CBSE Class 12 · 2015 · Delhi · Magnetic Effects of CurrentShort Answer

State the principle of working of a galvanometer. A galvanometer of resistance

G

is converted into a voltmeter to measure up to

V

volts by connecting a resistance

R_{1}

in series with the coil. If resistance

R_{2}

is connected in series instead, it can measure up to

V /2

volts. Find the resistance, in terms of

R_{1}

and

R_{2}

, to be connected to convert it into a voltmeter that can read up to

2 V

. Also find the resistance

G

of the galvanometer.

Answer: $R_{3} = 3 R_{1} - 2 R_{2}$ for the $2 V$ range, and $G = R_{1} - 2 R_{2}$ .

Solution

\textbf{Principle:} a current-carrying coil in a magnetic field experiences a torque proportional to the current. To read voltage, a high resistance is put in series:

R = \frac{V}{I _{g}} - G

R_{1} = \frac{V}{I _{g}} - G (i), R_{2} = \frac{V /2}{I _{g}} - G (ii) .

Subtracting:

\frac{V}{2 I _{g}} = R_{1} - R_{2}

, and from (i),(ii)

G = R_{1} - 2 R_{2}

. For the

2 V

range,

R_{3} = \frac{2 V}{I _{g}} - G = 4 (R_{1} - R_{2}) - (R_{1} - 2 R_{2}) = 3 R_{1} - 2 R_{2}

, with

G = R_{1} - 2 R_{2}

CBSE Class 12 Board · 2015 · Delhi · Subjective

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CBSE Class 12 · 2015 · Delhi · Magnetic Effects of CurrentShort Answer

Solution

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