Page 86 - Physics_Form_2

Physics for Secondary Schools
(b) Discuss what might happen to
the refrigerator if it is connected
to a supply of:
(i) 280 V.
(ii) 110 V.
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3. (a) Why are the electrical appliances
earthed?
– (b) Explain the precautions to take
during electrical wiring of a
house.
(c) Describe how electricity from a
fuse box is distributed to other
parts of a house.
4. (a) Explain briefly why most plugs
have three pins.
(b) In a three-pin plug, we find
wires labelled N, L and E:
(i) What do these letters
mean?
(ii) At present, what is the
internationally accepted
colour for wires labelled
N, L and E?
5. Give the differences between the tree
system and ring system methods of
wiring a house.
6. Why is the earth thicker and longer
in a three-pin plug?
Cells
A cell is a set-up used to cause a flow of
electric current in a conductor. The flow
of current is caused by reactions releasing
and accepting electrons at different ends
of a conductor. There are two common
types of electrochemical cells, namely
primary cells and secondary cells.
80
Physics Form 2 Final.indd 80
Primary cells
Primary cells, also known as voltaic
cells, are formed by dipping two different
electrodes (usually made of metals) into
an electrolyte. In a primary cell, reactants
are used up after some time and must be
replaced. Figure 2.54 shows features of a
simple primary cell.
A
+ –
Copper Zinc
Dilute H SO 4
2
Figure 2.54: A simple cell
Electrodes of a simple cell can be made
of copper (positive) and zinc (negative)
while an electrolyte is dilute sulphuric
acid.
The following are the processes that occur
when the cell is in operation:
The dilute sulphuric acid ionises into


2
sulphate ions SO 4 and hydrogen ions
( H ):
+
H SO  2 4 2H   SO 4 2
Zinc goes into solution as zinc ions
2  , releasing two electrons which
Zn
travel along the wire (external circuit)
to the copper electrode. The zinc ions
combine with the sulphate ions to form
zinc sulphate ZnSO . At the same

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