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Moving Charges and Magnetism — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsMoving Charges and Magnetism4 Marks
Question
A galvanometer can be converted into voltmeter of given range by connecting a suitable resistance R, in series with the galvanometer, whose value is given by,
$\text{R}_\text{s}=\frac{\text{V}}{\text{I}_\text{g}}-\text{G}$
where Vis the voltage to be measured, lg is the current for full scale deflection of galvanometer and G is the resistance of galvanometer.

Series resistor(R,) increases range of voltmeter and the effective resistance of galvanometer. It also protects the galvanometer from damage due to large current. Voltmeter is a high resistance instrument and it is always connected in parallel with the circuit element across which potential difference is to be measured. An ideal voltmeter has infinite resistance. In order to increase the range of voltmeter n times the value of resistance to be connected in series with galvanometer is Rs = (n - 1)G.
  1. 10mA current can pass through a galvanometer of resistance $25\Omega$ What resistance in series should be connected through it, so that it is converted into a voltmeter of 100V?
  1. $0.975\Omega$
  2. $99.75\Omega$
  3. $975\Omega$
  4. $9975\Omega$
  1. There are 3 voltmeter A, B, C having the same range but their resistance are $15000\Omega,10000\Omega$, and $5000\Omega$ respectively. 'Tile best voltmeter amongst them is the one whose resistance is
  1. $5000\Omega$
  2. $10000\Omega$
  3. $15000\Omega$
  4. all are equally good.
  1. A milliammeter of range 0 to 25mA and resistance of $10\Omega$ is to be converted into a voltmeter with a range of 0 to 25V. 'Tile resistance that should be connected in series will be:
  1. $930\Omega$
  2. $960\Omega$
  3. $990\Omega$
  4. $1010\Omega$
  1. To convert a moving coil galvanometer (MCG) into a voltmeter:
  1. A high resistance R is connected in parallel with MCG.
  2. A low resistance R is connected in parallel with MCG.
  3. A low resistance R is connected in series with MCG.
  4. A high resistance R is connected in series with MCG.
  1. To increase the current sensitivity of a moving coil galvanometer, we should decrease:
  1. Zero.
  2. Low.
  3. High.
  4. Infinity.
✓

Answer

  1. (d) $9975\Omega$

Explanation:

A galvanometer can be converted into a voltmeter of given range by connecting a suitable high resistance R in series of galvanometer, which is given by,

$\text{R}=\frac{\text{V}}{\text{I}_\text{g}}-\text{G}=\frac{100}{10\times10^{-3}}-25=10000-25=9975\Omega$

  1. (c) $15000\Omega$

Explanation:

An ideal voltmeter should have a very high resistance.

  1. (c) $990\Omega$

Explanation:

Resistance of voltmeter $=\frac{25}{25\times10^{-3}}=1000\Omega$

$\therefore\text{X}=1000-10=990\Omega$

  1. (d) A high resistance R is connected in series with MCG.

Explanation:

To convert a moving coil galvanometer into a voltmeter, it is connected with a high resistance in series. The voltmeter is connected in parallel to measure the potential difference. As the resistance is high, the voltmeter itself does not consume current.

  1. (d) Infinity.

Explanation:

The resistance of an ideal voltmeter is infinity.

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