Page 22 - Physics - XII

Principle/Theory K

When two resistances say R 1 + –
E
and R are connected in parallel, X R Rheostat
2
their combined resistance R is 1
P
given as X 2 B RB
1 1 1 R RR 2 E F
1
R P R 1 R 2 or P R R 2 G
1

The value of an unknown 0 10 20 30 40 50 60 70 80 90 100
resistance X can be determined Q D P
by using metre bridge as Jockey (J)

l A l cm C
X R (100–l) cm
100 l Fig. 3(b).1: Circuit diagram for parallel combination of resistors

R
1

where l is the length of metre bridge wire from zero end to balance point and
R is the resistance from resistance box.
R
Procedure Fig. 3(b).2: Parallel combination
2
1. Take two resistances.
2. Adopt the same steps as in experiment 2 to determine the values of X and X one by one.
1
2
3. Connect the two resistances X and X in parallel as shown in Fig. 3(b).1 and fi nd the resistance of the
2
1
parallel combination X . Take three sets of observations and record them in the observation table.
P
Observations and Calculations
Table for calculation of resistances X , X , and X P
2
1
Resistance Length Length Resistance Mean
Resistance used S. from resistance AD = l DC = (100 – l) X l R resistance
in the left gap No. box, R (cm) (cm) l X
100
(Ω) (Ω) (Ω)

1.
X 2. X =
1 1
3.
1.
X 2. X =
2 2
3.

1.
X and X in parallel 2. X =
1 2 P
3.

20