Download Lecture 2 - UniMAP Portal

ELECTRICAL TECHNOLOGY
EET 103/4
Define and explain the meaning of
current, voltage, resistance,
power, conductor, and insulator
1
Resistance (R)
• Opposition to current flow in a conductor –
collision and friction between free electrons, ions
and atoms.
• The energy used to move these charges is
converted into heat and raises the temperature of
the conductor.
• The opposition to the current flow is known as
resistance (R).
• The unit for resistance is ohm ()
2
Resistance (R)
• For large values kiloohm (k) and megaohm (M)
can be used.
1 k = 103 
1 M = 106 
• microohm () and milliohm (m) are used for
small values of resistance.
1 m = 10-3 
1  = 10-6 
3
Resistance (R)
• Material and its unique molecular
structure will react differently to
pressures to establish current through
its core.
– Conductors – Permit generous flow of
charge
– Insulators – Have high resistance
4
Resistance (R)
• The resistance of any material with a
uniform cross-sectional area is
determined by the following factors:
–
–
–
–
Material
Length
Cross-sectional Area
Temperature
5
Resistance (R)
• For a constant temperature, the
resistance (R) is dependent on :
– Material - resistivity ()
– Length (l)
– Cross-sectional Area (A)
R
l
A
6
Resistance (R)
R
l
A
7
Resistance (R)
• The higher the resistivity of a conductor, the
higher its resistance.
• The longer the length of a conductor, the
higher its resistance.
• The lower the cross-sectional area of a
conductor, the higher its resistance.
• The higher the temperature of a conductor,
the higher its resistance.
8
Resistance (R)
Effect of temperature
• Temperature has a significant effect on
the resistance of conductors,
semiconductors and insulators.
– For good conductors, an increase in temperature
will result in an increase in the resistance level.
Consequently, conductors have positive
temperature coefficients.
9
Resistance (R)
Effect of temperature
– For semiconductor materials, an increase in
temperature will result in a decrease in the
resistance level. Consequently, semiconductors
have negative temperature coefficients.
– As with semiconductors, an increase in
temperature will result in a decrease in the
resistance of an insulator. The result is a
negative temperature coefficient.
10
Resistor
• Component used in electric circuit to control
the current.
• General symbol.
11
Resistor types
• Resistors are made in many forms but all
belong in either of two groups:
Fixed resistors – are made of metal films,
high-resistance wire or carbon composition
Variable resistors – have a terminal
resistance that can be varied by turning a
dial, knob, screw, or anything else
appropriate for the application
12
Resistor types – fixed
Film resistors construction
13
Resistor types – fixed
Film resistors types
14
Resistor types – fixed
Fixed composition resistor construction
15
Resistor types – fixed
Fixed composition resistor appearance
16
Resistor types – variable
(a)Symbol
(b)Rheostat connection
(c)Rheostat connection
(d)Rheostat symbol
17
18
Resistor colour code
• Color coding was developed to identify
resistors that were too small for their
resistance value to be printed on them.
– Color bands are always read from the
end that has the bands closest to it.
19
Resistor colour code
– 1st and 2nd band represent the first two digits
– 3rd band determines the power-of-ten
multiplier (the number of zeros following the
second digit)
– 4th band is the manufacturer’s tolerance
(precision of the resistor)
20
Resistor colour code
21
Resistor colour code – example
22