flow from the earth to the electroscope.

Charging leaf electroscope positively by induction

Exercises

Prior knowledge

Lesson objectives

By the end of this lesson you should be able to

:

• Identify conductors and insulators
• Identify types of charge on materials

Identification of Charge

Experiment 1: To identify Charge

You are provided with three rods A, B and C and a negatively charged electroscope.

Procedure

On the next animation, observe the effect of each rod on the leaf of a negatively charged electroscope. Use the observations to identify the charge on each rod.

Discussion

An increase in divergence of the leaf indicates that the rod and electroscope have the same type of charge. That is, the rod has a negative charge. A decrease in divergence of the leaf shows that the rod is either neutral or positively charged. It follows that rod B is negatively charged (since it causes an increase in divergence). Rods A and C are either positively charged or neutral.

NB: An increase in divergence of the leaf is the only sure test of confirming the type of charge on a body.

Identification of Charge

Experiment 1: To identification of Charge

You are provided with three rods A, B and C and a negatively charged electroscope.

Procedure

Drag each rod at a time and hold it close but not touching the cap of the electroscope.
Observe what happens to the leaf, in each case.

Summary

An increase in divergence of leaf indicates that the rod and electroscope have the same type of charge.

A decrease in divergence of leaf indicates opposite charge or neutral, hence the

An increase in divergence of the leaf is the only sure test of confirming the type of charge on a body.

Repeat the procedure with a positively charged electroscope.

Identification of Charge

Experiment 1: To identification of Charge

You are provided with three rods A, B and C and a negatively charged electroscope.

Procedure

Drag each rod at a time and hold it close but not touching the cap of the electroscope.
Observe what happens to the leaf, in each case.

Summary

An increase in divergence of leaf indicates that the rod and electroscope have the same type of charge.

A decrease in divergence of leaf indicates opposite charge or neutral, hence the

An increase in divergence of the leaf is the only sure test of confirming the type of charge on a body.

Repeat the procedure with a positively charged electroscope.

Exercises

Activity 3:Identification of charge

You are provided with three rods A, B and C and a negatively charged electroscope

What to do:
Drag each rod at a time close but not touching the cap of the electroscope.
Observe what happens to the leaf, in each case.

Repeat the procedure with a positively charged electroscope.

Neutralization of Charge

The set-up below shows two electrically charged metal blocks carrying opposite charges. The blocks are connected using a copper wire. What do you observe?

Negative charges move from the negatively charged block to neutralize positive charges on the other. Positive charges do not move through solids.

Exercise

Conductors and Insulators

Activity To identify conductors and insulator

You are provided with the following uncharged materials: rubber, copper rod, wooden splint, plastic rod and brass rod.

Drag the materials, one at a time with the mouse to the cap of the charged electroscope. Observe what happens to the leaf.

Conclusion

Uncharged materials that cause the leaf to fall are good conductors. The leaf falls because the material conducts charges.Materials for which the leaf remains stationary are insulators (bad conductors).

Exercise

Electrostatics I

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Electrostatics I

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Electrostatics I

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