Upload
others
View
15
Download
0
Embed Size (px)
Citation preview
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 1 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
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 2 of 8
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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 3 of 8
(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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 4 of 8
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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 5 of 8
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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 6 of 8
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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 7 of 8
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.
________________________________________________________________________________
Mr. C. Busuttil Ch 15 Page 8 of 8
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?