1 electrical technology eet 103/4 define and explain the meaning of current, voltage, resistance,...
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ELECTRICAL TECHNOLOGY EET 103/4
Define and explain the meaning of current, voltage, resistance,
power, conductor, and insulator
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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 ()
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• For large values kiloohm (k) and megaohm (M) can be used.
Resistance (R)
1 k = 103 1 M = 106
• microohm () and milliohm (m) are used for small values of resistance.
1 m = 10-3 1 = 10-6
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• 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
Resistance (R)
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• The resistance of any material with a uniform cross-sectional area is determined by the following factors:– Material – Length– Cross-sectional Area– Temperature
Resistance (R)
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• For a constant temperature, the resistance (R) is dependent on :– Material - resistivity ()– Length (l)– Cross-sectional Area (A)
Resistance (R)
A
lR
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Resistance (R)
A
lR
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• 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.
Resistance (R)
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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.
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– 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.
Resistance (R)
Effect of temperature
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Resistor• Component used in electric circuit to control
the current.
• General symbol.
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• 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
Resistor types
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Resistor types – fixed
Film resistors construction
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Film resistors types
Resistor types – fixed
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Fixed composition resistor construction
Resistor types – fixed
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Fixed composition resistor appearance
Resistor types – fixed
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Resistor types – variable
(a)Symbol(b)Rheostat connection(c)Rheostat connection(d)Rheostat symbol
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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.
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– 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)
Resistor colour code
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Resistor colour code
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Resistor colour code – example