FORM 1E & 1W PHYSICS NOTES.

AUGUST HOLIDAY ASSIGNMENT.

BE CAREFUL TO:

i. FOLLOW THE GUIDE NUMBERS IN BRACKETS, e.g (1 ends here), (2 starts here)
ii. SKIP ALL EXERCISES OR QUESTIONS THAT ARE UNANSWERED. DO NOT COPY THEM.
iii. COMPLETE THE NOTES IN YOUR CLASSWORK NOTEBOOK. NOTES INCLUDE THE DIAGRAMS.

Chapter Eight ELECTROSTATICS I

START HERE..(1)
Introduction (2) STARTS HERE
 Electrostatics is a branch of physics that deals with
study of static electricity ,which is a non-moving electric EXERCISE
charge on the surface of an object. 1. Two balloons inflated with air are tied with strings and held 1
 A gain of electrons produces negative static charges metre apart. Both balloons are rubbed with fur. Why do the
while a loss of electrons produces positive charges. balloons move apart when brought close together?
 All substances are made of atoms .Three kinds of sub 2. For each situation below state whether the force between them
atomic particles are: is repulsive or attractive.
 Protons
 Neutrons
 Electrons
 Protons have a positive charge. Electrons have a
negative charge. Neutrons do not have any charge (i.e.
they are neutral).

Types of Charges
 Positive charges
 Negative charges
 The SI unit of charge is the coulomb, C. a) Conductors and Insulators in Terms of
 A body that has neither a negative charge nor a positive  Substances which do not allow electrons to pass through them
charge contains an equal number of protons and are called insulators.
electrons (Neutral atom). When these charges are equal  Substances which allow electrons to pass easily through them are
in number, a body is electrically neutral. called conductors. The reasons why they (metals) conduct easily
is because within any metal some electrons are not attached to
Electrostatic Charges of a Material specific atoms but are free to wander about.
 A hard rubber rod gains millions of electrons when it is  If you were to hold a metal rod and rub it with wool would it be
rubbed with a wool cloth. The cloth loses electrons to charged? NO However if you fit polythene handle onto the
the rod and becomes positively charged. The rubber metal rod it can be charged by rubbing it. We explain this by
becomes negatively charged. saying that the polythene is an insulator and will not allow
 When glass rod is rubbed with fur, the fur loses electrons to move through it on along its surface. When an
electrons to the glass rod. Glass rod becomes negatively insulator is rubbed, the electrons which are transferred come
charged and fur becomes positively charged. from atoms on the surface. The metals become charged by
 A body is said to be positively charged if it has an excess electrons from the wool. But if the metal is directly held, these
of positive charges or it has lost some electrons. electrons immediately ‘leak’ away to the earth through the
 A body is said to be negatively charged if it has an hand on the body whereas when the handle is insulated they
excess of electrons i.e. If it has gained some remain on the metal because they cannot pass through the
electrons. insulating handle.
 The following materials becomes negatively charged by
friction when rubbed: Charging Materials
 Polythene The following methods are used to charge materials:
 Most plastics (a) Induction
 Ebonite (b) Contact
 The following on the other hand becomes positively (c) Separation
charged when rubbed: (2 ENDS HERE)
 Acetate
 Perspex
  Glass
 Fur
The Basic Law of Electrostatics
 It states that like charges repel while unlike charges
 (3 CONTINUES)
(3) STARTS HERE b) Contact method
 Suspend the polystyrene ball coated with aluminium using dry
a) Induction silk thread.
Charging polystyrene ball negatively by induction  Bring a charged glass rod close to but not touching the
 Suspend the polystyrene ball coated with aluminium suspended ball.
using dry silk thread.
 Bring a charged glass rod close to but not touching the
suspended ball.

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

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.

 Withdraw the charged rod.

Note: Electrons flow from the earth to neutralize the positive

charge on the ball. Notes
 While holding the glass rod near the ball, withdraw the a) When a positive rod is rolled on the ball, some of the negative
finger and then the glass rod. 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.
Question: Explain how a material is charged negatively by contact
Note: The remaining negative charges redistribute method
themselves uniformly on the ball.
c) Separation Method
 When a body is charged by induction, it acquires the
 Place two spheres A and B together so that they form s single
charges that are opposite to the inducing charge.
conductor.
 Bring a charged polythene rod close to but not touching sphere
Question:
A.
Explain by use of diagrams how the above material can be
charged positively by induction method.
 (3) ENDS HERE
(4 STARTS HERE)
 Move sphere B away so as to break the contact, while (5 STARTS HERE)
holding the charged rod in position.  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.
 The rod is supported by a plug of good insulators which stops
 Withdraw the charged rod and test the two spheres using charges given to the cap from spreading to the case and leaking
a negatively charged rod for the presence and type of away.
charge in each sphere.  The cap is circular to ensure uniform distribution of charges 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 rubbing it along the
edge of the cap. In this way electrons are transferred from the
Exercise
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
polythene strip to the metal cap. These electrons are now on a
the charged rod is moved away from the sphere?
conductor because of their mutual repulsion immediately spread
them as far as possible.
The Gold Leaf Electroscope
 The metal cap, leaf and rod will therefore remain diverged.
 In damp weather, this process may be difficult. It is helped if a
(4 ENDS AFTER THE DIAGRAM BELOW) 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

 (6 STARTS HERE) Charge on Charge Effect on leaf
electroscope brought near divergence
 When the finger is withdrawn and the then the rod, the the cap
leaf diverges. + + Increase
- - Increase
Explanation
+ - Decrease
 When the rod is brought close to the cap, electrons are
- + Decrease
repelled to the plate and the leaf making the leaf to +or- Uncharged Decrease
diverge. body
 When the cap is touched, electrons flow to the earth (7 STARTS HERE)
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 Charges on Air
the electroscope becomes positively charged. Air can be charged by:
 Heating
Uses of Electroscope  Radiations
1. Detecting charge Applications of Electrostatic Charges
 Charged bodies brought near the cap will cause the  Electrostatics precipitators
leaf to rise (diverge).  Finger printing
2. Measuring the insulating properties of an object  Spray painting
 Charge the electroscope and bring the object to the
top of the electroscope and earth the electroscope. Danger of Electrostatics
The rate at which the leaf fall is the measure of the  Rubbing liquid molecules can be charged. If the liquid is
conducting ability of the object. inflammable, it can spark and explode. It is advisable to store
3. Measurement of voltages. fuels in metal cans so that any charges generated continually leak
 The electroscope can be used as an electrostatic out. This is the reason behind the presence of loose chain in
voltmeter. tankers carrying inflammable liquids.
4. Measurement of ionization current caused by
radioactivity (7 ENDS HERE)
 An electroscope slowly loses its charge in ionized air END OF TOPIC .
and the rate of charges or ionization current is the
measure of the activity of whatever caused the
ionization.

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
not a sure way but induction is.
(6 ENDS HERE)