Answer
Internal resistance rm=1.553 M Ω
Work Step by Step
Given :
Vs = 12v
Rs = 25 k Ω = 25 x 10³ Ω
Potential across is given as 11.81 V
Step 1 :
Circuit diagram is given as and it is shown in above figure.
Step 2 :
Current across voltmeter will be zero
Therefore, Rs and rm will be in series connection.
When resistances are connected in series then voltage divider rule can applied.
Step 3 :
By voltage divider rule voltage across is ;
(Vs x rm)/(rm + Rs )
Therefore;
11.81 = (12 x rm ) / (25x10³ + rm)
11.81(25 x 10³+rm ) = 12 rm
295250 + 11.81 rm = 12 rm
295250 = 12 rm - 11.81 rm
295250 = 0.19 rm
rm = 295250 / 0.19
rm = 1553947.368
rm = 1.553 M Ω
Therefore internal resistance rm = 1.553 M Ω
