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M3 > 3.7 Resistance and Resistors > Flashcards

3.7 Resistance and Resistors Flashcards

1
Q
A
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2
Q

What is resistance in the context of electrical properties?

A

Resistance is an electrical property of a conductor.

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3
Q

How does resistance affect voltage and current in a conductor?

A

A higher resistance requires a higher voltage to be applied across the conductor for a given current.

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4
Q

List the factors that influence the electrical resistance of a material.

A
  • Length of the object
  • Cross-sectional area of the object
  • Material from which the object is made
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5
Q

How does the length of a resistor affect its resistance?

A

Resistance increases in direct proportion to the length of a resistor.

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6
Q

What happens to resistance if the length of a resistor is doubled?

A

The resistance is also doubled.

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7
Q

How does the cross-sectional area of a resistor affect its resistance?

A

Resistance increases in inverse proportion to the cross-sectional area of a resistor.

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8
Q

What happens to resistance if the cross-sectional area of a resistor is doubled?

A

The resistance is halved.

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9
Q

Do all materials oppose the flow of electric current?

A

Yes, all materials oppose the flow of an electric current to some extent.

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10
Q

What conditions must be met to compare the resistances of various materials?

A
  • Length: L = 1 m
  • Cross-sectional area: q = 1 mm²
  • Temperature: 20°C
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11
Q

Fill in the blank: Resistance is determined by the _______ of the object.

A

[length, cross-sectional area, material]

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12
Q

True or False: Resistance decreases if the cross-sectional area of a resistor is increased.

A

True

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13
Q

What is specific resistance?

A

Specific resistance, also known as resistivity, is an electrical property of a material that measures the resistance of the material.

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14
Q

What factors affect the electrical resistance of an object?

A
  • Length of the object
  • Cross-sectional area of the object
  • Material from which the object is made
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15
Q

How does resistance change with the length of a resistor?

A

Resistance increases in direct proportion to the length of a resistor.

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16
Q

How does resistance change with the cross-sectional area of a resistor?

A

Resistance increases in inverse proportion to the cross-sectional area of a resistor.

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17
Q

What are the standard dimensions used for comparing resistances of various materials?

A
  • Length: L = 1 m
  • Cross-sectional area: q = 1 mm²
  • Temperature: 20 °C
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18
Q

What is the mathematical relationship between resistance, length, and cross-sectional area?

A

R = ρ * (L/A), where R is resistance in ohms (Ω), L is length in metres (m), A is cross-sectional area in square metres (m²), and ρ is specific resistance.

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19
Q

What symbol is used to represent specific resistance?

A

The symbol for specific resistance is ⍴ (rho).

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20
Q

What are the units of specific resistance?

A

The units of specific resistance are ohm metres (Ωm).

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21
Q

Fill in the blank: Ohm metres (Ωm) is the _______ unit for specific resistance.

A

SI

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22
Q

What alternative unit is commonly used in engineering for specific resistance?

A

Ohm millimetres squared per metre (Ωmm²/m)

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23
Q

What are fixed resistors designed to provide?

A

Fixed resistors are designed to provide a constant value of resistance.

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24
Q

What are variable resistors designed to provide?

A

Variable resistors are designed to provide a range of resistance values.

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25
Q

What is the specific resistance of copper at 20 °C?

A

0.01786 Ωmm²/m

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26
Q

What is the specific resistance of iron at 20 °C?

A

0.10 to 0.15 Ωmm²/m

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27
Q

True or False: A copper wire that is twice as long as another copper wire with the same cross-sectional area will have double the resistance.

A

True

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28
Q

True or False: The specific resistance of a material is independent of its dimensions.

A

True

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29
Q

What is the specific resistance of silver at 20 °C?

A

0.016 Ωmm²/m

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30
Q

List the resistivities of the following substances at 20 °C: Copper, Aluminium, Iron, Silver, Gold.

A
  • Copper: 0.01786 Ωmm²/m
  • Aluminium: 0.02857 Ωmm²/m
  • Iron: 0.10 to 0.15 Ωmm²/m
  • Silver: 0.016 Ωmm²/m
  • Gold: 0.023 Ωmm²/m
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31
Q

What do resistor colour codes indicate?

A

Resistance, wattage rating, and tolerance

Engineers must be able to read these codes to build or maintain electrical circuits.

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32
Q

What does the tolerance of a resistor represent?

A

The extent to which it can differ from the rated value

The tolerance is indicated in the colour code and affects the interval between available resistance values.

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33
Q

What is the maximum consequence of exceeding a resistor’s wattage rating?

A

Damage to the circuit and risk of fire

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34
Q

How do manufacturers indicate resistance and tolerance on resistors?

A

Using a colour code

Big resistors may have additional resistance values printed.

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35
Q

What is the tolerance percentage for the E12 range of resistors?

A

10%

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36
Q

What are preferred values in resistor manufacturing?

A

Values of the E12 and E24 range

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37
Q

What is the factor for the E12 range?

A

1.21

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38
Q

What is the factor for the E24 range?

A

1.12

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39
Q

What happens if the result of a calculation is 500 ohms?

A

Use the closest standard value, i.e., 470 ohms

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40
Q

What is the power rating of a resistor?

A

The maximum power that can be dissipated without affecting its function

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41
Q

What is the typical power rating range for small resistor components?

A

50 to 500 mW

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42
Q

What can large wire-wound resistors handle in terms of power?

A

100 W and more

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43
Q

What effect does ambient temperature have on a resistor’s power rating?

A

Ambient temperatures above the stated value will invalidate the power rating

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44
Q

Fill in the blank: The electrical energy applied to a resistor is converted completely into _______.

A

heat

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45
Q

What is one way to prevent overheating in a resistor?

A

Increase the surface area while maintaining the cross-sectional area

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46
Q

True or False: All resistors have the same tolerance.

A

False

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47
Q

What are the IEC standard ranges for resistors?

A

E6, E12, and E24

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48
Q

How many values are there in the E12 range?

A

12 values

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49
Q

How many values are there in the E24 range?

A

24 values

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50
Q

What is the total resistance in a series connection of resistors?

A

The total resistance, RTotal, is the sum of the individual resistances.

This is expressed as RTotal = R1 + R2 + … + Rn.

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51
Q

What is a potential divider?

A

A series connection of resistors where the voltage is divided among the resistors.

Each resistor receives a portion of the total voltage, which can be tapped at a certain point.

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52
Q

What happens to the current in a series circuit?

A

The current is the same through all components.

This is analogous to water flowing through different sections of a hose pipe.

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53
Q

What is the relationship between voltage and resistance in a series circuit according to Ohm’s law?

A

V = I * R for each resistor.

The total voltage is the sum of the voltages across each resistor.

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54
Q

What is the total resistance in a parallel connection of resistors?

A

The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances.

This can be expressed as 1/RTotal = 1/R1 + 1/R2 + … + 1/Rn.

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55
Q

What is a characteristic of resistors connected in parallel?

A

Voltage is the same across each resistor.

This means each resistor experiences the same potential difference.

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56
Q

What happens to the total current in a parallel circuit?

A

The total current is the sum of the currents in each branch.

This allows for more current paths, effectively increasing the conductor cross-section.

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57
Q

True or False: In a series circuit, the total resistance is always greater than the smallest individual resistance.

A

True.

This is because resistances add up in series connections.

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58
Q

Fill in the blank: In a series circuit, the _____ provided by the battery is divided between the resistors.

A

voltage

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59
Q

What is a key feature of resistors connected in series?

A

The individual resistances must be added together to find the total resistance.

This is a fundamental property of series circuits.

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60
Q

What does Kirchhoff’s second law state regarding series resistors?

A

The total voltage across all resistors is equal to the sum of the voltages across each resistor.

This law is essential for analyzing series circuits.

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61
Q

How does a parallel connection affect total resistance?

A

The total resistance is lower than the lowest individual resistance.

This is due to the multiple paths available for current flow.

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62
Q

What is the method to calculate total resistance for resistors in series?

A

Add the resistances together

The formula for total resistance in series is R_total = R1 + R2 + … + Rn.

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63
Q

What is the formula for calculating total resistance in parallel?

A

Add the reciprocals of the individual resistances

The formula for total resistance in parallel is 1/R_total = 1/R1 + 1/R2 + … + 1/Rn.

64
Q

What is the effective resistance when a 3 Ω resistor is placed in series with a 4 Ω resistor?

A

7 Ω

Calculation: R_total = 3 Ω + 4 Ω.

65
Q

What is the effective resistance when a 3 Ω resistor is placed in parallel with a 4 Ω resistor?

A

1.2 Ω

Calculation: 1/R_total = 1/3 + 1/4 leads to R_total = 1.2 Ω.

66
Q

How do you approach circuits with series-parallel combinations?

A

Calculate effective resistance of series or parallel groups first

Continue applying the respective formulas until reduced to a single series or parallel combination.

67
Q

What is the effective resistance of a series-parallel combination circuit with three resistors resulting in 4 Ω?

A

4 Ω

This is the final effective resistance after calculations for the series-parallel combination.

68
Q

What is the effective resistance of a series-parallel combination circuit with three resistors resulting in 5 Ω?

A

5 Ω

This is the final effective resistance after calculations for the second series-parallel combination.

69
Q

Fill in the blank: When resistors are connected in series, the total resistance is equal to the sum of the _______.

A

individual resistances

70
Q

True or False: In a parallel circuit, the total resistance is always less than the smallest individual resistance.

A

True

71
Q

What is the first step in calculating the effective resistance of a series-parallel circuit?

A

Calculate the combined resistance of series or parallel resistors

This involves simplifying the circuit by solving one section at a time.

72
Q

What is a variable resistor?

A

A resistor with adjustable resistance values

It typically works by sliding a contact (wiper) over a resistive element.

73
Q

What is a potentiometer?

A

A three-terminal device used to control voltage

It functions as a potential divider.

74
Q

What is a rheostat?

A

A two-terminal device used to control current

It functions as a variable resistance.

75
Q

How are potentiometers and rheostats different?

A

Potentiometers have three terminals, rheostats have two

The difference is more about connection than construction.

76
Q

What are the typical applications of potentiometers?

A
  • Volume controls in audio circuits
  • Brightness in lighting circuits
  • Varying speed of DC motors
77
Q

What is the function of the wiper in a potentiometer?

A

It moves over a resistive element to adjust voltage

The position of the wiper determines the output voltage.

78
Q

What is the symbol for a potentiometer?

A

A symbol representing a three-terminal device with a wiper

It indicates the connections for input voltage and output voltage.

79
Q

What arrangement can a rheostat have?

A

Two alternative arrangements with two terminals

Option 1: terminal 3 to input current, terminal 2 to output current. Option 2: terminal 3 to input current, terminals 1 and 2 to output.

80
Q

What is the main use of a rheostat in a circuit?

A

To restrict current by increasing resistance

This can be achieved by reducing voltage or increasing resistance (Ohm’s Law).

81
Q

Fill in the blank: A digital potentiometer is an electronically controlled _______.

A

resistor

82
Q

What is a pre-set potentiometer?

A

A potentiometer mounted on printed circuit boards with limited range

Usually set during manufacture or maintenance.

83
Q

What is the typical construction of a high-power rheostat?

A

A wiper moving over copper windings

It is designed to handle higher currents.

84
Q

True or False: Rheostats can be used in high-power applications.

A

True

85
Q

What does a rheostat control within a circuit?

A

The current

It does this by varying resistance, which affects current flow.

86
Q

What happens to the current when the resistance in a rheostat increases?

A

The current decreases

According to Ohm’s Law, as resistance increases, current decreases.

87
Q

What happens to the resistivity of conductors as temperature increases?

A

Resistivity increases, which decreases conductivity.

88
Q

What does PTC stand for?

A

Positive Temperature Coefficient.

89
Q

What characterizes a Positive Temperature Coefficient (PTC) conductor?

A

Resistance increases with increasing temperature.

90
Q

What is indicated by a negative temperature coefficient?

A

Resistance decreases with increasing temperature.

91
Q

How is the proportional change in resistance calculated?

A

Proportional change in resistance = change in temperature x temperature coefficient.

92
Q

What is a typical value for the temperature coefficient of a carbon film resistor?

A

Around 0.0008/K.

93
Q

What does ppm/K stand for?

A

Parts per million per kelvin.

94
Q

Convert 0.0008/K to ppm/K.

A

800 ppm/K.

95
Q

If a 10 kΩ resistor has a temperature coefficient of +0.0008/K and the temperature increases by 100 degrees, what is the increase in resistance?

A

800 Ω.

96
Q

What type of graph do PTC conductors follow?

A

A linear upward gradient.

97
Q

What are semiconductors with decreasing resistivity as temperature increases called?

A

Negative Temperature Coefficient (NTC) materials.

98
Q

True or False: PTC conductors have a downward curve on their graph.

A

False.

99
Q

What is the primary characteristic of fixed resistors?

A

Fixed electrical resistance that is not adjustable.

100
Q

What are variable resistors?

A

Resistors that can be set at different resistance values or change in response to environmental factors such as temperature or light.

101
Q

What is the purpose of resistors in a circuit?

A

To control voltage or current and to detect changes in the environment.

102
Q

What types of packages can fixed resistors have?

A

Axial leads, surface mount packages, or customized packages.

103
Q

What type of resistor is currently the most popular?

A

Surface Mounted Device (SMD) resistors.

104
Q

Define a perfect resistor.

A

A perfect resistor has a constant ohmic resistance independent of frequency, voltage, or temperature.

105
Q

True or False: All resistors have perfect resistance.

A

False.

106
Q

What factors determine the extent to which external factors affect resistance values?

A

The type of resistor and its material composition.

107
Q

What should the choice of resistor be based on?

A

Required accuracy, power dissipation, and noise considerations.

108
Q

What is the most common type of axial lead resistor?

A

Carbon film resistors.

109
Q

What is the typical application for metal film resistors?

A

Higher-precision applications.

110
Q

What type of resistors are typically used for high-power applications?

A

Wire wound resistors.

111
Q

What is often used for power ratings from 1 to 200 W?

A

Aluminium housing.

112
Q

What materials are SMD resistors typically constructed from?

A

Thick or thin film material.

113
Q

Fill in the blank: The _______ film is the most common axial lead resistor.

A

carbon

114
Q

What are the main limitations of fixed resistors?

A

Resistance values vary within defined tolerance limits.

115
Q

List the three common types of fixed resistors.

A
  • Carbon film
  • Metal film
  • Wire wound
116
Q

What is the primary function of variable resistors?

A

Variable resistors have adjustable resistance values.

117
Q

What is the difference between a potentiometer and a rheostat?

A

A potentiometer has three terminals and is used as a potential divider, while a rheostat has two terminals and functions as a variable resistance.

118
Q

How can variable resistors be controlled?

A

Variable resistors can be controlled electronically or mechanically.

119
Q

What are thermistors?

A

Thermistors are temperature-dependent resistors that change resistance with temperature.

120
Q

What are the two types of thermistors based on their temperature response?

A

Positive Temperature Coefficient (PTC) Resistors and Negative Temperature Coefficient (NTC) Resistors.

121
Q

What does a Positive Temperature Coefficient (PTC) resistor do?

A

PTC resistors have a positive temperature coefficient, meaning their resistivity increases proportionately with temperature.

122
Q

What is the characteristic of NTC thermistors?

A

NTC thermistors have a negative temperature coefficient, meaning their resistance decreases as temperature increases.

123
Q

What materials are typically used to make NTC thermistors?

A

NTC thermistors are made of polycrystalline mixed crystals of iron oxides, cobalt oxides, titanium compounds, and specific impurities.

124
Q

What happens to the conductivity of NTC thermistors as temperature rises?

A

As temperature rises, more electrons escape from their orbits, increasing the conductivity of the material.

125
Q

What is the purpose of voltage-dependent resistors (VDR)?

A

The resistance value of VDR changes according to the applied voltage.

126
Q

How does the resistance of a VDR change with voltage?

A

The resistance of a VDR decreases as the voltage increases, irrespective of the polarity of the voltage.

127
Q

Fill in the blank: A variable resistor with three terminals is called a _______.

A

potentiometer

128
Q

Fill in the blank: A variable resistor with two terminals is called a _______.

A

rheostat

129
Q

True or False: NTC thermistors are primarily used for temperature stabilization in semiconductor connections.

A

True

130
Q

What is the significance of the characteristic conductance curves for PTC thermistors?

A

PTC thermistors undergo a large and highly predictable change in resistance as temperature increases.

131
Q

What are the two common types of mechanically varied resistors?

A

Potentiometers and rheostats

Both can be configured for different applications.

132
Q

What is the primary function of a potentiometer?

A

To make changes to the output of a circuit

Example: rotary volume control.

133
Q

What type of control is typically used for variable resistors that do not need adjustments during normal operation?

A

Trimmers

Adjusted with fine screwdrivers or dedicated tools.

134
Q

What is the sliding contact in a variable resistor called?

A

Wiper

It slides over a resistive path.

135
Q

What are the types of resistive materials that can be used in variable resistors?

A
  • Carbon composition
  • Wire-wound
  • Conductive plastic
  • Ceramic resistor

The choice depends on the application.

136
Q

What is a key characteristic of rheostats in terms of current?

A

They tend to carry a significant amount of current

This is why wire-wound resistive paths are often used.

137
Q

How are potentiometers and rheostats different in terms of circuit configuration?

A

They are wired into a circuit in slightly different ways

Both are essentially the same device.

138
Q

What types of motion do the sliding contacts of potentiometers typically have?

A
  • Linear
  • Rotational

Some devices may use both types of motion.

139
Q

What is a helipot?

A

A potentiometer that uses a resistor in the form of a helix

It allows for a different type of motion for the wiper.

140
Q

What are the two parts of the construction of variable resistors?

A
  • Sliding parts
  • Non-sliding parts

The wiper is part of the sliding components.

141
Q

True or False: The construction of a rheostat is similar to that of a potentiometer.

A

True

Both use a fixed terminal and a moving terminal sliding over a resistive path.

142
Q

What is a Wheatstone bridge?

A

A type of electrical scale that compares voltage in two branches of a circuit to determine unknown resistance.

143
Q

What components are connected in series in the first branch of a Wheatstone bridge?

A

Resistors R1 and R2.

144
Q

What components are connected in series in the second branch of a Wheatstone bridge?

A

Resistors R3 and R4.

145
Q

What does the voltage at point a depend on in a Wheatstone bridge?

A

The ratio of R1 and R2.

146
Q

What does the voltage at point b depend on in a Wheatstone bridge?

A

The ratio of R3 and R4.

147
Q

What is the role of the galvanometer in a Wheatstone bridge?

A

It connects points a and b and measures the current.

148
Q

What does it mean when the Wheatstone bridge is ‘balanced’?

A

The potential difference between points a and b is zero, and no current is measured by the galvanometer.

149
Q

What is the balance condition for a Wheatstone bridge?

A

R1/R2 = R3/R4.

150
Q

In a Wheatstone bridge, if R1 is replaced by a potentiometer and R2 is an unknown resistor Rx, how can Rx be calculated?

A

By reading the resistance from the potentiometer scale.

151
Q

In the worked example, what are the values of R3 and R4?

A

R3 = 10 Ω, R4 = 40 Ω.

152
Q

What value does the potentiometer VR1 read when the galvanometer in the example shows zero?

A

12.5 Ω.

153
Q

What is the formula used to rearrange the balance condition to make Rx the subject?

A

Rx = (R2 * R3) / R1.

154
Q

Fill in the blank: The circuit diagram for a Wheatstone bridge is often drawn in a _______ shape.

A

diamond

155
Q

True or False: A Wheatstone bridge can only measure resistances greater than zero.

A

True

156
Q

What is the primary application of a Wheatstone bridge?

A

To determine the value of unknown resistance.

M3 (19 decks)

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