How does a fuse in lighting circuit differ  
from that in a heating circuit? What is the  
importance of a fuse in electrical appliance?  
Example 05  
Fuse against circuit breaker  
Answer:  
Answers  
While a fuse prevents devices and homes  
Lighting circuit  
Heating circuit  
The fuse in alighting The fuse in alighting  
circuit has a lower circuit has a higher  
against  
power overloads only,  
circuit  
breakers protect homes and devices against  
power overloads and short-circuiting  
current  
because  
circuit  
rating current  
lighting because  
rating  
lighting  
carry high  
carry  
low circuit  
current to power the current to produce  
bulbs heat  
The fuse in lighting The fuses can  
circuits is usually tolerate brief over-  
Example 06  
(a) State  
five  
advantages  
of  
using  
faster in response to current because they  
high current surges are designed for  
to prevent damage to appliances that may  
accumulators  
✓ It is not expensive to manufacture  
compared to other types of cells  
✓ It is reliable in terms of always  
providing the required voltage when  
required  
sensitive  
components  
electronic draw higher initial  
current  
Importance of a fuse  
A fuse protects the circuit from overloading  
and short circuits by breaking the circuit  
when a specific current exceeds the fuse  
rating  
✓ It can give very high currents  
compared to the simple cell  
✓ If stored without electrolyte, it has a  
very long shelf life  
✓ It has a wide range of sizes and  
capacities available  
2.6 CHEMICAL CELLS  
A cell is a set up to cause flow of electric  
current in a conductor. The flow of current  
is caused by reactions, releasing and  
accepting electron at the different end of  
a conductor.  
(b) Give three disadvantages of using  
accumulators  
(i)  
It is heavy and very large in size  
(ii) It can overheat and even explode  
during charging if overcharged.  
Chemical cells: - these are cells in which a  
chemical reaction generates electricity  
(iii) Once the electrolyte has been poured  
into the cells, it has to be kept in a  
charged condition throughout to  
There are two types of chemical cells  
1.Primary cells  
avoid  
weakening of the  
2.Secondary cells  
chemicals  
Example 07  
Primary cells: - are chemical cells which  
cannot be recharged (renewed) once are  
exhausted.  
These are polarization and local  
action.  
Examples of primary cells are  
ACTION OF A SIMPLE CELL  
(i)  
Simple cell  
Dry cell  
(ii)  
The following are the processes that occur  
when the cell is in operation:  
Secondary cells: -are chemical cells which  
can be recharged (renewed) when they are  
exhausted.  
Dilute sulphuric acid ionizes into sulphate  
ions (푆푂4 ) and hydrogen ions (퐻+):  
2−  
2−  
2퐻+ + 푆푂4  
Example of secondary cells area  
퐻2푆푂4  
(i)  
Lead acid accumulator  
The alkaline battery  
(ii)  
Zinc also goes into solution as zinc ions  
(푍푛2+) releasing two electrons which travel  
along the wire (external circuit) to the  
copper electrode. The zinc ions combine  
with the sulphate ions to form zinc sulphate  
(푍푛푆푂4). The same time when zinc ions go  
into solution, an equivalent number of  
hydrogen ions move to the copper electrode  
where they gain electrons and are liberated  
as hydrogen gas (bubbles).  
THE SIMPLE CELL  
The simple cell consists of:  
(i)  
Zinc metal as the negative  
electrode  
(ii)  
(iii)  
Copper metal as the positive  
electrode  
Dilute sulphuric acid as the  
electrolyte  
2퐻+ + 2푒−  
퐻2(푔)  
When the circuit is complete, electrons flow  
from zinc to copper while current flows  
from copper to zinc.  
By  
losing  
electrons,  
copper  
becomes  
positively charged and enables it to attract  
electrons from zinc thorough connecting  
wire. This movement of electrons through  
the wire is called electric current.  
Note  
When a bulb is connected to the two  
electrodes, it lights brightly. After some  
time, the hydrogen ions in the electrolyte  
pick up electrons and form an insulating  
layer of hydrogen gas bubbles around the  
copper plate making it difficult for the  
electrons to flow. This increases the  
resistance and reduce the current flow which  
makes the bulb to dim. The process by  
✓ Simple cell cannot supply current for  
a long time because it suffers from  
two defects while in operation.  
which the hydrogen bubbles forms around  
POLARIZATION  
the copper plate is called polarization.  
Polarization is the defect of a simple cell  
caused by the formation of hydrogen  
bubbles around the copper plate.  
As zinc reacts with sulphuric acid, it  
dissolves and exposes hidden impurities of  
carbon and iron. These impurities form  
small cells called local cells. These local  
cells cause the zinc to be used up even when  
current is not being supplied. This defect is  
called local action.  
These hydrogen bubbles form insulation  
layer around the copper therefore blocking  
the easy flow of current  
HOW TO MINIMIZE POLARISATION  
polarization can be minimized by adding a  
depolarizer, like potassium dichromate and  
manganese oxide. The depolarizer donates  
oxygen which combines with hydrogen to  
form water.  
Example 01  
When the bulb is connected between the two  
plates of the simple cell, the bulb lights up  
however, the brightness of the bulb fades  
after about a minute. Explain why its  
brightness fades.  
A good depolarizer should not react with the  
electrolyte.  
Answers  
This is due to polarization of a simple cell  
The brightness of the bulb fades because of  
the formation of hydrogen bubbles which  
forms an insulating layer on the copper  
plate, making it difficult for the electrons to  
flow. This makes the bulb to dim.  
Note  
The hydrogen bubbles formed insulate the  
copper plate and prevent other positive  
hydrogen ions from receiving electrons from  
the copper plate to become neutral. This  
increases the resistance inside the cell.  
THE CELL DEFECTS  
Also, hydrogen ions accumulating at the  
copper plate repel other Hydrogen ions (This  
defect is called Back e.m.f and opposes or  
weakens the main e.m.f of the cell).  
Cell defects: - is the failure of a simple cell  
to function properly due to formation of  
hydrogen bubbles on the copper plate or  
small local cells on the zinc plate.  
Simple cell has two main defects which  
cause the current to diminish quickly when  
the cell is being used.  
LOCAL ACTION  
Local action is the process by which a cell is  
used up when no external current is flowing.  
Two defects of simple cell are:  
As zinc reacts with sulphuric acid, it  
dissolves and exposes hidden impurities of  
carbon and iron. These impurities form  
small cells called local cells. These local  
(i) Polarization  
(ii) Local Action  
cells cause the zinc to be used up even when  
current is not being supplied. This defect is  
called local action.  
Also, hydrogen ions accumulating at the  
copper plate repel other Hydrogen ions (This  
defect is called Back e.m.f and opposes or  
weakens the main e.m.f of the cell).  
HOW TO MINIMIZE LOCAL ACTION  
THE LECLANCHE CELL  
Local action can be minimized by using  
pure zinc or by coating zinc with a layer of  
The Leclanché cell: is the cell uses an  
aqueous solution of ammonium chloride (sal  
ammoniac) as the electrolyte, amalgamated  
zinc rod as cathode.  
mercury  
(amalgamation).The  
mercury  
dissolves the zinc metal leaving behind the  
impurities.  
Example 01  
(a) A student using a simple cell to light a  
bulb observed that the bulb lit only for a  
short time then went off. Explain why the  
bulb went off after only a short time.  
answers  
The bulb went off due to polarization effect,  
in which hydrogen bubbles were formed on  
The carbon rod as anode fixed in a porous  
pot, containing a powdered mixture of  
the  
copper  
electrode  
which  
formed  
carbon  
and  
manganese  
makes the  
(IV)  
oxide.  
more  
insulation around the copper electrode  
therefore blocking the easy flow of current.  
Thecarbon  
mixture  
conducting, and the manganese (IV) oxide  
(manganese dioxide, MnO2) acts as a  
depolarizer.  
(b) State a way in which polarization  
reduces the current in a simple cell.  
Answers  
The Leclanché cell is called a primary cell  
and this type of cell current is produced  
from a non-recoverable or irreversible  
chemical reaction.  
By forming the hydrogen layerwhich  
insulates the copper plate increasing the  
resistance inside the cell.  
THE DRY CELL (DRY LECLANCHE  
CELL)  
The hydrogen bubbles formed insulate the  
copper plate and prevent other positive  
hydrogen ions from receiving electrons from  
the copper plate to become neutral. This  
increases the resistance inside the cell.  
It is a modified Leclanché cell in which the  
main electrolyte is a paste of starch and  
ammonium chloride.  
A dry cell, also called Leclanché cell,  
consists of a carbon rod (positive terminal)  
which is surrounded by manganese (IV)