Page 56 - Physics_Form_2

Physics for Secondary Schools

Solution The effective resistance of the resistors in
For three resistors in parallel, parallel, i.e., 4 Ω and 1.5 Ω, is now in

1 1 1 1 series with resistors of 15  and 20  .
= + +
R T R 1 R 2 R 3 Therefore, the total resistance is given
FOR ONLINE READING ONLY
For the first set of three resistors in by,
parallel,
R 
1 1 1 1 1 4  1.5  20  15   40.5 
T
   
12  4
R T 24  T 8  R   1.5 4   20  15 40.5  
R  T 4  The current is obtained through,

For the second set of resistors in V
parallel, I =
1 1 1 1 R T
= + +
R T R 4 R 5 R 6  9V  0.22 A

1  1  1  1  2 40.5 

R T 6  3  6  3  Therefore, the current reading on the

R  T 1.5  ammeter in the circuit is 0.22 A.

Internal resistance of a cell
A cell always resists the passage of current in a circuit. This resistance is called internal
resistance. The internal resistance of a cell acts as if it is a resistor connected in series
with the cell, so when current flows through the cell, there is a potential drop across
this resistor. Consider a cell of e.m.f, E, and internal resistance, r connected across an
external resistor, R, as shown in Figure 2.19 (a).

E E
S S

R R

(a) (b)

Figure 2.19: A simple circuit; (a) switch open and (b) switch clossed

50
Student’s Book Form One

Physics Form 2 Final.indd 50 25/10/2025 10:26