3.7 Resistance and Resistors Flashcards
1
Q
A
2
Q
What is resistance in the context of electrical properties?
A
Resistance is an electrical property of a conductor.
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.
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
5
Q
How does the length of a resistor affect its resistance?
A
Resistance increases in direct proportion to the length of a resistor.
6
Q
What happens to resistance if the length of a resistor is doubled?
A
The resistance is also doubled.
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.
8
Q
What happens to resistance if the cross-sectional area of a resistor is doubled?
A
The resistance is halved.
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.
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
11
Q
Fill in the blank: Resistance is determined by the _______ of the object.
A
[length, cross-sectional area, material]
12
Q
True or False: Resistance decreases if the cross-sectional area of a resistor is increased.
A
True
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.
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
15
Q
How does resistance change with the length of a resistor?
A
Resistance increases in direct proportion to the length of a resistor.
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.
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
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.
19
Q
What symbol is used to represent specific resistance?
A
The symbol for specific resistance is ⍴ (rho).
20
Q
What are the units of specific resistance?
A
The units of specific resistance are ohm metres (Ωm).
21
Q
Fill in the blank: Ohm metres (Ωm) is the _______ unit for specific resistance.
A
SI
22
Q
What alternative unit is commonly used in engineering for specific resistance?
A
Ohm millimetres squared per metre (Ωmm²/m)
23
Q
What are fixed resistors designed to provide?
A
Fixed resistors are designed to provide a constant value of resistance.
24
Q
What are variable resistors designed to provide?
A
Variable resistors are designed to provide a range of resistance values.
25
What is the specific resistance of copper at 20 °C?
0.01786 Ωmm²/m
26
What is the specific resistance of iron at 20 °C?
0.10 to 0.15 Ωmm²/m
27
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.
True
28
True or False: The specific resistance of a material is independent of its dimensions.
True
29
What is the specific resistance of silver at 20 °C?
0.016 Ωmm²/m
30
List the resistivities of the following substances at 20 °C: Copper, Aluminium, Iron, Silver, Gold.
* 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
31
What do resistor colour codes indicate?
Resistance, wattage rating, and tolerance
## Footnote
Engineers must be able to read these codes to build or maintain electrical circuits.
32
What does the tolerance of a resistor represent?
The extent to which it can differ from the rated value
## Footnote
The tolerance is indicated in the colour code and affects the interval between available resistance values.
33
What is the maximum consequence of exceeding a resistor's wattage rating?
Damage to the circuit and risk of fire
34
How do manufacturers indicate resistance and tolerance on resistors?
Using a colour code
## Footnote
Big resistors may have additional resistance values printed.
35
What is the tolerance percentage for the E12 range of resistors?
10%
36
What are preferred values in resistor manufacturing?
Values of the E12 and E24 range
37
What is the factor for the E12 range?
1.21
38
What is the factor for the E24 range?
1.12
39
What happens if the result of a calculation is 500 ohms?
Use the closest standard value, i.e., 470 ohms
40
What is the power rating of a resistor?
The maximum power that can be dissipated without affecting its function
41
What is the typical power rating range for small resistor components?
50 to 500 mW
42
What can large wire-wound resistors handle in terms of power?
100 W and more
43
What effect does ambient temperature have on a resistor's power rating?
Ambient temperatures above the stated value will invalidate the power rating
44
Fill in the blank: The electrical energy applied to a resistor is converted completely into _______.
heat
45
What is one way to prevent overheating in a resistor?
Increase the surface area while maintaining the cross-sectional area
46
True or False: All resistors have the same tolerance.
False
47
What are the IEC standard ranges for resistors?
E6, E12, and E24
48
How many values are there in the E12 range?
12 values
49
How many values are there in the E24 range?
24 values
50
What is the total resistance in a series connection of resistors?
The total resistance, RTotal, is the sum of the individual resistances.
## Footnote
This is expressed as RTotal = R1 + R2 + ... + Rn.
51
What is a potential divider?
A series connection of resistors where the voltage is divided among the resistors.
## Footnote
Each resistor receives a portion of the total voltage, which can be tapped at a certain point.
52
What happens to the current in a series circuit?
The current is the same through all components.
## Footnote
This is analogous to water flowing through different sections of a hose pipe.
53
What is the relationship between voltage and resistance in a series circuit according to Ohm's law?
V = I * R for each resistor.
## Footnote
The total voltage is the sum of the voltages across each resistor.
54
What is the total resistance in a parallel connection of resistors?
The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances.
## Footnote
This can be expressed as 1/RTotal = 1/R1 + 1/R2 + ... + 1/Rn.
55
What is a characteristic of resistors connected in parallel?
Voltage is the same across each resistor.
## Footnote
This means each resistor experiences the same potential difference.
56
What happens to the total current in a parallel circuit?
The total current is the sum of the currents in each branch.
## Footnote
This allows for more current paths, effectively increasing the conductor cross-section.
57
True or False: In a series circuit, the total resistance is always greater than the smallest individual resistance.
True.
## Footnote
This is because resistances add up in series connections.
58
Fill in the blank: In a series circuit, the _____ provided by the battery is divided between the resistors.
voltage
59
What is a key feature of resistors connected in series?
The individual resistances must be added together to find the total resistance.
## Footnote
This is a fundamental property of series circuits.
60
What does Kirchhoff's second law state regarding series resistors?
The total voltage across all resistors is equal to the sum of the voltages across each resistor.
## Footnote
This law is essential for analyzing series circuits.
61
How does a parallel connection affect total resistance?
The total resistance is lower than the lowest individual resistance.
## Footnote
This is due to the multiple paths available for current flow.
62
What is the method to calculate total resistance for resistors in series?
Add the resistances together
## Footnote
The formula for total resistance in series is R_total = R1 + R2 + ... + Rn.
63
What is the formula for calculating total resistance in parallel?
Add the reciprocals of the individual resistances
## Footnote
The formula for total resistance in parallel is 1/R_total = 1/R1 + 1/R2 + ... + 1/Rn.
64
What is the effective resistance when a 3 Ω resistor is placed in series with a 4 Ω resistor?
7 Ω
## Footnote
Calculation: R_total = 3 Ω + 4 Ω.
65
What is the effective resistance when a 3 Ω resistor is placed in parallel with a 4 Ω resistor?
1.2 Ω
## Footnote
Calculation: 1/R_total = 1/3 + 1/4 leads to R_total = 1.2 Ω.
66
How do you approach circuits with series-parallel combinations?
Calculate effective resistance of series or parallel groups first
## Footnote
Continue applying the respective formulas until reduced to a single series or parallel combination.
67
What is the effective resistance of a series-parallel combination circuit with three resistors resulting in 4 Ω?
4 Ω
## Footnote
This is the final effective resistance after calculations for the series-parallel combination.
68
What is the effective resistance of a series-parallel combination circuit with three resistors resulting in 5 Ω?
5 Ω
## Footnote
This is the final effective resistance after calculations for the second series-parallel combination.
69
Fill in the blank: When resistors are connected in series, the total resistance is equal to the sum of the _______.
individual resistances
70
True or False: In a parallel circuit, the total resistance is always less than the smallest individual resistance.
True
71
What is the first step in calculating the effective resistance of a series-parallel circuit?
Calculate the combined resistance of series or parallel resistors
## Footnote
This involves simplifying the circuit by solving one section at a time.
72
What is a variable resistor?
A resistor with adjustable resistance values
## Footnote
It typically works by sliding a contact (wiper) over a resistive element.
73
What is a potentiometer?
A three-terminal device used to control voltage
## Footnote
It functions as a potential divider.
74
What is a rheostat?
A two-terminal device used to control current
## Footnote
It functions as a variable resistance.
75
How are potentiometers and rheostats different?
Potentiometers have three terminals, rheostats have two
## Footnote
The difference is more about connection than construction.
76
What are the typical applications of potentiometers?
* Volume controls in audio circuits
* Brightness in lighting circuits
* Varying speed of DC motors
77
What is the function of the wiper in a potentiometer?
It moves over a resistive element to adjust voltage
## Footnote
The position of the wiper determines the output voltage.
78
What is the symbol for a potentiometer?
A symbol representing a three-terminal device with a wiper
## Footnote
It indicates the connections for input voltage and output voltage.
79
What arrangement can a rheostat have?
Two alternative arrangements with two terminals
## Footnote
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
What is the main use of a rheostat in a circuit?
To restrict current by increasing resistance
## Footnote
This can be achieved by reducing voltage or increasing resistance (Ohm’s Law).
81
Fill in the blank: A digital potentiometer is an electronically controlled _______.
resistor
82
What is a pre-set potentiometer?
A potentiometer mounted on printed circuit boards with limited range
## Footnote
Usually set during manufacture or maintenance.
83
What is the typical construction of a high-power rheostat?
A wiper moving over copper windings
## Footnote
It is designed to handle higher currents.
84
True or False: Rheostats can be used in high-power applications.
True
85
What does a rheostat control within a circuit?
The current
## Footnote
It does this by varying resistance, which affects current flow.
86
What happens to the current when the resistance in a rheostat increases?
The current decreases
## Footnote
According to Ohm’s Law, as resistance increases, current decreases.
87
What happens to the resistivity of conductors as temperature increases?
Resistivity increases, which decreases conductivity.
88
What does PTC stand for?
Positive Temperature Coefficient.
89
What characterizes a Positive Temperature Coefficient (PTC) conductor?
Resistance increases with increasing temperature.
90
What is indicated by a negative temperature coefficient?
Resistance decreases with increasing temperature.
91
How is the proportional change in resistance calculated?
Proportional change in resistance = change in temperature x temperature coefficient.
92
What is a typical value for the temperature coefficient of a carbon film resistor?
Around 0.0008/K.
93
What does ppm/K stand for?
Parts per million per kelvin.
94
Convert 0.0008/K to ppm/K.
800 ppm/K.
95
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?
800 Ω.
96
What type of graph do PTC conductors follow?
A linear upward gradient.
97
What are semiconductors with decreasing resistivity as temperature increases called?
Negative Temperature Coefficient (NTC) materials.
98
True or False: PTC conductors have a downward curve on their graph.
False.
99
What is the primary characteristic of fixed resistors?
Fixed electrical resistance that is not adjustable.
100
What are variable resistors?
Resistors that can be set at different resistance values or change in response to environmental factors such as temperature or light.
101
What is the purpose of resistors in a circuit?
To control voltage or current and to detect changes in the environment.
102
What types of packages can fixed resistors have?
Axial leads, surface mount packages, or customized packages.
103
What type of resistor is currently the most popular?
Surface Mounted Device (SMD) resistors.
104
Define a perfect resistor.
A perfect resistor has a constant ohmic resistance independent of frequency, voltage, or temperature.
105
True or False: All resistors have perfect resistance.
False.
106
What factors determine the extent to which external factors affect resistance values?
The type of resistor and its material composition.
107
What should the choice of resistor be based on?
Required accuracy, power dissipation, and noise considerations.
108
What is the most common type of axial lead resistor?
Carbon film resistors.
109
What is the typical application for metal film resistors?
Higher-precision applications.
110
What type of resistors are typically used for high-power applications?
Wire wound resistors.
111
What is often used for power ratings from 1 to 200 W?
Aluminium housing.
112
What materials are SMD resistors typically constructed from?
Thick or thin film material.
113
Fill in the blank: The _______ film is the most common axial lead resistor.
carbon
114
What are the main limitations of fixed resistors?
Resistance values vary within defined tolerance limits.
115
List the three common types of fixed resistors.
* Carbon film
* Metal film
* Wire wound
116
What is the primary function of variable resistors?
Variable resistors have adjustable resistance values.
117
What is the difference between a potentiometer and a rheostat?
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
How can variable resistors be controlled?
Variable resistors can be controlled electronically or mechanically.
119
What are thermistors?
Thermistors are temperature-dependent resistors that change resistance with temperature.
120
What are the two types of thermistors based on their temperature response?
Positive Temperature Coefficient (PTC) Resistors and Negative Temperature Coefficient (NTC) Resistors.
121
What does a Positive Temperature Coefficient (PTC) resistor do?
PTC resistors have a positive temperature coefficient, meaning their resistivity increases proportionately with temperature.
122
What is the characteristic of NTC thermistors?
NTC thermistors have a negative temperature coefficient, meaning their resistance decreases as temperature increases.
123
What materials are typically used to make NTC thermistors?
NTC thermistors are made of polycrystalline mixed crystals of iron oxides, cobalt oxides, titanium compounds, and specific impurities.
124
What happens to the conductivity of NTC thermistors as temperature rises?
As temperature rises, more electrons escape from their orbits, increasing the conductivity of the material.
125
What is the purpose of voltage-dependent resistors (VDR)?
The resistance value of VDR changes according to the applied voltage.
126
How does the resistance of a VDR change with voltage?
The resistance of a VDR decreases as the voltage increases, irrespective of the polarity of the voltage.
127
Fill in the blank: A variable resistor with three terminals is called a _______.
potentiometer
128
Fill in the blank: A variable resistor with two terminals is called a _______.
rheostat
129
True or False: NTC thermistors are primarily used for temperature stabilization in semiconductor connections.
True
130
What is the significance of the characteristic conductance curves for PTC thermistors?
PTC thermistors undergo a large and highly predictable change in resistance as temperature increases.
131
What are the two common types of mechanically varied resistors?
Potentiometers and rheostats
## Footnote
Both can be configured for different applications.
132
What is the primary function of a potentiometer?
To make changes to the output of a circuit
## Footnote
Example: rotary volume control.
133
What type of control is typically used for variable resistors that do not need adjustments during normal operation?
Trimmers
## Footnote
Adjusted with fine screwdrivers or dedicated tools.
134
What is the sliding contact in a variable resistor called?
Wiper
## Footnote
It slides over a resistive path.
135
What are the types of resistive materials that can be used in variable resistors?
* Carbon composition
* Wire-wound
* Conductive plastic
* Ceramic resistor
## Footnote
The choice depends on the application.
136
What is a key characteristic of rheostats in terms of current?
They tend to carry a significant amount of current
## Footnote
This is why wire-wound resistive paths are often used.
137
How are potentiometers and rheostats different in terms of circuit configuration?
They are wired into a circuit in slightly different ways
## Footnote
Both are essentially the same device.
138
What types of motion do the sliding contacts of potentiometers typically have?
* Linear
* Rotational
## Footnote
Some devices may use both types of motion.
139
What is a helipot?
A potentiometer that uses a resistor in the form of a helix
## Footnote
It allows for a different type of motion for the wiper.
140
What are the two parts of the construction of variable resistors?
* Sliding parts
* Non-sliding parts
## Footnote
The wiper is part of the sliding components.
141
True or False: The construction of a rheostat is similar to that of a potentiometer.
True
## Footnote
Both use a fixed terminal and a moving terminal sliding over a resistive path.
142
What is a Wheatstone bridge?
A type of electrical scale that compares voltage in two branches of a circuit to determine unknown resistance.
143
What components are connected in series in the first branch of a Wheatstone bridge?
Resistors R1 and R2.
144
What components are connected in series in the second branch of a Wheatstone bridge?
Resistors R3 and R4.
145
What does the voltage at point a depend on in a Wheatstone bridge?
The ratio of R1 and R2.
146
What does the voltage at point b depend on in a Wheatstone bridge?
The ratio of R3 and R4.
147
What is the role of the galvanometer in a Wheatstone bridge?
It connects points a and b and measures the current.
148
What does it mean when the Wheatstone bridge is 'balanced'?
The potential difference between points a and b is zero, and no current is measured by the galvanometer.
149
What is the balance condition for a Wheatstone bridge?
R1/R2 = R3/R4.
150
In a Wheatstone bridge, if R1 is replaced by a potentiometer and R2 is an unknown resistor Rx, how can Rx be calculated?
By reading the resistance from the potentiometer scale.
151
In the worked example, what are the values of R3 and R4?
R3 = 10 Ω, R4 = 40 Ω.
152
What value does the potentiometer VR1 read when the galvanometer in the example shows zero?
12.5 Ω.
153
What is the formula used to rearrange the balance condition to make Rx the subject?
Rx = (R2 * R3) / R1.
154
Fill in the blank: The circuit diagram for a Wheatstone bridge is often drawn in a _______ shape.
diamond
155
True or False: A Wheatstone bridge can only measure resistances greater than zero.
True
156
What is the primary application of a Wheatstone bridge?
To determine the value of unknown resistance.
