Chapter 1
Electrostatics
The Electric Charge
Electric charge, or ’electricity’, can come from batteries and generators. But some materials
become charged when they are rubbed. Their charge is sometimes called electrostaticcharge or ’static electricity’. It causes sparks to jump from your finger to a metal doorknob
after you walk across a wool carpet.
Figure 1.1:
2
3
1.0.1 Negative and Positive charges
Figure 1.2: Attraction and Repulsion
There are two distinctly different types of electric charges. These were given the arbitrary
names positive and negative by Benjamin Franklin. One may observe that, when comparing
objects bearing these different charges, like charges repel each other and opposite charges
attract each other.
1.0.2 Where charges come from
Everything is made of tiny particles called atoms. These have electric charges inside them.
A simple model of the atom is shown below. There is a central nucleus made up of protonsand neutrons. Orbiting the nucleus are much lighter electrons:
Electrons have a negative (-) charge.
Protons have an equal positive (+) charge.
Neutrons have no charge.
Figure 1.3:
4 CHAPTER 1. ELECTROSTATICS
Normally, atoms have equal numbers of electrons and protons, so the net (overall)
charge on a material is zero. However, when two materials are rubbed together, electrons
may be transferred from one to the other. One material ends up with more electrons than
normal and the other with less. So one has a net negative charge, while the other is left with
a net positive charge. Rubbing materials does not make electric charge. It just separates
charges that are already there.
Figure 1.4:
1.0.3 Conductors and Insulators
Different materials have varying abilities to conduct electrical charge. Those that allow
electric charges to move freely are called conductors, and those that do not transport
charges freely are called insulators.
When some materials gain charge, they lose it almost immediately. This is because
electrons flow through them or the surrounding material until the balance of negative and
positive charge is restored. Conductors are materials that let electrons pass through them.
Metals are the best electrical conductors. Some of their electrons are so loosely held to
their atoms that they can pass freely between them. These free electrons also makes metals
good thermal conductors. Good conductors include carbon and metals (especially silver,
copper and aluminium). Poor conductors include water, the human body and the earth.
5
In an insulator the electrons are tightly held to atoms and are not free to move - al-
though they can be transferred by rubbing. Insulators are easy to charge by rubbing be-
cause any electrons that get transferred tend to stay where they are. Common insulators
include plastics, glass and rubber – when such materials are rubbed, only the area rubbed
may change in charge.
Thus, if you hold and rub rods of polythene and copper, the polythene may attract paper
because its surface is charged, but the copper will not, since the charge you rub into it will
become dispersed throughout your body.
A different type of material is the semiconductor. Semiconductors include silicon and
germanium, and their electrical properties can be changed by adding various amounts of
foreign atoms to the material. They are poor conductors when cold, but much better con-
ductors when warm. These semiconductive properties make possible most of the technol-
ogy invented in the last twenty years.
1.0.4 Attraction of uncharged objects
A charged object will attract any uncharged object close to it. For example, the charged
screen of a TV will attract dust.
Figure 1.5:
The diagram above shows what happens if a positively charged rod is brought near a
small piece of aluminium foil. Electrons in the foil are pulled towards the rod, which leaves
the bottom of the foil with a net positive charge. As a result, the top of the foil is attracted
6 CHAPTER 1. ELECTROSTATICS
to the rod, while the bottom is repelled. However, the attraction is stronger because the
attracting charges are closer than the repelling ones.
1.0.5 Induction
If the earth can be considered an infinite sink for electrical charge, and a conductor is
connected to the earth, the object is said to be grounded or ’earthed’. Building on this idea,
we can see how a conductor may become charged by a process called induction.
When, for example, a positively charged rod is brought near (not touching) a neutral,
non-grounded sphere, the electrons in the sphere near the rod will migrate towards the rod.
However, if the sphere is grounded, some of the electrons will migrate from the earth.
Then, when the ground is removed, the sphere retains an induced negative charge. When
the rod is removed, the positive charge is distributed evenly within the sphere. Throughout,
the rod loses none of its charge.While
charging
by in-
duction
does not
require
contact,
charging
by con-
duction
does nec-
cessitate
contact.
Insulators can undergo a process similar to charging by induction called polarization.
In this case, the presence of a charged external object causes charges within individual
molecules to realign in an insulator. This is why things like combs and balloons can attract
neutral object after being rubbed.
Figure 1.6:
7
Coulomb’s LawWhen
inputing
the values
for the
charges
in Equ.
1.1 only
put the
magni-
tude and
NOT the
sign.
Coulomb’s law is based on experiments which verified that the electric force between two
particles is proportional to the product of the charges q1 and q2 of the two particles and
inversely proportional to the square of their separation r. It is attractive if the particles have
like charges and repulsive if charges have a different sign.
F = k|q1| |q2|
r2 (1.1)
Where k is a constant which depends on the unit used for charge. In SI, this unit is
the coulomb (C), which is defined in terms of a unit current called the ampere (A), current
being the rate of flow of charge. If the current passing through a wire is 1A, the amount of
charge which will pass through any point on that wire in one second is 1C. So, in SI, the
coulomb constant is:
k = 8.9875×109 N ·m2
C2 (1.2)
(k is often approximated as 9.0×109 for simplicity in calculations.) The constant can
also be written as:
k =1
4πε0(1.3)
Where ε0 is the permittivity of free space and is:
ε0 = 8.8542×10−12 C2
N ·m2 (1.4)
Note that electromagnetic forces are still vectors.
S. S. Calbio http://www.izifundo.weebly.com 2015
BISHOP ANSTEY HIGH SCHOOL & TRINITY COLLEGE EAST
SIXTH FORM
CXC CAPE PHYSICS, UNIT 2
Ms. S. S. CALBIO – Homework #1
Question 1 & 2 Due: 3rd September 2015 @ 10:45am, Question 3 is a bonus.
Electric Fields
http://izifundo.weebly.com/objective-3-electric-fields.html
1) Interact with the PHET Simulation of John Travolta and explain (in approx. one
paragraph) what is occurring and why.
2) Force between charges
The figure to the right shows three small
charges A, B and P in a line. The charge at A is
positive, that at B is negative and that at P is
positive. The values are those shown.
(a)Calculate the force on the charge at P due to A and B.
(b)At what point X on the line AB could there be no force on the charge P due to A and B if
P were placed there?
3) Force between charges
In the figure to the right, two small equal charges 2 x 10-8C
are placed at A and B, one positive and the other negative.
AB is 6cm.
Find the force on a charge +1 x 10-8C placed at P, where P is 4cm from the line AB along
the perpendicular bisector XP.