ELECTRIC CELLS
ELECTRIC CELLS
Cells and
batteries are portable sources of electrical energy. They are used in areas
where a normal electrical supply is not available. Generally, in the rural areas,
people use dry cells for their torches.
A cell comprises an arrangement of chemically active materials
whose reaction produces electric energy when the external electric circuit is
completed.
Basic parts of an electric cell are:
(a) a positive electrode
(anode), (b) a
negative electrode (cathode), (c)
an electrolyte (active reagent).
The electrolyte reacts with either one or both electrodes to
produce electric energy. Reaction stops when the electric circuit is opened.
There are two types of cells:
1.
Primary Cells 2. Second ary Cells
Primary Cell It is not
rechargeable. After it is exhausted or depleted, it is discarded. The
reason is that the chemical action that take place is not reversible.
Most primary cells utilize electrolytes that are
contained within absorbent material or a separator (i.e.
no free or liquid electrolyte), and are thus termed dry cells.
The most common
primary cell types are the simple cell (zinc acid cell), Leclanché cell,
Manganese-alkaline cell, mercury cell, silver oxide and lithium-air cell.
1. simple cell (The
Zinc-acid Cell)
The diagram below
illustrates a zinc-acid cell also referred to as the simple cell. It consists
of:
(a)
zinc as the negative electrode,
(b) copper as the positive electrode, and (c) dilute sulphuric acid as the
electrolyte.
Zinc + Sulphuric
acid Zinc sulphate + Hydrogen + Electric energy.
The hydrogen gas
collects in bubbles around the copper electrode. The e.m.f. of this cell is
1.5V. This cell has many defects and for these, it not viably produced
commercially.
Defects of a simple Electric Cell
The practical value and performance of
simple cells is limited by the following defects:
(a)
Polarization
In the reaction in the simple cell,
hydrogen gas is evolved. It collects in bubbles around the positive electrode
and-eventually insulate the positive electrode from the solution.
This stops the reaction. This process is known as polarization.
It is minimized by use of a
depolarizer. This is a chemical which reacts with the hydrogen to produce
water e.g. manganese dioxide.
(b)
Local Action
Impurities such as iron and lead
embedded in the zinc electrode form small local cells. The impurity
acts as the positive electrode and zinc as the negative. The formation of
these local cells between the impurities and the zinc electrode is referred to
as local action. It tends to wear the zinc electrode and the electrolyte; this
happens even when the cell is not in use.
Local action may be minimized by using
pure zinc, but zinc in its pure state is very expensive. Instead, a
cheaper option is used alloying the zinc electrode with mercury.
This process is referred to as amalgamation and resultant alloy is called
zinc amalgam.
2.
Leclanché cell
The most common type of primary cell in use is the Leclanché cell.
In its wet form, it consists of a glass jar containing:
a saturated solution of sal ammoniac (ammonium chloride) as the
electrolyte,
a zinc rod as the negative electrode, and a plate of carbon as the
positive electrode.
a. Wet Leclanché Cell
A
mixture of manganese dioxide and powdered carbon is pressed around the carbon
rod and then enclosed in a porous pot which the solution can soak through. The
manganese dioxide acts as the depolarizer, while the powdered carbon gives
greater conductivity.
The
e.m.f. of the Leclanché cell is 1.5 V and the internal resistance of a normal
size cell is 1 Ohm. The wet cells are now going obsolete. They were once
a majorly used to supply power
to land-line telephone installations in remote areas. The wet
cell experience two defects:
i.
polarization ii. it is difficult to
carry about without spilling the liquid content.
Advantages
of wet Leclanche cell
1.
The
chemical used are very cheap and easily available
2.
It
gives longer current for short time.
Disadvantages
of wet Leclanche cell
i.
The
electrolyte evaporates quickly and thus requires addition of water always.
ii.
It
is not suitable for giving long duration current.
b.
Dry Cell
The
dry Leclanché cell or simply dry cell is useful because it is compact and
portable. It is constructed with the same materials as the wet cell, except
that the electrolyte is in the form of a paste or jelly. The zinc electrode is
in the form of a can, as shown below.
The
chemical action is the same as in the wet cell and its e.m.f. is 1.5 V, too.
Advantages
of dry leclanche cell
i.
relatively high e.m.f ii.
Portability iii. Cheapness of its chemical.
3. Daniel cell: it consists of a copper vessel filled with a saturated copper sulphate
solution. It has a porous pot containing dilute sulphuric acid (H2SO4) which is
the electrolyte for this cell. Amalgamated zinc rod immersed in the acid. The
zinc rod acts as the negative terminal of the cell while copper vessel acts as
the positive terminal. The copper sulphate solution acts as the depolarizer for
the cell. The cell has a e.m.f of about 1.1V.
Advantages of Daniel cell
1.
It has
a longer working time.
2.
It is
not affected by polarization
A secondary cell or battery is one
that can be electrically recharged after use to their original pre-discharge
condition, by passing current through the circuit in the opposite direction to
the current during discharge. The following graphic evidences the recharging
process.
Secondary cells are rechargeable
units. When they are depleted, they can be replenished by charging.
The most common secondary cells are:
1. The lead-acid
cells which make the car battery.
2. The
alkaline cells, such as nickel-cadmium cells.
3. The lithium ion cell, which makes the cellphone battery and portable computer batteries are most recent.
Because they are rechargeable, they are used as standby and emergency power
supplies for hospitals, factories, schools, cellphones, tablets pcs and many
others. They are also important for situations where higher currents are
required, for example, in motor vehicles, aircrafts and mobile equipment used
in remote areas.
Secondary batteries fall into two sub-categories
depending on their intended applications.
Cells that are utilized as energy storage
devices, delivering energy on demand. Such cells are typically connected to
primary power sources so as to be fully charged on demand. Examples of these
type of secondary cells include emergency no-fail and standby power sources,
aircraft systems and stationary energy storage systems for load-leveling.
Advantages of alkaline accumulators over lead-acid accumulators
1. Large currents can be
drawn from them
2. They require little maintenance 3.
They are portable
4. They can remain discharged for a long time without getting
ruined.
Disadvantages
1. They are very expensive
2. They have lower e.m.f per cell.
SUMMARY
|
CELL/COMPONENT |
+ve pole |
-ve pole |
Electrolytes |
Defects |
Depolarizer |
|
SIMPLE CELL |
Cu rod/ plate |
Zn rod / plate |
Dil. H2SO4 |
local action polarization |
|
|
DANIEL CELL |
Cu vessel |
Zn rod |
Dil. H2SO4 |
local action |
CuSO4 |
|
WET LECLANCHE |
Cu rod |
Zn rod |
Dil. NH4Cl |
Polarization Difficult to carry |
MnO2 |
|
DRY LECLANCHE |
Cu rod |
Zn container |
Moist NH4Cl |
local action polarization |
MnO2 |
|
NICKEL-IRON ACCUMULATOR |
Iron (Fe) |
Cadmium(Cd) |
Alkaline sol. |
|
|
|
LEAD-ACID ACCUMULATOR |
Lead Oxide (Pb) |
Lead (Pb) |
Dil. H2SO4 |
|
|
PRESENTATION
Step I: The teacher tests the ability of the students
on ss2 work (welcome test)
Step II: The teacher does the corrections on the
chalkboard for the students.
Step III: The teacher introduces the new topic to the
students.
Step IV: The teacher explains the types of cell
Step V: The students mention the different examples of
types of cell.
EVALUATION
The teacher evaluates the lessons by asking these questions:
i.
Define electric cell
ii.
Explain zinc acid cell
iii.
State the components of dry cells
ASSIGNMENT
Differentiate between primary and secondary cell, at least 4
points.
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