1. This unit covers basic electrical concepts including charge, current, energy, power, voltage, resistors and circuits. It defines key terms and formulas for calculations. 2. Resistors are introduced as circuit elements that limit current. Different types are described including fixed, variable, and those identified by color codes. 3. Kirchhoff's laws and Thevenin's theorem are presented as methods for analyzing circuits to calculate current, voltage, resistance and simplify complex networks.

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Unit 2: Basic Concepts
Objectives
After studying this unit, you should be able to:
• Measure voltage, current and resistance.
• Identify the different types of resistors.
• Identify resistors by using the colour code
• Do calculation by applying Ohm’s-,
Kirchhoff’s law and Thevenin’s theorem.
1. Charge: The attraction or repelling ability of
a particle. Opposite charges attract

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Unit 2: Basic Concepts
The same charges repel
2. Current: The rate is the flow of charge
A higher flow of current means an increase in charge
• Energy: Joule is the work done when a force of one newton displace an
object through a distance of one meter.

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Unit 2: Basic Concepts
1 Joule = 1 Newton X 1 meter
1 Newton
1 meter

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Unit 2: Basic Concepts
4. Power: The watt is the power derived from
producing one joule of energy / second.
5. Volt: One volt is the difference in potential
across a load which causes a one joule of
energy to be released when a charge of
one coulomb flows through it.
6. Electric power: Power is measured in watt
and the watt is the power generated when
producing one joule of energy/second.
Q = I x t
W = V x I x t
P = V x I

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Unit 2: Basic Concepts
7. Electric circuits
Electric circuits are the conveyer of energy. It
conveys the energy from the source, namely the
battery to the load. The load consumes the
energy when it is connected to an external
device, for instance a radio.

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Formula for Current
Current is measured as the amount of
charge that is flowing in a second.
Charge => Coulombs=> Q
Current => Amperes => I
Time => Seconds => t
I = Q/t

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Electrical power formula
1 Volt = 1joule / 1 coulomb
Or
V = W / Q

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Unit 2: Basic Concepts
E
Source
Load
BT2
BATTERY
R
E is the electromotive force (emf) which is present across
the battery.
V is the potential difference (pd) across the load.
Both the emf and pd is measured in volt.
8. Ohm’s Law
George Ohm discovered that the relationship between the
current through a resistance and the voltage applied
across it is directly proportional.

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Unit 2: Basic Concepts
9. Resistors
The resistance value of resistors depends on the
material used during manufacturing, the length
and cross-sectional area as well as the
temperature.
Resistors limit electric current in a circuit.
9.1 Types of resistors
There are different fixed resistors available:
The high power types are wire wound to
withstand high heat levels.
The low power types have their tracks made of
carbon film, coated onto a cylindrical former.

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Unit 2: Basic Concepts
9.2 Fixed resistors
Some resistors are
cylindrical, with the actual
resistive material in the
center or on the surface of
the cylinder (film) resistors.
There are carbon film and
metal film resistors. Power
resistors come in larger
packages designed to
dissipate heat efficiently.
High power resistors is wire
wound resistors.
The sketch shows the
construction of a carbon film
resistor.

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Unit 2: Basic Concepts
9.3 Wirewound
Resistors are made by winding thin
wire onto a ceramic rod. They can be
made extremely accurately for use in
multimeters, oscilloscopes and other
measuring equipment. Some types of
wirewound resistors can pass large
currents without overheating and are
used in power supplies and other high
current circuits.
9.4 Variable resistors
The variable resistor can be adjusted
by turning a shaft or sliding a control.
They are also called potentiometers or
rheostats and allow resistance of the
device to be altered by hand.
It is worthwhile to have a look at a few
of the common pot types that are
available. Figure 1 shows an array of
conventional pots - both PCB and
panel mounting.

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Unit 2: Basic Concepts
9.6 Linear potentiometers
A linear pot has a resistive element of constant cross-
section, resulting in a device where the resistance
between the wiper and one end terminal is proportional
to the distance between them. Linear describes the
electrical 'law' of the device, not the geometry of the
resistive element.
9.7 Logarithmic potentiometers
A log pot has a resistive element that either 'tapers' in
from one end to the other, or is made from a material
whose resistivity varies from one end to the other. This
results in a device where output voltage is a logarithmic
(or inverse logarithmic depending on type) function of the
mechanical angle of the pot.

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Unit 2: Basic Concepts
9.8 Colour code
• How can the value of a resistor be worked out from the
colours of the bands? Each colour represents a number
according to the following scheme:
• Resistance is measured in ohms, the symbol for ohm is
an omega (Ω).
1 is quite small so resistor values are often given in kΩ
and MΩ.
1 k = 1000Ω 1 M = 1000000Ω.
• Resistor values are normally shown using coloured
bands.
Each colour represents a number as shown in the table.

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Unit 2: Basic Concepts
Colour Digit 1 Digit 2 Multiplier Tolerance
Black 0 0 0x0
Brown 1 1 1x0 1%
Red 2 2 2x0 2%
Orange 3 3 3x0
Yellow 4 4 4x0
Green 5 5 5x0
Blue 6 6 6x0
Violet 7 7 7x0
Grey 8 8 8x0

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Unit 2: Basic Concepts
Colour Digit 1 Digit 2 Multiplier Tolerance
White 9 9 9x0
Gold 0.1 5%
Silwer 0.01 10%
None 20%

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Unit 2: Basic Concepts
• The first band gives the first digit.
• The second band gives the second digit.
• The third band indicates the number of
zeros.
• The fourth band is used to shows the
tolerance (precision) of the resistor, this
may be ignored for almost all circuits but
further details are given.

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Unit 2: Basic Concepts
9.9 Resistor E-12 series
To limit the range of resistor values to a manageable
number a preferred range only is available.
These are
10 12 15 18 22 27 33 39 47 56 68 82
This means that 1 ohm, 12 ohm, 180 ohm, 2200 ohm
resistors etc are available.
10.Series and parallel circuits
10.1 Series Circuit
R3
R2
R1
RT = R1 + R2 + R3

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Unit 2: Basic Concepts
10.2 Parallel Circuit
0,8A
R2
4R
R1
10R

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Unit 2: Basic Concepts
10. Voltage Sources
DS1
LAMP
1
2
Vs
The internal resistance of a voltage source should be very low.
Rint
RL
13V
Vs
13.5V

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2: Basic Unit Concepts
11 Current Source
1A
3A
2A
Rload
Rint
The internal resistance of a current source should be very high

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2: Basic Unit Concepts

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Unit 2: Basic Concepts
11. Kirchhoff’s Law
11.1 Current Law: For a given junction or
node in a circuit, the sum of the currents
entering equals the sum of the currents
leaving.
I1 = I2 + I3

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Unit 2: Basic Concepts
11.2 Kirchhoff’s Voltage law
At any instant the algebraic sum of the emf’s in a
closed loop is equal to the algebraic sum of the
pd’s around the same loop.
E1 + E2 = VR1 + VR2 + VR3 + VR4
R2
3V
E1
20V
3V
1k E2
8V
R3
4V
R1
2V
20 + (-8) = 2 + 3 + 4 + 3

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Unit 2: Basic Concepts
11.2 The Voltage Law state that around any closed loop
in a circuit, the sum of the potential differences across
all elements is zero.
12. Voltage divider
Voltage divider circuits consist of series connected
resistors and each will develop a voltage across it,
which is lower than the supply voltage.

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Unit 2: Basic Concepts
13. Voltage Divider
E
V2
R2
R1
2
1
2
CC
2
R
x
V


R
R
V

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Unit 2: Basic Concepts
14. Current divider
In the case of current
dividers the resistors
are connected in
parallel.
I2 = IT x
It
R1 R2
2
1
1
R

R
R

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Unit 2: Basic Concepts
15. Thevenin’s Theorem
Any combination of batteries and
resistances with two terminals can be
replaced by a single Voltage source (e)
and a single series resistor ®. The value
of e is the open circuit voltage at the
terminals, and the value of r is e divided
by the current with the terminals short
circuited.

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Unit 2: Basic Concepts
15.1 Thevenin’s equivalent voltage VTH
The Thevenin voltage e used in Thevenin's Theorem is an ideal
voltage source equal to the open circuit voltage at the terminals.
In the example below, the resistance R2 does not affect this
voltage and the resistances R1 and R3 form a voltage divider,
giving

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Unit 2: Basic Concepts
The Thevenin resistance r used in Thevenin's Theorem is the resistance
measured at terminals AB with all voltage sources replaced by short circuits
and all current sources replaced by open circuits. It can also be calculated
by dividing the open circuit voltage by the short circuit current at AB, but the
previous method is usually preferable and gives

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Unit 2: Basic Concepts
15.2 Thevenin’s equivalent resistance RTH
The Thevenin resistance r used in Thevenin's Theorem is the
resistance measured at terminals AB with all voltage sources
replaced by short circuits and all current sources replaced by
open circuits. It can also be calculated by dividing the open
circuit voltage by the short circuit current at AB, but the previous
method is usually preferable and gives