3.5 DC Sources of Electricity Flashcards
What are primary cells?
Cells that supply voltage immediately without charging beforehand.
The electrochemical processes in primary cells are not reversible.
Can primary cells be recharged?
No, primary cells cannot be recharged.
They can be recycled.
What is the typical initial voltage of zinc-carbon dry cells?
About 1.5 V.
The voltage decreases as the battery is discharged.
What is a disadvantage of zinc-carbon batteries?
The zinc container can oxidize and leak.
These batteries should not be stored in devices for an extended period.
How much energy can alkaline batteries deliver compared to zinc-carbon batteries?
Up to five times the energy.
Alkaline batteries are also prone to leaking potassium hydroxide.
What is the voltage of a lead acid cell when charged?
2.2 V.
The final discharge voltage is 1.83 V.
What materials make up the positive and negative plates of a lead acid cell?
Positive plates: lead peroxide (PbO2); Negative plates: pure spongy lead (Pb).
What is the electrolyte composition in a lead acid cell?
30% sulphuric acid and 70% distilled water.
What happens to sulphate ions during discharge in a lead acid cell?
They combine with the negative lead plate to form lead sulphate.
What is the voltage of a nickel cadmium cell?
1.2 V, falling to 1.0 V when discharged.
What is the electrolyte in nickel cadmium cells?
Potassium hydroxide.
What is a key advantage of nickel cadmium batteries over lead acid batteries?
Higher energy density and greater number of charge/discharge cycles.
What is the risk associated with nickel cadmium batteries?
Risk of thermal runaway due to decreasing internal resistance as temperature rises.
What is the rated voltage of mercury oxide coin cells?
1.35 V.
What are silver oxide coin cells used for?
Watches, calculators, photoelectric exposure devices, and hearing aids.
What is the voltage of lithium coin cells?
Up to 3.5 V.
What is the main characteristic of sealed nickel cadmium batteries?
The electrolyte does not change during charging.
What is the purpose of the zinc container in a zinc-carbon dry cell?
Acts as both a package to hold active chemicals and as the negative electrode.
Fill in the blank: The electrochemical reactions in _______ cells are reversible.
secondary
True or False: Alkaline primary batteries can be recharged.
True, but only if they are designed to be rechargeable.
What is the capacity rating for batteries commonly expressed in?
Ampere hours (Ah).
What happens to the electrolyte level in a nickel cadmium battery during charging?
It rises.
What does a battery’s capacity depend on?
Its construction and size.
What are the two types of galvanic cells mentioned?
Round cells and flat cells (coin cells).
What is a series connection of cells?
Connecting unlike terminals of power sources creates a series connection
What happens to the total voltage when cells are connected in series?
The total voltage is the sum of the voltages of the individual cells
What is the effect of a series connection on current?
The current through a series circuit is constant
How does connecting cells in series affect internal resistance?
The internal resistance of a battery is equal to the sum of the internal resistance of the cells
What happens to terminal voltage when a battery is connected to an external load?
The internal resistance causes the terminal voltage of the battery to drop
What defines a parallel connection of cells?
Connecting like terminals of power sources to each other creates a parallel connection
What is the voltage output of cells connected in parallel?
Provides the same voltage
How does connecting cells in parallel affect total internal resistance?
It lowers the total internal resistance of the supply
What is the effect of connecting resistors in parallel?
Lowers the total resistance
What happens to total current when cells are connected in parallel?
The total current is the sum of the individual currents
What is the voltage across parallel branches?
Constant
What is a practical application of parallel connections in power generation?
Generators in power plants are connected in parallel
True or False: Connecting unlike power sources in parallel is recommended.
False
In a setup with two cells in parallel, what happens when both switches are closed?
A current of only 1 A flows through each of the two power sources
Fill in the blank: A battery with cells in parallel can supply a larger _______.
current
What is the total voltage for each power source in parallel?
20 V
What is the relationship between total resistance in a parallel connection and individual resistances?
The total resistance is lower than the lowest individual resistance
What is the term used for the voltage generated by a power source such as a battery?
Source voltage
Source voltage is the voltage generated by the power source before any load is applied.
What is the voltage available at the terminals of a battery called?
Terminal voltage
Terminal voltage is the voltage that is actually available to the load when the battery is connected.
What primarily causes internal resistance in batteries?
Resistance of the electrolyte and the plates
Internal resistance affects the performance and efficiency of the battery.
What happens to terminal voltage when a load is connected to a power source?
Terminal voltage falls
The fall in terminal voltage is due to the potential difference across the internal resistance.
How is terminal voltage related to Electromotive Force (EMF) when a load is connected?
VT = EMF - (I × Ri)
Where VT is terminal voltage, EMF is the source voltage, I is the load current, and Ri is internal resistance.
True or False: In an open-circuit operation, the terminal voltage is equal to the source voltage.
True
No current flows in open-circuit operation, making terminal voltage equal to source voltage.
What occurs in a voltage generator when a load is applied?
Terminal voltage decreases as current increases
This is due to the increase in potential difference across the generator’s internal resistance.
What is the effect of short-circuiting a battery’s terminals?
High current flows limited by internal resistance
Short-circuiting can lead to very high currents, potentially damaging the battery.
What components can be included in a power supply unit to stabilize terminal voltage?
Voltage regulators and load resistors
These components help maintain a constant voltage and current for connected electrical equipment.
How is a battery’s internal resistance calculated?
Ri = (OCV - CCV) / I
OCV is Open Circuit Voltage, CCV is Closed-Circuit Voltage, and I is load current.
Fill in the blank: A battery’s internal resistance ______ as the battery discharges.
increases
This increase in internal resistance can affect the battery’s performance over time.
What is a thermocouple?
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.
What principle does a thermocouple operate on?
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.
Name the materials used in the construction of a thermocouple.
Common materials include:
* Chromel
* Alumel
* Bismuth
* Antimony
* Constantan
* Copper
* Platinum
* Platinum Rhodium
Different combinations of these materials yield varying thermoelectric voltages.
State an aeronautical application for a thermocouple.
Thermocouples are primarily used to measure exhaust gas temperatures of engines in aviation.
This application is crucial for monitoring engine performance and safety.
What happens to electrons in a heated metal?
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.
Fill in the blank: The thermoelectric voltage generated by a thermocouple depends on the _______ between the junction and the two free ends.
temperature difference
What is the thermoelectric voltage generated by a Chromel-Alumel thermocouple per 100 °C?
4.2 mV per 100 °C.
This is one of the common combinations used in thermocouples.
True or False: A thermocouple can measure temperatures ranging from -200 °C to 2300 °C.
True
What effect is greater at higher temperatures when two different metals are in contact?
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.
List the thermoelectric voltage generated by Bismuth-Antimony per 100 °C.
10.0 mV per 100 °C.
This combination provides a higher voltage compared to others.
List the thermoelectric voltage generated by Constantan-Copper per 100 °C.
4.2 mV per 100 °C.
This is another common combination used in thermocouples.
What is the thermoelectric voltage generated by Platinum-Platinum Rhodium per 100 °C?
1.0 mV per 100 °C.
This combination is used for high-temperature measurements.
What is the principle of operation of photo-cells?
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.
What materials are commonly used in the construction of photo-cells?
Silicon and selenium
Silicon PV cells are particularly significant due to their large surface areas.
Name some aeronautical applications of photo-cells.
Automatic light level adjustment in aircraft
Photo-cells help adjust working illumination based on surrounding light intensity.
What are the components of a depletion layer photo-cell (photovoltaic or PV cell)?
A metallic base plate, a semiconductor layer, and a thin translucent metal skin as a counter electrode.
What voltage is typically produced by solar cells when exposed to sunlight?
Approximately 0.6 V
Solar cells can also produce approximately 10 mW per cm² of active area.
Fill in the blank: A typical use of photocells is in _______.
exposure meters, alarm systems, counters, and low power devices.
What happens to the Light Dependent Resistor (LDR) resistance when ambient light decreases?
The resistance increases.
True or False: The voltage generated by photo-cells is independent of light intensity.
False.
What is the purpose of the circuit involving a Light-Emitting Diode (LED) and a transistor in photo-cells?
To turn on the LED or vary its brilliance based on light intensity.
What occurs to the potential at the base of the transistor when the LDR resistance increases?
It changes, switching the transistor on.
What is the function of the photo-cell in an automatic light adjustment system?
To sense surrounding light intensity and adjust illumination automatically.
