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UKCAA M3 - electrical fundamentals > Resistance 7a > Flashcards

Resistance 7a Flashcards

1
Q

What is resistance in the context of electricity?

A

Resistance is an electrical property of a conductor that determines how much current flows for a given voltage.

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2
Q

What is the relationship between current, resistance, and voltage in a conductor?

A

For a given current, a higher resistance requires a higher voltage to be applied across the conductor.

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3
Q

What factors determine the numerical value of a conductor’s resistance?

A

The resistance of a conductor depends on the material from which it is made, its shape, and its size.

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4
Q

How does the length of a resistor affect its resistance?

A

Resistance increases in direct proportion to the length of a resistor; if the material is doubled in length, the resistance is also doubled.

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4
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; if the resistor is doubled in cross-sectional area, the resistance is halved.

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5
Q

Why is it important to compare samples of different materials with the same dimensions?

A

Comparing samples with the same dimensions ensures a fair comparison of the resistances of various materials.

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5
Q

How do all materials relate to the flow of electric current?

A

All materials oppose the flow of an electric current to some extent.

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6
Q

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

A

If the length of a conductor is doubled, its resistance also doubles.

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7
Q

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

A

If the cross-sectional area of a conductor is doubled, its resistance is halved.

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8
Q

What should be done to fairly compare the resistances of different materials?

A

To fairly compare the resistances of different materials, samples should be prepared with the same dimensions.

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9
Q

What is the effect of the material on the resistance of a conductor?

A

The material from which a conductor is made significantly affects its resistance.

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10
Q

Why does resistance increase with length and decrease with cross-sectional area?

A

Resistance increases with length because the electrons encounter more collisions over a longer path. It decreases with cross-sectional area because a larger area allows more paths for the electrons to flow through, reducing collisions.

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11
Q

Q: How is the relationship between length, cross-sectional area, and resistance mathematically expressed?

A

R=ρ x L/A

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12
Q

What does the symbol 𝜌 represent in the resistance formula?

A

The symbol
𝜌
ρ represents the specific resistance or resistivity.

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13
Q

What are the units of specific resistance (resistivity)?

A

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

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14
Q

How should the cross-sectional area be measured when using the formula for specific resistance?

A

The cross-sectional area should be measured in square metres (m²) when using ohm metres (Ωm), or in square millimetres (mm²) when using ohm millimetres squared per metre (Ωmm²/m).

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15
Q

What are fixed resistors and variable resistors?

A

Fixed resistors are designed to provide a constant value of resistance, while variable resistors are designed to provide a range of resistance values.

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16
Q

How do you calculate the resistance of a wire made from a material with a given specific resistance?

A

Use the formula
𝑅=𝜌𝐿/𝐴
ensuring the units of specific resistance and cross-sectional area are compatible.

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17
Q

What is the resistance of a 10 m copper wire with a cross-sectional area of 2 mm² and a specific resistance of 0.020 Ωmm²/m?

A

R= 0.020×10/2 = 0.1Ω

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18
Q

What is the resistance of a 10 m iron wire with a cross-sectional area of 2 mm² and a specific resistance of 0.100 Ωmm²/m?

A

𝑅=0.100×10/2 = 0.5Ω

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19
Q

What is the resistance of a copper wire that is 1 m long with a cross-sectional area of 1 mm² at 20 °C?

A

0.0178 Ω

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20
Q

How does doubling the length of a copper wire affect its resistance, if the cross-sectional area remains the same?

A

Doubling the length of a copper wire doubles its resistance.

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21
Q

What is the resistivity of silver at 20 °C?

A

The resistivity of silver at 20 °C is 0.016 Ωmm²/m.

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22
Q

What is the resistivity range of iron at 20 °C?

A

The resistivity range of iron at 20 °C is 0.10 to 0.15 Ωmm²/m.

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23
Q

Why might engineering often quote specific resistance in ohm millimetres squared per metre (Ωmm²/m)?

A

Ohm millimetres squared per metre (Ωmm²/m) is easier to relate to a standard piece of wire with dimensions of 1 m in length and 1 mm² in cross-sectional area.

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24
Q

What happens to the resistance of a copper wire three times as long as a 1 m wire, with the same cross-sectional area?

A

The resistance of a copper wire three times as long will be three times the resistance of a 1 m wire.

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25
Q

List the resistivity of copper, aluminium, and gold at 20 °C.

A

Copper: 0.01786 Ωmm²/m
Aluminium: 0.02857 Ωmm²/m
Gold: 0.023 Ωmm²/m**

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26
Q

What must be considered when using the formula for calculating resistance with specific resistance?

A

Ensure the units of specific resistance are compatible with the units of the cross-sectional area being used.

27
Q

How do you calculate the total resistance of a group of resistors?

A

The method depends on how the resistors are connected. Use different formulas for series and parallel connections.

28
Q

What happens to the current in resistors connected in series?

A

The current flows through all resistors in turn and has the same value for each resistor.

29
Q

How do you calculate the total resistance for resistors in series?

A

Add the values of the resistors together.
𝑅 Total=𝑅1+𝑅2+𝑅3+…

30
Q

What are the three typical features of a series connection?

A

The voltage is divided between the resistors.
The current is the same through all components.
The individual resistances are added together to find the total resistance.**

31
Q

What is a potential divider (or voltage divider)?

A

A series connection where each resistor has only a part of the total voltage across it. The output voltage can be tapped at a certain point called the “pick-up.”

32
Q

What happens to the current in resistors connected in parallel?

A

The current splits up, and a smaller current flows through each available path.

33
Q

How do you calculate the total resistance for resistors in parallel?

A

Add the reciprocals of the individual resistances to find the reciprocal of the total resistance.
1/𝑅Total=1/𝑅1+1/𝑅2+1/𝑅3 + …

34
Q

What are the three typical features of a parallel connection?

A

Voltage is the same across each resistor.
The total current is the sum of the currents in each branch.
The total resistance is lower than the lowest individual resistance.**

35
Q

How does the area through which the current can flow change in a parallel connection?

A

The area increases, which decreases the resistance.

36
Q

How is the total voltage in a series circuit related to the individual voltages across each resistor?

A

The total voltage is the sum of the voltages across each resistor.
𝑉 Total=V1+𝑉2+𝑉3+…

37
Q

In a series circuit, how is the voltage across each resistor related to its resistance?

A

The voltage across each resistor is proportional to its resistance.

38
Q

How do you describe the current in a parallel circuit using Kirchhoff’s first law?

A

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

39
Q

What effect does adding more resistors in parallel have on the total resistance?

A

Adding more resistors in parallel decreases the total resistance.

40
Q

What happens to the voltage across resistors in parallel?

A

The voltage across each resistor is the same.

41
Q

What analogy can describe the current flow in a series circuit?

A

Current in a series circuit is like water flowing through different sections of the same hose pipe.

42
Q

How do you determine the total resistance for a combination of series and parallel resistor networks?

A

Apply the series and parallel formulas in turn, step by step.

43
Q

What law can be used to show the relationship between the total and individual resistances in parallel?

A

Kirchhoff’s first law and Ohm’s law.

44
Q

What is the formula for the total resistance
𝑅Total in a parallel circuit with resistors
𝑅1,𝑅2,…,𝑅𝑛?

A

1/R Total=1/R1+1/R2+…+ 1/Rn

45
Q

how do you calculate the resistance of a circuit with both parallel and series resistors?

A

first calculate parallel resistors so as to turn it into a series resistance then calculate the series resistors

46
Q

What is a variable resistor?

A

An electro-mechanical transducer that has adjustable resistance values, usually working by sliding a contact (wiper) over a resistive element.

47
Q

What is a potentiometer?

A

A three-terminal variable resistor used to control voltage.

47
Q

What is a rheostat?

A

A two-terminal variable resistor used to control current.

48
Q

How is a potentiometer connected?

A

Terminal 3 is connected to the input voltage, terminal 1 to the ground, and terminal 2 is the variable output voltage.

49
Q

What is the function of a potentiometer?

A

To control voltage by acting as an adjustable voltage divider, commonly used in volume controls and lighting circuits.

50
Q

How is a rheostat connected?

A

Option 1: Terminal 3 is connected to the input current, and terminal 2 provides the output current, with terminal 1 not connected. Option 2: Terminal 3 is connected to the input current, and terminals 1 and 2 form the output terminal.

51
Q

What is the difference between a potentiometer and a rheostat?

A

The difference lies in their connections and applications: potentiometers control voltage with three terminals, while rheostats control current with two terminals.

52
Q

What is a digital potentiometer?

A

An electronically controlled variable resistor.

53
Q

What are some types of variable resistors other than mechanically varied resistors?

A

Positive or negative temperature coefficient resistors and voltage-dependent resistors.

54
Q

What is a pre-set potentiometer?

A

A potentiometer mounted on printed circuit boards, with a limited range, usually set during manufacture or maintenance.

55
Q

What is a pre-set rheostat?

A

A rheostat mounted on printed circuit boards, with a limited range, usually set during manufacture or maintenance.

56
Q

How does a potentiometer vary the speed of DC motors?

A

By adjusting the voltage supplied across the motor, which changes the motor’s speed.

57
Q

What is the purpose of a rheostat in an electric circuit?

A

To restrict the current by increasing the resistance in the circuit.

58
Q

How is the resistance in a potentiometer adjusted?

A

The position of the wiper determines the resistance ratio, forming a potential divider and controlling the voltage.

59
Q

How is the resistance in a rheostat adjusted?

A

The position of the wiper determines the resistance between the terminals, which adjusts the current in the circuit.

60
Q

What symbol represents a potentiometer in circuit diagrams?

A
61
Q

What symbol represents a rheostat in circuit diagrams?

A
62
Q

In which devices are potentiometers commonly used?

A

Volume controls in audio circuits and brightness controls in lighting circuits.

63
Q

In which devices are rheostats commonly used?

A

Circuits where current needs to be controlled or restricted.

64
Q

How does a pre-set rheostat function?

A

It allows for adjustment of the resistance, set during manufacture or maintenance, to control current in the circuit.

65
Q
A
66
Q
A

UKCAA M3 - electrical fundamentals (11 decks)

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