When a current of 5 A flows through an ammeter, it gives full deflection. The resistance of ammeter is 20 Ω. If the same ammeter is to be used for measuring a maximum current of 10 A, then the ammeter must be connected to a resistance of :
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 20 Ω in parallel
To make an ammeter read a larger current, a low resistance called a shunt is connected in parallel with it. The shunt takes the extra current, and the ammeter still carries only its full-scale current \( I_g = 5\,\mathrm{A} \). Both are across the same potential difference.
\( I_g = 5\,\mathrm{A},\quad G = 20\,\Omega,\quad I = 10\,\mathrm{A} \)
\( I_gG = (I - I_g)S \)
\( \displaystyle S = \frac{I_gG}{I - I_g} = \frac{5\times20}{10 - 5}\,\Omega = 20\,\Omega \)