Page 89 - Physics_Form_2

Current electricity

The internal resistance of the battery is means that the external loop is made up
different from that of a single cell. In this of 3 batteries, each with a voltage of 3
case, the internal resistances of four cells V. The total voltage (across AB) is the
are parallel, as shown in Figure 2.57. As same as that of a single battery (3 V).
a result, the total internal resistance is
given by, Cell defects
FOR ONLINE READING ONLY
1 = 1 + 1 + 1 + 1 When the simple cell is in operation,

r T r 1 r 2 r 3 r 4 the current drops to a very small value
1  1  1  1  1  4 after some time. A simple cell has two
r T 1  1  1  1  1  main defects which cause the current to

1  4 diminish quickly when the cell is being
used. These defects are local action and
r T 1  polarization.

r  0.25 
T
Local action
Thus, the battery has an e.m.f. of 1.5 V The local action of a cell is the
and an internal resistance of 0.25. deterioration of the battery due to currents
that are flowing from and to the same

Example 2.7 electrode. The deterioration is caused
by embedded impurities such as iron,
Determine the voltage across AB in the lead and carbon in a zinc electrode. The
circuit shown in Figure 2.58. impurities act as positive electrodes and

create electric currents between zinc and
this positive (impurities) electrode. Local

15 v 15 v 15 v action can be reduced by using pure zinc
A or by rubbing mercury on the zinc plate to
form an amalgam.
B
15 v 15 v 15 v Polarization

Polarization is a defect that occurs
in simple electric cells due to the
accumulation of hydrogen gas around
Figure 2.58
a positive electrode. The hydrogen gas
Solution produced in chemical reactions inside the
Note that, all the internal loops are cell can accumulate around the electrodes
connected in series while the external resulting in insulation. Polarization in cells
loops are connected in parallel. can be minimized using depolarizers such
as potassium dichromate or manganese
For the internal loops, the total voltage oxide, which oxidizes hydrogen to water.
in each loop = (1.5 + 1.5) V = 3 V. This Simple cells, Daniel and Leclanché cells,

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Physics Form 2 Final.indd 83 25/10/2025 10:26