Download 1 Coulomb = 6.242*10 18 electrons

1
Electronics
Lesson-1
Basic Electricity
1.1 Current flow
If you connect a dry cell to a torch bulb using two pieces of
wire, the bulb will glow. That shows current flowing through
the circuit.
Fig-1.1
The dry cell or battery is made of chemicals and conductors.
Flexible wire is normally made of copper and is covered with
insulating material like plastic or PVC.
1.1.1 Direction of current
Electric current always flows from positive terminal to the
negative terminal through the external circuit. But inside the
battery, it flows from negative terminal to the positive
terminal.
You can get a clear idea of the current flow using a LED
(Light Emitting Diode)
Fig 1.2
1.2
Conductors & Insulators
Materials in which electric current can flow are called
conductors, while those that refuse such movement are called
nonconductors or insulators.
Examples:Conductors:gases
Insulators:Dry air
any metals, Carbon, Acids, Salts, ionized
Glass, Mica,
Rubber,
Porcelain, Dry wood,
2
1.2.1 Good & Poor Conductors
Conductors are divided into two; good conductors and poor
conductors.
Good conductors:-Electricity can flow easily through good
conductors. It is found to vary with the resistance of the
material.
A few examples of good conductors are Silver,
Copper, Gold, Aluminum etc.
Poor Conductors:-Electricity can flow through poor conductors,
but not flow easily as in good conductors. A few examples are
Nicrom, Manganin, Tungsten and Iron.
1.3
Batteries in series
If we connect a dry cell to a small bulb it will glow, but not
very brightly because it uses 1.5 Volt. If we connect two
cells in a series, the brightness will increase because the
voltage increases to 3 Volts. If we connect a few more cells
the filament of the bulb will get destroyed and the current
will stop. That means due to the high temperature of the
tungsten filament, it melts.
Fig 1.3
1.4
Resistance
According to the diagram in
Fig 1.4 if we connect a
bulb with three cells, it
will glow brightly. If we
connect a nicrome wire, “R”
as shown in the diagram the
brightness of the bulb will
decrease.
Fig 1.4
That means the current through the circuit is decreasing. The
reason for this phenomenon is defined as the resistance of the
nicrome wire.
Usually
bad
conductors
have
more
resistance
or
low
conductivity and good conductors have low resistance or high
conductivity.
3
1.5 Electric Charge
Everything physical is built up of atoms. Atoms are very small
particles. They are so small that they cannot be seen even
through the most powerful microscope. But the atom in turn
consists of several different kinds of still smaller
particles. Electron and proton are the most important
particles of electricity. They have negative and positive
electric charges respectively.
The important fact about these two opposite kinds of
electricity is that they are strongly attracted each other.
Also there is a strong force of repulsion between the two
charges of the same kind. Opposite charges attract each other
with a strong force.
An ordinary atom consists of a central core called nucleus,
carrying a number of protons. The positive charge on the
nucleus is exactly balanced by the negative charges on the
electrons, orbiting around the nucleus. However, it is
possible for an atom to lose one of its electrons. When that
happens the atom has a little less negative charge than it
should. That is, it has a net positive charge. Such an atom
is said to be ionized, and in this case the atom is a positive
ion. If an atom picks up an extra electron, it is called a
negative ion. The diagram in Fig-1.5 shows a neutral atom,
negative ion and a positive ion.
Fig-1.5
Atoms and ions are not moving in solid conductors, but
they are moving in gases and liquids. The current flows
through solid conductors by the movement of electrons.
1.5.1 Coulomb (C)
The practical unit (SI-unit) of electric charge is "Coulomb".
This is equivalent to the many billions of electrons.
1 Coulomb = 6.242*1018 electrons
4
1.6 Electric Current (Ampere)
A positive ion will attract any stray electron in the
vicinity, including the extra one that may be attached to a
nearby negative ion. In this way it is possible for electrons
to travel from an atom to atom. The movement of ions or
electrons constitutes the electric current. The flow of
electric current is measured in Amperes. One "Ampere" is
equivalent to the movement of one Coulomb passing a point in
the circuit in one second. This is the practical unit (SIunit) for the electric current, but for the sake of
convenience we use smaller units as milliampere (mA) and
microampere (µA).
1000 µA = 1 mA
1000 mA = 1 A
1.7 Electro Motive Force ( E.M.F.)
The electric current flows due to the electrical pressure or
force generated by the source. This is called electromotive
force.(e.m.f.) That causes current flow. It may develop in
several ways - the chemical reaction in a cell, the movement
of electrons in a solar cell or the magnetic effect in a
generator (alternator).
The unit of e.m.f. is "Volt". A larger unit is called
kilovolt (kV) and smaller units are called millivolt(mV) and
microvolt(µV).
1.7.1 Potential difference & Voltage
The electrical pressure difference between two points of an
electrical circuit is called potential difference. When it is
measured by Volts it's called voltage. The units are as same
as e.m.f.
1.7.2
E.M.F. and Voltage Drop
Fig-1.6
E.M.F. of a cell is equal to the voltage between two
terminals, when there is no current flow. When there is a
current flow or energy consumed by an external circuit, the
voltage will be reduced. If the external circuit
is
5
disconnected the voltage will increase up to the value of
E.M.F.
The reduced amount is called the voltage drop. The voltage
drop depends on the internal resistance of the battery and the
resistance of the external circuit.
1.8
Capacity of a battery ( Ah. or mAh.)
Normally large batteries have more capacity. Capacity means
how much charge (like energy) can be taken from the battery.
If 2 Amp current can take 5 hours continually from a battery,
the capacity is little more than 10 Ampere-hour (Ah.)
If a battery has a high capacity, it’s internal resistance
will be low.
1.9 Resistivity
If two conductors of the same size and shape are used with
different materials, the amount of current that will flow when
a given EMF is applied will be found to vary with the
resistance of the material. The lower the resistance, the
greater the current for a given value of EMF.
Unit of resistance is "Ohm". Smaller units are micro Ohm
(µΩ) and milli Ohm (mΩ) and larger units are kilo Ohm (kΩ) and
Mega Ohm (MΩ).
1000 µΩ = 1 mΩ
1000 mΩ = 1 Ω
1000 Ω = 1 kΩ
1000 kΩ = 1 MΩ
Regarding the earlier example, if both conductors have
the same size and same shape, a property varies with the type
of material, it's called resistivity. The resistivity of a
material is defined as the resistance of a cube of the
material measuring between two opposite faces. But practically
this is impossible to measure.
The resistance of a wire with uniform cross section is
directly proportional to its length and inversely proportional
to its cross sectional area.
A wire with a certain resistance for a given length will
have twice as much resistance for a given length of the wire
is doubled. If you consider two wires having same material and
same length but the cross sectional area is twice, while
doubling the cross sectional area will halve the resistance.
The formula related with above factors is:
R = ρl / A
R
l
A
ρ
=
=
=
=
resistance of the wire
length of the wire
area of the cross section of the wire
specific resistance or resistivity of the material of
the wire
(ρ is a Greek letter “rho”)
6
Units:-
SI-unit is Ω-m. This is useful for calculations.
But µΩ-m is more convenient.
If “R” is in Ohms, “l” is in meters and “A” is in
square meters, then “ρ” is in Ohm-meter (Ω-m)
If “R” is in micro Ohms, “l” is in meters and “A” is
in square meters, then “ρ” is in micro Ohm-meter (µΩm)
Other units are defined same as above:
micro Ohm centimeter (µΩ-cm)
and
Ohm centimeter (Ω-cm)
1.9.1 Relative Resistivity
One of the best conductors is copper. It is convenient to
compare the resistance of the material under consideration
with that of a copper conductor of the same size and shape.
The ratio of these two is the relative resistivity of the
material.
In other word the relative resistivity of a material is
equal to the resistivity of that material divided by the
resistivity of copper.
1.9.2 Relative Resistivity of Metals
Material
Silver
Copper
Gold
Aluminum
Chromium
Tungsten
Zinc
Brass
Cadmium
Nickel
Iron
Platinum
Steel
Lead
Manganin
Nicrome
Relative resistivity
Resistivity (μΩ-cm)
0.94
1.0
1.4
1.6
1.8
3.2
3.4
3.7 - 4.9
4.4
5.1
5.68
6.5
7.6 - 12.7
12.8
25.6
58.1
1.59
1.7
2.4
2.7
3.1
5.4
5.8
6.3 – 8.3
7.5
8.7
9.7
11
12.9 – 21.6
21.8
43.5
98.8
Example:What is the resistance of a one meter long, 20 SWG copper
wire? (resistivity of copper is 1.7 µΩ-cm, diameter of 20
swg wire is 0.9144mm)
7
Apply the formula R = ρl / A
ρ = 1.7 µΩ-cm,
A = cross section of the wire = π r2
π = 3.14, r = 0.9144/2 = 0.4572 mm = 0.04572 cm,
l = 100cm
Therefore R = 1.7 x 100 /(3.14 x 0.045722)
= 25887 µΩ
= 0.025887 Ω or 25.887 mΩ
EXERCISES
1.1 What is the relationship between Coulomb and Ampere?
1.2 What is the difference between voltage and electro motive force?
1.3 Capacity of a 1.5 V dry cell is 1000 mA-h. 300mA current consumed for 30 minutes.
What is the remaining capacity of the cell?
1.4 A 12volts soldering iron element made by using nicrom wire having 0.5 mm diameter.
If the resistance of the element is 4 Ohms, what is the length of the nicrom wire.
(resistivity of nicrom is 98.8 µΩ-cm.
1.5 What is the resistance of 1m of 18 gauge copper wire?
(resistivity of copper is 1.7 µΩ-cm, diameter of 18 SWG is 1.219mm)
Answers
1.1
1.2
1.3
1.4
1.5
One Ampere is equal to one coulomb passing a point in one second.
EMF is the voltage between two terminals of a source, when there is no current flow.
850 mA-h
79.5 cm
11.064 mΩ