A resistance of 980 Ω is connected in series with a voltmeter, after which the scale division becomes 50 times larger. The resistance of voltmeter is
Let voltmeter resistance be
R. Since scale becomes 50 times larger, current reduces by 50 times. So, total resistance becomes 50R.
Given: 50R = R + 980
⇒ 49R = 980 ⇒ R = 20 Ω.
Answer: (B) 20 Ω.
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Let the resistance of the voltmeter be Rv . When a 980 Ω resistor is connected in series, the total resistance becomes
Rv + 980 and the sensitivity (scale division) becomes 50 times larger, meaning the current is reduced to
1/50 of original.
Using the current division rule:
Rv/Rv + 980 = 1/50 ⇒ 50R
v = R
v +980 ⇒ 49R
v = 980 ⇒ R
v = 20Ω
Answer: (B) 20Ω.
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