Ee1 chapter7 cells

Aug 16, 2012

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IT2001PAEngineering Essentials (1/2)

Chapter 7 - Cells

Chapter 7 - Cells

IT2001PA Engineering Essentials (1/2) 2

Lesson Objectives

Upon completion of this topic, you should be able to:

Describe the constructional features and uses of common batteries.

Chapter 7 - Cells


Specific Objectives

Define the function of a cell.

State the differences between a primary and a secondary cell.

Describe the construction and operating principles of a simple voltaic cell.

State the factors that effect the electromotive force of a cell.

Identify the following types of cells: Dry Lelanche cells

Mercury cells

Lead-acid cells

Alkaline cells.

Chapter 7 - Cells


Operating Principles of Cells

Cell

- A device that converts chemical energy to electrical energy.

Battery

- A number of cells connected together.

Types of Cells

- Primary cell

- Secondary cell

Chapter 7 - Cells


Primary Cell and Secondary Cell

PRIMARY CELL

Can only convert chemical energy into electrical energy.

- CANNOT BE RECHARGED -

SECONDARY CELL

Converts chemical energy into electrical energy and vice versa.

- CAN BE RECHARGED -

Chapter 7 - Cells


Simple Voltaic Cell

Primary cell

Consists of: a) Copper rod (+ve electrode)

b) Zinc rod (-ve electrode)

c) Dilute sulphuric acid

(electrolyte)

d) Container or casing

Chapter 7 - Cells


Simple Voltaic Cell

Chapter 7 - Cells


Operating Principles:

When the zinc and copper rods are connected externally by a wire, a current flows from copper to zinc. Hence, copper is the positive pole (ANODE) and zinc is the negative pole (CATHODE). This current flow is due to the chemical reaction of the cell.

Chapter 7 - Cells


Polarization

The gathering of Hydrogen bubbles around the copper rod.

This increases the internal resistance and decreases the emf of the cell.

Can be prevented by Depolarizers (eg. Manganese Dioxide)

Chapter 7 - Cells


Polarization

Chapter 7 - Cells


Local Action

Due to the impurities (eg. lead, carbon, iron) in the zinc rod.

This reduces the strength of the current.

Can be prevented by coating the zinc rod with mercury.

Chapter 7 - Cells


Local Action

Chapter 7 - Cells


Types of Primary CellsTypes of Primary Cells

1.Wet Leclanche Cell

2.Dry Leclanche Cell

3.Mercury Cell

4.Carbon-Zinc Dry Cell

5.Alkaline-Manganese Cell

Chapter 7 - Cells


Wet Leclanche CellWet Leclanche Cell

Chapter 7 - Cells


Dry Leclanche CellDry Leclanche Cell

Chapter 7 - Cells


Mercury CellMercury Cell

Chapter 7 - Cells


Carbon-Zinc Dry CellCarbon-Zinc Dry Cell

Chapter 7 - Cells


Alkaline-Manganese CellAlkaline-Manganese Cell

Chapter 7 - Cells


Types of Secondary CellsTypes of Secondary Cells

1. Lead-Acid Cell

2. Nickel-Cadmium Cell

Chapter 7 - Cells


Lead-Acid CellLead-Acid Cell

Chapter 7 - Cells


Nickel-Cadmium CellNickel-Cadmium Cell

Chapter 7 - Cells


Functions of the Components of a Cell Positive Electrode

Higher potential of the battery terminal.

Negative Electrode

Lower potential of the battery terminal.

Electrolyte

Causes the electrodes to react and produce emf.

Container or Casing

This is the one that holds the electrolyte. The electrodes are not touching the casing.

Chapter 7 - Cells


Capacity of Secondary Cell

It is expressed in Ampere-hour (Ah).

Capacity of a cell depends on:

a) size of electrodes (plates)

b) number of electrodes

c) amount of acid

d) rate of discharge

Chapter 7 - Cells


Specific Gravity of Electrolyte

Definition:

This is referring to how acidic the electrolyte is.

A Hydrometer is used to measure the S.G.

Fully-charged battery - S.G. = 1.28

Fully-discharged battery - S.G. = 1.15

Chapter 7 - Cells


Measuring Specific Gravity of Electrolyte using a Hydrometer

Chapter 7 - Cells


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