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M3 > 3.5 DC Sources of Electricity > Flashcards

3.5 DC Sources of Electricity Flashcards

1
Q
A
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2
Q

What are primary cells?

A

Cells that supply voltage immediately without charging beforehand.

The electrochemical processes in primary cells are not reversible.

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3
Q

Can primary cells be recharged?

A

No, primary cells cannot be recharged.

They can be recycled.

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4
Q

What is the typical initial voltage of zinc-carbon dry cells?

A

About 1.5 V.

The voltage decreases as the battery is discharged.

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5
Q

What is a disadvantage of zinc-carbon batteries?

A

The zinc container can oxidize and leak.

These batteries should not be stored in devices for an extended period.

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6
Q

How much energy can alkaline batteries deliver compared to zinc-carbon batteries?

A

Up to five times the energy.

Alkaline batteries are also prone to leaking potassium hydroxide.

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7
Q

What is the voltage of a lead acid cell when charged?

A

2.2 V.

The final discharge voltage is 1.83 V.

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8
Q

What materials make up the positive and negative plates of a lead acid cell?

A

Positive plates: lead peroxide (PbO2); Negative plates: pure spongy lead (Pb).

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9
Q

What is the electrolyte composition in a lead acid cell?

A

30% sulphuric acid and 70% distilled water.

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10
Q

What happens to sulphate ions during discharge in a lead acid cell?

A

They combine with the negative lead plate to form lead sulphate.

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11
Q

What is the voltage of a nickel cadmium cell?

A

1.2 V, falling to 1.0 V when discharged.

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12
Q

What is the electrolyte in nickel cadmium cells?

A

Potassium hydroxide.

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13
Q

What is a key advantage of nickel cadmium batteries over lead acid batteries?

A

Higher energy density and greater number of charge/discharge cycles.

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14
Q

What is the risk associated with nickel cadmium batteries?

A

Risk of thermal runaway due to decreasing internal resistance as temperature rises.

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15
Q

What is the rated voltage of mercury oxide coin cells?

A

1.35 V.

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16
Q

What are silver oxide coin cells used for?

A

Watches, calculators, photoelectric exposure devices, and hearing aids.

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17
Q

What is the voltage of lithium coin cells?

A

Up to 3.5 V.

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18
Q

What is the main characteristic of sealed nickel cadmium batteries?

A

The electrolyte does not change during charging.

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19
Q

What is the purpose of the zinc container in a zinc-carbon dry cell?

A

Acts as both a package to hold active chemicals and as the negative electrode.

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20
Q

Fill in the blank: The electrochemical reactions in _______ cells are reversible.

A

secondary

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21
Q

True or False: Alkaline primary batteries can be recharged.

A

True, but only if they are designed to be rechargeable.

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22
Q

What is the capacity rating for batteries commonly expressed in?

A

Ampere hours (Ah).

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23
Q

What happens to the electrolyte level in a nickel cadmium battery during charging?

A

It rises.

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24
Q

What does a battery’s capacity depend on?

A

Its construction and size.

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25
Q

What are the two types of galvanic cells mentioned?

A

Round cells and flat cells (coin cells).

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26
Q

What is a series connection of cells?

A

Connecting unlike terminals of power sources creates a series connection

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27
Q

What happens to the total voltage when cells are connected in series?

A

The total voltage is the sum of the voltages of the individual cells

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28
Q

What is the effect of a series connection on current?

A

The current through a series circuit is constant

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29
Q

How does connecting cells in series affect internal resistance?

A

The internal resistance of a battery is equal to the sum of the internal resistance of the cells

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30
Q

What happens to terminal voltage when a battery is connected to an external load?

A

The internal resistance causes the terminal voltage of the battery to drop

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31
Q

What defines a parallel connection of cells?

A

Connecting like terminals of power sources to each other creates a parallel connection

32
Q

What is the voltage output of cells connected in parallel?

A

Provides the same voltage

33
Q

How does connecting cells in parallel affect total internal resistance?

A

It lowers the total internal resistance of the supply

34
Q

What is the effect of connecting resistors in parallel?

A

Lowers the total resistance

35
Q

What happens to total current when cells are connected in parallel?

A

The total current is the sum of the individual currents

36
Q

What is the voltage across parallel branches?

A

Constant

37
Q

What is a practical application of parallel connections in power generation?

A

Generators in power plants are connected in parallel

38
Q

True or False: Connecting unlike power sources in parallel is recommended.

A

False

39
Q

In a setup with two cells in parallel, what happens when both switches are closed?

A

A current of only 1 A flows through each of the two power sources

40
Q

Fill in the blank: A battery with cells in parallel can supply a larger _______.

A

current

41
Q

What is the total voltage for each power source in parallel?

A

20 V

42
Q

What is the relationship between total resistance in a parallel connection and individual resistances?

A

The total resistance is lower than the lowest individual resistance

43
Q

What is the term used for the voltage generated by a power source such as a battery?

A

Source voltage

Source voltage is the voltage generated by the power source before any load is applied.

44
Q

What is the voltage available at the terminals of a battery called?

A

Terminal voltage

Terminal voltage is the voltage that is actually available to the load when the battery is connected.

45
Q

What primarily causes internal resistance in batteries?

A

Resistance of the electrolyte and the plates

Internal resistance affects the performance and efficiency of the battery.

46
Q

What happens to terminal voltage when a load is connected to a power source?

A

Terminal voltage falls

The fall in terminal voltage is due to the potential difference across the internal resistance.

47
Q

How is terminal voltage related to Electromotive Force (EMF) when a load is connected?

A

VT = EMF - (I × Ri)

Where VT is terminal voltage, EMF is the source voltage, I is the load current, and Ri is internal resistance.

48
Q

True or False: In an open-circuit operation, the terminal voltage is equal to the source voltage.

A

True

No current flows in open-circuit operation, making terminal voltage equal to source voltage.

49
Q

What occurs in a voltage generator when a load is applied?

A

Terminal voltage decreases as current increases

This is due to the increase in potential difference across the generator’s internal resistance.

50
Q

What is the effect of short-circuiting a battery’s terminals?

A

High current flows limited by internal resistance

Short-circuiting can lead to very high currents, potentially damaging the battery.

51
Q

What components can be included in a power supply unit to stabilize terminal voltage?

A

Voltage regulators and load resistors

These components help maintain a constant voltage and current for connected electrical equipment.

52
Q

How is a battery’s internal resistance calculated?

A

Ri = (OCV - CCV) / I

OCV is Open Circuit Voltage, CCV is Closed-Circuit Voltage, and I is load current.

53
Q

Fill in the blank: A battery’s internal resistance ______ as the battery discharges.

A

increases

This increase in internal resistance can affect the battery’s performance over time.

54
Q

What is a thermocouple?

A

A thermocouple is a type of power supply that generates a voltage when two different metals are connected and their junction is heated.

It is based on the principle of thermoelectric voltage generated due to temperature differences.

55
Q

What principle does a thermocouple operate on?

A

The principle of thermoelectric voltage, which occurs when two different metals are joined and their junction is heated, creating a voltage difference.

This voltage depends on the temperature difference between the junction and the free ends, as well as the types of metals used.

56
Q

Name the materials used in the construction of a thermocouple.

A

Common materials include:
* Chromel
* Alumel
* Bismuth
* Antimony
* Constantan
* Copper
* Platinum
* Platinum Rhodium

Different combinations of these materials yield varying thermoelectric voltages.

57
Q

State an aeronautical application for a thermocouple.

A

Thermocouples are primarily used to measure exhaust gas temperatures of engines in aviation.

This application is crucial for monitoring engine performance and safety.

58
Q

What happens to electrons in a heated metal?

A

Electrons in a heated metal move faster, similar to gas particles, due to increased temperature.

This behavior leads to variations in electron density across different metals.

59
Q

Fill in the blank: The thermoelectric voltage generated by a thermocouple depends on the _______ between the junction and the two free ends.

A

temperature difference

60
Q

What is the thermoelectric voltage generated by a Chromel-Alumel thermocouple per 100 °C?

A

4.2 mV per 100 °C.

This is one of the common combinations used in thermocouples.

61
Q

True or False: A thermocouple can measure temperatures ranging from -200 °C to 2300 °C.

A

True

62
Q

What effect is greater at higher temperatures when two different metals are in contact?

A

The movement of electrons from the region of higher electron density to the region of lower electron density.

This phenomenon is crucial for the operation of thermocouples.

63
Q

List the thermoelectric voltage generated by Bismuth-Antimony per 100 °C.

A

10.0 mV per 100 °C.

This combination provides a higher voltage compared to others.

64
Q

List the thermoelectric voltage generated by Constantan-Copper per 100 °C.

A

4.2 mV per 100 °C.

This is another common combination used in thermocouples.

65
Q

What is the thermoelectric voltage generated by Platinum-Platinum Rhodium per 100 °C?

A

1.0 mV per 100 °C.

This combination is used for high-temperature measurements.

66
Q

What is the principle of operation of photo-cells?

A

Photo-cells generate a voltage when exposed to light, as electrons are released from a semiconductor layer and flow to a metal skin, creating a charge difference.

67
Q

What materials are commonly used in the construction of photo-cells?

A

Silicon and selenium

Silicon PV cells are particularly significant due to their large surface areas.

68
Q

Name some aeronautical applications of photo-cells.

A

Automatic light level adjustment in aircraft

Photo-cells help adjust working illumination based on surrounding light intensity.

69
Q

What are the components of a depletion layer photo-cell (photovoltaic or PV cell)?

A

A metallic base plate, a semiconductor layer, and a thin translucent metal skin as a counter electrode.

70
Q

What voltage is typically produced by solar cells when exposed to sunlight?

A

Approximately 0.6 V

Solar cells can also produce approximately 10 mW per cm² of active area.

71
Q

Fill in the blank: A typical use of photocells is in _______.

A

exposure meters, alarm systems, counters, and low power devices.

72
Q

What happens to the Light Dependent Resistor (LDR) resistance when ambient light decreases?

A

The resistance increases.

73
Q

True or False: The voltage generated by photo-cells is independent of light intensity.

A

False.

74
Q

What is the purpose of the circuit involving a Light-Emitting Diode (LED) and a transistor in photo-cells?

A

To turn on the LED or vary its brilliance based on light intensity.

75
Q

What occurs to the potential at the base of the transistor when the LDR resistance increases?

A

It changes, switching the transistor on.

76
Q

What is the function of the photo-cell in an automatic light adjustment system?

A

To sense surrounding light intensity and adjust illumination automatically.

M3 (19 decks)

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