Cells and Batteries

Question 1

3 functions of a cell

Answer 1

1. it applies emf
2. it stores energy
3. It is itself a link in the circuit and current passes through it

Question 2

What is the EMF of a cell?

Answer 2

The electromotive force (emf) is the total potential difference generated by a source, such as a battery or cell, when no current is flowing. It represents the maximum voltage the source can provide due to chemical or physical processes occurring within it.

Question 3

What is the PD of a cell?

Answer 3

The voltage measured across the terminals of a cell under load.

Question 4

Terminal Voltage

Answer 4

The terminal voltage is the voltage measured across the terminals of a battery or cell when it is connected to a load, meaning current is flowing through the circuit. It is the actual voltage that the battery delivers to the rest of the circuit, which is lower than the EMF or Open circuit voltage of the cell

Terminal Voltage = open circuit Voltage - (Ir)

Question 5

What is the difference between the emf of a cell and it’s terminal voltage?

Answer 5

The emf of a cell is the total voltage it produces, while the terminal voltage is the it’s total voltage minus the voltage drop across it’s internal resistance

Question 6

What is Open circuit voltage?

Answer 6

The voltage measured across the terminals of a device or cell, when their is no current flowing through it.

Question 7

What is the difference between connecting cells in series and in parallel?

Answer 7

Series:

Connection: Positive to Negative
Voltage: Additive
Internal Resistance: Additive
Current: Stays the same.

Parallel:

Connection: All Positive electrodes connected together, and all negative electrodes connected together
Voltage: The same as a single cell
Internal Resistance: Lower - 1/Tr = 1/r1 + 1/r2 + 1/r3
Current: Higher current due to lower internal resistance.

Question 8

What is a battery?

Answer 8

A number of cells connected together in series

Question 9

What is a primary battery

Answer 9

A non-rechargeable battery or cell. The chemical reactions in the battery are non-reversable.

Question 10

What is a secondary battery?

Answer 10

A chargeable battery.

Question 11

What is the difference between a dry and wet cell or battery?

Answer 11

In a wet cell the electrolyte is a liquid, such as hydrochloric acid, or brine. When the cell is no longer serviceable the electrodes and electrolyte fluid can be replaced.

In a dry cell the electrolyte is a paste, such as a manganese oxide and potassium hydroxide mixture. When the cell is no longer serviceable it needs to be thrown away.

Question 12

What are the attributes of a good cell?

Answer 12

1. High EMF
2. Low internal resistance
3. low Polarization (constant emf over long periods)
4. minimum local action (to extend shelf life)
5. Cost effective.

Question 12

Why do batteries or cells need an electrolyte?

Answer 12

The electrolyte is a medium that contains ions and can be in the form of a liquid, gel, or solid. It allows ions to move between the electrodes, enabling the chemical reactions to occur. It also serves as a separator to prevent direct contact between the anode and cathode, which would cause a short circuit.

Question 12

What are electrodes?

Answer 12

There are two electrodes in a battery - the anode (negative electrode) and the cathode (positive electrode). The anode undergoes oxidation (loses electrons), and the cathode undergoes reduction (gains electrons).

Question 13

What is a per hour amperage rating?

Answer 13

the constant current which a battery can deliver for a specified amount of hours before the terminal PD drops below a certain voltage

An 20 hour amperage- hour rating means a battery can supply a constant amount of current for 20 hours of continues use before the terminal PD begins to drop.

Question 14

What is polarization in batteries or cells?

Answer 14

Polarization in batteries refers to the reduction in efficiency and performance of a battery due to the buildup of reaction products on the electrodes or other factors that impede the electrochemical reactions. This can result in a voltage drop and increased internal resistance.

Question 15

Definition of Local Action in Batteries

Answer 15

Local Action in batteries refers to the self-discharge of the battery due to internal chemical reactions that occur even when the battery is not connected to an external circuit. These unwanted reactions typically involve impurities in the electrode materials and result in a continuous, gradual loss of charge.

Attributes of Local Action
Self-Discharge: Local action leads to the battery losing charge over time, even when not in use.
Internal Reactions: These are chemical reactions occurring within the battery, unrelated to any external load.
Impurities: Impurities in the electrode materials often catalyze these reactions, contributing to local action.
Heat Generation: The reactions can generate heat, which may further accelerate the self-discharge process.
Decreased Efficiency: Local action reduces the overall efficiency and lifespan of the battery by wasting stored energy.

Examples of Local Action
Zinc-Carbon Batteries: In traditional zinc-carbon batteries, impurities in the zinc can cause local action, leading to self-discharge. The zinc reacts with the electrolyte even when the battery is not in use.
Lead-Acid Batteries: Impurities in the lead can cause local action, where the lead slowly corrodes and reacts with the sulfuric acid electrolyte, leading to self-discharge.
Primary Batteries: Many primary (non-rechargeable) batteries exhibit local action due to impurities in the anode material.

Non-Examples of Local Action
High-Purity Materials: Batteries made with high-purity electrode materials and electrolytes exhibit minimal local action because there are fewer impurities to catalyze unwanted reactions.
Inactive Batteries: Batteries stored in optimal conditions (e.g., low temperature, dry environment) where the chemical reactions are significantly slowed down, leading to minimal self-discharge.
Secondary (Rechargeable) Batteries in Ideal Conditions: Well-maintained rechargeable batteries that are regularly used and recharged tend to show less local action because their design minimizes impurities and unwanted reactions.

Summary
Local Action is an important concept in battery technology that refers to the self-discharge caused by internal chemical reactions, primarily due to impurities in the electrode materials. It leads to a gradual loss of charge, decreased efficiency, and reduced battery lifespan. Understanding local action and its attributes is crucial for improving battery design and material purity, thereby enhancing battery performance and longevity.

Question 16

Concept: Electrodes

Answer 16

Electrodes are conductive materials that allow electric current to enter or leave a non-metallic part of a circuit, such as an electrolyte, a semiconductor, or a vacuum. They play a crucial role in electrochemical cells, batteries, and various electronic devices by facilitating the movement of electrons and ions.

Attributes of Electrodes

Conductivity: Electrodes must be made of materials that can conduct electricity, such as metals (copper, zinc, silver), graphite, or conductive polymers.

Role in Reactions:
Anode: The electrode where oxidation occurs (loss of electrons).
Cathode: The electrode where reduction occurs (gain of electrons).

Electrochemical Activity: Electrodes participate in electrochemical reactions, either gaining or losing electrons depending on their role (anode or cathode).

Material Properties: The choice of electrode material affects the efficiency, voltage, and lifespan of the electrochemical cell or device. Important properties include reactivity, stability, and overpotential.

Physical Structure: The surface area, porosity, and shape of electrodes can influence their performance, particularly in batteries and fuel cells.

Examples of Electrodes

Zinc and Copper Electrodes in a Voltaic Cell:
Zinc (Anode): In a zinc-copper voltaic cell, zinc serves as the anode, where it oxidizes and releases electrons.
Copper (Cathode): Copper serves as the cathode, where it reduces and gains electrons.

Graphite Electrodes in Lithium-Ion Batteries:
Anode: Often made of graphite, where lithium ions intercalate during charging.
Cathode: Commonly made of lithium cobalt oxide (LiCoO₂), where lithium ions de-intercalate during discharging.

Platinum Electrodes in Fuel Cells:
Anode: Platinum catalyzes the oxidation of hydrogen, producing protons and electrons.
Cathode: Platinum also catalyzes the reduction of oxygen, combining with protons and electrons to form water.

Non-Examples of Electrodes

Insulating Materials: Materials such as plastic, wood, or rubber, which do not conduct electricity, cannot serve as electrodes.

Pure Electrolytes: While electrolytes contain ions that facilitate the movement of charge, they themselves are not electrodes; they are the medium through which the ions move.

Non-Conductive Ceramics: Ceramics that do not conduct electricity (e.g., porcelain) cannot be used as electrodes in electrochemical cells.

Summary
Electrodes are essential components in any system that involves the flow of electric current and electrochemical reactions. They come in various materials and structures depending on their application, such as batteries, fuel cells, and electrolyzers. Understanding the properties and roles of electrodes helps in designing efficient and durable electrochemical systems.