3. ELECTRICAL CHARGE
3
An electron is the smallest particle that exhibits negative electrical
charge. When an excess of electrons exists in a material, there is a net
negative electrical charge. When a deficiency of electrons exists,
there is a net positive electrical charge.
The charge of an electron and that of a proton are equal in
magnitude.
Electrical charge, an electrical property of matter that exists because
of an excess or deficiency of electrons, is symbolized by Q.
Static electricity is the presence of a net positive or negative charge in
a material. Everyone has experienced the effects of static electricity
from time to time. for example, when attempting to touch a metal
surface or another person or when the clothes in a dryer cling
together.
4. ELECTRICAL CHARGE
4
Materials with charges of opposite polarity are attracted to each
other, and materials with charges of the same polarity are repelled,
as indicated in Figure 2-5. A force acts between charges, as
evidenced by the attraction or repulsion. This force, called an electric
field, consists of invisible lines of force, An electron is the smallest
particle that exhibits negative electrical charge. When an excess of
electrons exists in a material, there is a net negative electrical charge.
When a deficiency of electrons exists, there is a net positive electrical
charge.
8. VOLTAGE
8
As you have seen, a force of attraction exists between a positive
and a negative charge. A certain amount of energy must be
exerted, in the form of work, to overcome the force and move the
charges a given distance apart. All opposite charges possess a
certain potential energy because of the separation between them.
The difference in potential energy per charge is the potential
difference or voltage. Voltage is the driving force in electric circuits
and is what establishes current.
As an analogy. consider a water tank that is supported several
feet above the ground. A given amount of energy must be exerted in
the form of work to pump water up to fill the tank. Once the water is
stored in the tank, it has a certain potential energy which, if released.
can be used to perform work.
Voltage, symbolized by V, is defined as energy or work per unit
charge.
10. VOLTAGE
10
If 50 J of energy are available for every 10 C of charge, what is
the voltage.
Solution:
11. CURRENT
11
Voltage provides energy to electrons, allowing them to move
through a circuit. This movement of electrons is the current. which
results in work being done in an electrical circuit
free electrons are available in all conductive and semi conductive
materials. These electrons drift randomly in au directions, from
atom to atom, within the structure of the material, as indicated in
figure.
Random motion of free electrons in a motion
12. CURRENT
12
If a voltage is placed across a conductive or semi conductive
material, one end becomes positive and the other negative, as
indicated in Figure. The repulsive force produced by the negative
voltage at the left end causes the free electrons (negative charges)
to move toward the right. The attractive force produced by the
positive voltage at the right end pulls the free electrons to the right.
The result is a net movement of the free electrons from the negative
end of the material to the positive end, as shown in Figure
13. CURRENT
13
The movement of these free electrons from the negative end of the
material to the positive end is the electrical current, symbolized by I.
Electrical current is the rate of flow of charge.
Current in a conductive material is determined by the number of
electrons (amount of charge) that flow past a point in a unit of time.
15. CURRENT
15
Ten coulombs of charge flow past a given point in a wire in 2 s. What
is the current in amperes.
Solution:
16. RESISTANCE
16
When there is current through a material, the free electrons move
through the material and occasionally collide with atoms. These
collisions cause the electrons to lose some of their energy, thus
restricting their movement. The more collisions, the more the flow of
electrons is restricted. This restriction varies and is determined by the
type of material.
The property of a material to restrict or oppose the flow of electrons
is called resistance, R.
Resistance is the opposition to current.
Resistance is expressed in ohms, symbolized by the Greek letter
omega (Ω).
17. RESISTANCE
17
One ohm (1 Ω) of resistance exists if there is one ampere (1 A) of
current in a material when one volt (1V) is applied across the
material. The schematic symbol for resistance is shown in Figure
18. ENERGY & POWER
18
When there is current through a resistance, electrical energy is
converted to heat or other form of energy, such as light. A common
example of this is a light bulb that becomes too hot to touch. The
current through the filament that produces light also produces
unwanted heat because the filament has resistance. Electrical
components must be able to dissipate a certain amount of energy in a
given period of time.
Energy is the ability to do work, and power is the rate at which
energy is used.
Power (P) is a certain amount of energy (W) used in a certain
length of time (t), expressed as follows:
20. ENERGY & POWER
20
An amount of energy equal to 100 J is used in 5 s. What is the power
in watts.
Solution:
21. THE KILOWATT-HOUR (KWH) UNIT OF
ENERGY
21
The Joule has been defined as a unit of energy. However, there is
another way to express energy. Since power is expressed in watts and
time in seconds, units of energy called the watt-second (Ws), watt-hour
(Wh), and kilowatt-hour (kWh) can be used.
When you pay your electric bill, you are charged on the basis of the
amount of energy you use, not the power. Because power companies
deal in huge amounts of energy, the most practical unit is the kilowatt-
hour. You use a kilowatt-hour of energy when you use one thousand
watts of power for one hour. For example, a 100 W light bulb burning
for 10 h uses 1 kWh of energy.
22. THE KILOWATT-HOUR (KWH) UNIT OF
ENERGY
22
Determine the number of kilowatt-hours (kWh) for each of the
following energy consumptions:
(a) 1400 W for l h, (b) 2500 W for 2 b, (c) 100,000 W for 5 h
Solution:
