ch15 electrostatics - notes

________________________________________________________________________________

Mr. C. Busuttil Ch 15 of 8

15.1 The Atom

Objects charge up because they gain or lose

electrons. The electron is the smallest particle

of the atom. The electron carries a fixed

negative charge. Every atom has a positively

charged nucleus that is surrounded by electrons.

An atom is uncharged because it contains

equal amount of positive and negative charge.

15.2 Electric Charge

Rub a plastic pen or comb on your sleeve

and then hold it near some tiny pieces of

paper. You can see that the pen or comb

attracts the pieces of paper like a magnet

would do to some iron filings.

Similarly, rub a plastic object on your

sleeve and then hold it near a thin stream

of water from a tap. You can see that the

stream bends towards the pen or comb.

Because of friction with the sleeve, the plastic object has been charged with

stationary or static electricity.

Chapter

15

ElectrostaticsElectrostaticsElectrostaticsElectrostatics

________________________________________________________________________________


Rub a strip of polythene on a dry

woollen cloth and hang it in a

paper stirrup. We say the

electricity on polythene is a

negative charge.

Rub another strip of polythene

and bring it close. We see that

both strips repel each other.

Now repeat the experiment with

one strip of charged polythene and

one strip of charged cellulose

acetate.

This time, both strips attract each

other because each strip is

charged oppositely.

Because it behaves differently,

we say that cellulose acetate

(Perspex) is charged positively.

Remember that:

� Like electric charges repel.

� Unlike electric charges attract.

The amount of charge that is stored is measured in coulombs, C. We often

use micro-coulombs )101( 6C

−× , µµµµc, and nano-coulombs )101( 9

C−

× , nc, since

the coulomb is a very large amount of charge.

Problem:

Two small balls coated in metallic paint are suspended by long insulating strings

from A and B as shown in figure 1 below.

________________________________________________________________________________


(a) Both balls in figure 1 are given a negative charge.

(i) Complete figure 2 above to show the new positions of the balls.

(ii) Choose the appropriate word to complete the sentence below from

the following list: attract, unlike, force, repel, like, small. The balls in figure 2 _______________ each other since________

charges repel each other.

(b) The ball suspended from B is carefully moved and suspended from C

without changing the size of the charges on both balls.

(i) Complete the above figure to show the new positions of the balls.

(ii) As the distance between the two balls carrying the same charge

increases, the force of repulsion between them _____________.

15.3 Conductors and Insulators

Certain substances such as metals, water and the human body allow

electricity to flow through them with ease; these substances are

conductors of electricity. Other materials such as wood, glass, rubber and

Perspex do not conduct electricity; they are electrical insulators.

The atoms of electrical conductors have one or more outer electrons that are

relatively free from the attraction of their nuclei. Therefore, they can move

with relative ease through the body of their materials when subjected to

certain conditions. For instance, when a battery is connected between two

points on a conductor, ‘free’ electrons drift through the conductor.

All electrons in the atoms of electrical insulators are firmly attracted to the

nuclei of their atoms. Hence, electrons are not free to move through an

insulator.

________________________________________________________________________________


15.4 Electrostatic Induction

If a charged body R is brought near an insulated conductor X, charges on the

conductor are re-distributed. This phenomenon is known as electrostatic

induction. The conductors are not charged since they have neither gained nor

lost charge; only displacement of electrons has occurred. Protons are at the

centre of the atom and are not displaced by induction.

15.5 Charging a Conductor by Electrostatic Induction

15.5.1 Charging by direct contact

Direct contact between the charged insulator and the conductor transfers

charge to the conductor from the charged insulator. It gives the conductor

the same type of charge as the insulator.

15.5.2 Charging by induction – separating conductors

Step 1: Bring a charged polythene strip near a metal sphere A.

Step 2: Move sphere B away while the polythene stays near sphere A.

Step 3: Finally remove the polythene.

________________________________________________________________________________


15.5.3 Charging by induction – earthing a conductor

Step 1: Bring a charged strip S near an insulated conductor A.

Step 2: Touch A with a finger to earth completely.

Step 3: Finally remove the charged strip S.

Problem:

The following advert is seen on a magazine:

(a) The static magic duster becomes ____________ by rubbing it with the

polythene bag.

(b) Name the charges that are being transferred from the magic duster to

the polythene bag. _______________________.

(c) The magic duster is brought close to a neutral dust particle. Draw the

charges on the dust particle when it gets close to the magic duster.

(d) Why is the dust particle attracted to the magic duster?

(e) If the dust particle sticks to the magic duster, what charge will it have

now? ____________.

(f) What happens to the dust particle after some time? Explain.

________________________________________________________________________________


Problem:

When a polythene strip is rubbed with a dry cloth, both the polythene and

cloth become charged because electrons are being transferred.

On the above diagram:

(i) Mark + and – to show the charge on the cloth and on the polythene.

(ii) Draw an arrow to show the direction in which the electrons are

moving.

An insulated copper strip was charged by rubbing. The charged copper and

polythene strips were then earthed. Only the copper strip lost its charge.

Which of the two strips was:

(iii) the conductor? ______________.

(iv) the insulator? ______________ .

The above method shows one way of charging 2 insulated metal spheres, by

separation. The strip is negatively charged.

(i) Mark the charges on A and B in figure 1.

(ii) Underline the correct phrase in the brackets:

The charge on sphere A is (greater than, equal to, smaller than) the

charge on sphere B.

(iii) Sphere B was momentarily earthed. First the earth connection and

then the strip was removed. Then sphere B was separated from sphere

A. Mark the charges on spheres A and B in figure 2.

________________________________________________________________________________


15.6 Uses of Electrostatics

Bicycles and cars are painted using an electrostatic

paint spray. The nozzle is given a charge and this makes

a better spray. The droplets all have the same charge

and repel each other so that the paint spreads out to

form a large even cloud. Less paint is needed because

the charged droplets are all attracted to the object

because it has an opposite charge. The same idea is

used to make crop-spraying more efficient for farmers.

A similar method can be used to show up fingerprints

on paper. The paper is placed near a charged wire and

a fine black powder is used instead of smoke. The

powder sticks to the fingerprint but not to the clean

paper. Photocopiers are based on a similar method.

Power stations and factories produce huge

amounts of smoke pollution. This smoke is a

cloud of small dust particles or ash. It can

be removed by using static electricity.

Some thin wires are stretched across the

centre of the chimney: These wires are

charged positively to about 50,000V and

they cause the gas around them to be

charged or ionised. Because of this, the smoke particles become positively

charged. These positive particles are then repelled by the wires, towards the

earthed metal plates, where the dust sticks.

A mechanical hammer hits the plates every few minutes and the ash falls down

into a bin. Later, it is used to make house-bricks.

________________________________________________________________________________


15.7 Dangers of Electrostatics

A liquid through a pipe can become charged in

the same way as two objects rubbed

together. This can be dangerous if it causes a

spark and the liquid is inflammable. For this

reason, whenever an aeroplane is being re-

fuelled by a tanker lorry, they are always

connected together by a copper wire. For the

same reason, spare petrol for cars should

always be carried in metal cans, never plastic.

Another dangerous situation posed by static

electricity comes around during a

thunderstorm. It is very important in such

situations to channel lightning energy through

paths along which the least damage is

suffered. It is here where we use what are

known as lightning conductors.

Problem:

(a) Louisa rubs a polythene rod with a piece of cloth. The polythene becomes

_______________ charged while the cloth is _______________

charged. This happens because polythene _________________

electrons, while the cloth ____________ electrons.

(b) (i) A trailing strip attached to the back of a car and hanging so that it

touches the ground, stops it from overcharging with static electricity.

The strip is made of _______________.

(ii) In places where it rains all the time, the strip is not needed. Why?

ch15 electrostatics - notes

Documents