Download Electricity and Circuitry

PDT 180
ENGINEERING SCIENCE
ELECTRICITY AND CIRCUITRY
MISS MUNIRA MOHAMED NAZARI
SCHOOL OF BIOPROCESS ENGINEERING
UNIMAP
CO 4
Ability to analyze
basic electrical
circuitry.
2
SESSION 2012/2013
TOPIC OUTLINE
 Concepts of electricity,
 Insulators and conductors,
 Electric fields,
 Coulomb’s Law.
 Concepts of electric current,
 Ohm’s Law,
 Electric power,
 Series and parallel wiring
3
SESSION 2012/2013
BACIS CONCEPTS OF ELECTRICITY
OBJECTIVES
-To define electricity
- To identify electrical charges
- To define electrical fields
4
SESSION 2012/2013
Concept of Electricity
 Electricity
Greek word electrons which mean
“amber” which is a petrified tree
resin.
 If an amber rod is rubbed with a piece of cloth, the amber
attracts small pieces of leave or dust.
 If u rub a plastic ruler and bring it close to some tiny pieces
of papers, the ruler will eventually attract the papers.
This phenomenon is called “static electricity”
5
SESSION 2012/2013
Concept of Electricity
 Object can be charged by rubbing.
6
SESSION 2012/2013
Electric charge
 Electrostatics is the study of interaction between electric
charges which are not moving.
 Two types of electric charges:
Positive
7
SESSION 2012/2013
Carried by
particles called
electrons.
Negative
Carried by
particles called
protons.
Electric charge
 The force between the charges
can be either attraction or
repulsion.
 Unit for charge is Coulomb, C
where;
1 Coulomb  6.25 x 108 electrons or protons
or
1 e  1.6 x 10-19 coulomb
 Mass of electron = 9.11 x 10-31 kg
 Mass of proton
8
SESSION 2012/2013
= 1.672 x 10-27 kg
Conservation of electric charge
 Law of conservation of electric charge states that the net
amount of electric charge produced in any process is zero.
 It means that in any process, electric charge cannot be created
or destroyed.
 It can be transfer to one object to another.
 For example, when a plastic ruler is rubbed with a paper towel,
the plastic acquires a negative charge and the towel an equal
number of positive charges. The charges are separated but the
sum of the two is zero.
9
SESSION 2012/2013
Conservation of electric charge
 Can be illustrated using a simple
model of atom.
 A neutral atom contains an equal
number of protons and electrons.
 If the number of protons and
electrons are not same, the atom will
have a net positive or negative
charge.This atom is called an ion.
10
SESSION 2012/2013
Insulators and conductors
 Materials can be either insulating or conducting materials.
 Insulating
Material contains electrons which are not free
to move through the material. Eg: glass & wood
 Conductors
Material contains electrons are free to move in
the material. Eg: steel & gold
 Semiconductors
11
SESSION 2012/2013
Material in which there are a few free electrons
and the material is a poor conductor or
electricity. Eg: silicon & carbon.
Coulomb’s law
 Experiment shows that the electric force between two
charges is proportional to the product of the charges and
inversely proportional to the distance between them.
12
SESSION 2012/2013
Coulomb’s Law
 Coulomb’s Law states that two point charges exert a force
(F) on one another that is directly proportional to the
product of the magnitudes of the charges (Q) and inversely
proportional to the square of the distance (r) between their
centers.
Q1
Q2
r
 We can rewrite the Coulomb’s Law in term of equation as:
Q1 Q 2
Fk 2
r
k is a constant which has a value of 8.988 x 109 Nm2 / C2
13
SESSION 2012/2013
Coulomb’s Law
 The force is along the line connecting the charges and is
attractive if the charges are opposite, and repulsive if they are
the same.
14
SESSION 2012/2013
Example 1
 Find the magnitude and direction of the force on the
electron.
Q1 Q 2
Fk 2
r
k  8.988 x 109 Nm2 / C 2
r  0.53 x 10-10 m
F  8.2 x 10 N
-8
15
SESSION 2012/2013
Example 2
 Which charge exerts the greater force?
These two forces have equal magnitude. F21 = F12.
The force on Q2 exerted by Q1 is the same as the
force on Q1 exerted by Q2 except that Q1 and Q2 are
reversed.
16
SESSION 2012/2013
Example 3
 Three charges in a line.
 Three charged particles are arranged in a line as shown below.
Calculate the net electrostatic force on particles 3 (the -4.0 µC
on the right) due to the other two charges.
0.30 m
-
0.20 m
+
Q1 = - 8.0 µC
Q2 = +3.0 µC
Q3 = - 4.0 µC
F = - F32 + F31 = -2.7 N + 1.2 N = - 1.5 N
17
SESSION 2012/2013
Electric field
 An electric field present if an electric charge experiences an
electric force at any particular point in space.
 In order to visualize the path taken by a charged particle in an
electric field; electric field lines (lines of force) are drawn.
 These line start on a positive charge and end on negative
18
charge.
SESSION 2012/2013
Electric field
 The number of field lines starting (ending) on a positive
(negative) charge is proportional to the magnitude of the
charge.
 The electric filed is stronger where the field lines are closer
together.
19
SESSION 2012/2013
Electric field
 Electric dipole : two equal charges,
opposite in sign.
 The electric field between two
closely spaced, oppositely parallel
plates is constant.
20
SESSION 2012/2013
Electric field
 Summary of field lines:
 Field lines indicate the direction of the field; the field is tangent
to the line.
 The magnitude of the field is proportional to the density of the
lines.
 Field lines start on positive charges and end on negative
charges; the number is proportional to the magnitude of the
charge.
21
SESSION 2012/2013
BACIS CONCEPTS OF CIRCUITRY
OBJECTIVES
-To define electric current, voltage, resistance and electric power
- To define electric circuit
- To analyze the series and parallel wiring.
22
SESSION 2012/2013
Concepts of Electric Current
 An electric current exists whenever electric charge flows
from the battery terminals through a region or circuit like a
light bulb circuit as shown in figure below.
 The magnitude of the current is measures in amperes (A).
23
SESSION 2012/2013
Concepts of Electric Current
 Above figure represent symbol of electric current flow.
 What actually happened is that free electron charges moving
from the battery terminal and flow or crossing through the
wire.The flow charge is known as electric current.
24
SESSION 2012/2013
Concepts of Electric Current
 Definition
 Electric current in a wire is defined as the net amount of charge
that passes through the wire per unit time at any point.
 The average current is defined as:
 Unit for electric current = ampere (A)  (coulomb/sec)
25
SESSION 2012/2013
Example : Electric charge flow
 A steady current of 2.5 A flows in a wire for 4.0 min.
Calculate charge passed through any point in the circuit.
How many electrons would this be?
26
SESSION 2012/2013
Ohm’s Law
 Ohm’s law discusses on resistance in electrical circuit.
 Electric resistance is the resistance of electrical current flow.
Electrical resistance is important since it can control the
amount of current flow. Ohm defines resistance, R as the
ratio of the voltage applied across the circuit.
 Resistance, R of the piece of material or wire is given as
 Unit for resistance, R = ohm (Ω)
27
SESSION 2012/2013
Example : Resistance in a bulb
 A small flashlight bulb draws 300 mA current from 1.5 volt
battery. Determine the resistance of the flashlight bulb.
R = V/I
= 1.5 V / 0.30 A
= 5.0 Ω
28
SESSION 2012/2013
Ohm’s Law
 Resistors
 All electrical devices have resistance to the flow of current.
 For instance, the connecting wires of a circuit have resistance.
However, in electronic devices, resistors are used to control the
amount of current.
 The types of resistor varied from fixed resistor to variable
resistor. When we draw a circuit diagram, we indicate the
resistance or the resistor in the circuit by the symbol:
Fixed Resistor
29
SESSION 2012/2013
Variable Resistor
Ohm’s Law
 Resistance factors in a wire.
 Wire in an electric circuit can be thick or thin wire and the
resistance is not the same. There ate 3 factors that affect the
resistance of a wire which are:
 Length of wire
 Cross sectional area
 Resistivity of the wire.
 It was found that the relationship of these factors is as
30
SESSION 2012/2013
Ohm’s Law
 The longer the wire, the higher the resistance and if the cross
sectional areas of the wire is smaller, the resistance is also
higher.
 Meanwhile, the symbol ρ represents the resistivity of the
material. Resistivity of the material depends on the property of
the material.
 For conductors, the resistivity value is higher compared to
insulators.
 Meanwhile, resistivity for semiconductor materials depends on
the temperature of the material.
31
SESSION 2012/2013
Electric Power
 Power as in kinematics, is the energy transformed by a device
per unit time.
 Unit for power = watt, W
32
SESSION 2012/2013
Electric Power
 If the devices have resistors, then the electrical power of the
device is transformed into other form of energy.
 For instance, a toaster, irons, stoves, etc become hot when
provided with electrical power.
 If we want to express the power in term of resistance, then
the formula becomes as:
33
SESSION 2012/2013
Example : Flashlight
 Determine the resistance of the bulb
V = 1.5 Volt
P = 0.15 Watt
I = P/V = 0.1 A
R = V/I = 15 Ω
 Determine the power if V = 1.2 volt
P = V²/R = 0.096 Watt
34
SESSION 2012/2013
Series and Parallel Wiring
 There are 2 methods by which connection can be made:
 Series wiring
 Parallel wiring
35
SESSION 2012/2013
Series and Parallel Wiring
 Series wiring
 Means that the devices are connected in such a way that there is
the same amount of current flow through each device.
 In the series wiring, if one of the devices is disconnected, the
current will not be able to flow through another device, which
means that the device is interrupted too.
 Because of the series wiring, the voltage supplied by the battery
id divided between the devices.
 For instance, if we have 3 resistors, then, the voltage is divided
between the 3 resistors.
36
SESSION 2012/2013
Series and Parallel Wiring
 Voltage, V of the battery,
V = V1 + V2 + V3 = IR2 + IR3
V = I (R1 + R2 + R3)
 Equivalent resistance in the circuit,
Req = R1 + R2 + R3
37
SESSION 2012/2013
Series and Parallel Wiring
 Parallel wiring
 Means that the devices are connected in such a way that the
same voltage is applied across each device.
 Parallel wiring is the most popular wiring method. This is
because, if the current in one of the devices is interrupted (by
opened or broken wire), then the current in the other devices
are not interrupted.
 In parallel circuit, the total current I that leave the battery break
into each branch.
 Eg: If we have two branches, then the current is I1 and I2.
38
SESSION 2012/2013
Series and Parallel Wiring
 Total current is
I = I1 + I2
 Devices in parallel circuit each experience the same voltage
from the main voltage (battery). Therefore, each of the current
can be represented as
I1 = V/R1
39
SESSION 2012/2013
and
I2 = V/R2
Series and Parallel Wiring
 Similar to series circuit, we need to determine the equivalent
resistance of circuit. This can be done by adding all the current
in each branch.
 For instance, in a 3 branch parallel circuit:
I = I 1 + I2 + I3
= V/R1 + V/R2 + V/R3
= V(1/R1 + 1/R2 + 1/R3)
 Therefore
1/Req = V(1/R1 + 1/R2 + 1/R3)
40
SESSION 2012/2013
Example
 Two resistors of 200 Ω are connected (a) in series and (b) in
parallel to a 24.0 volt battery. Determine the equivalent
resistance and the current through each resistor.
41
SESSION 2012/2013
Summary
 A battery is a source of constant potential difference.
 Electric current is the rate of flow of electric charge.
 Conventional current is in the direction that positive charge
would flow.
 Resistance is the ratio of voltage to current:
 Power in an electric circuit:
42
SESSION 2012/2013