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
Given,
full deflection current, $I_{full}=5A$
Resistance of the ammeter, $R=20Ω$
Maximum current to be measured, $I_{max}=10A$
$V_{ammeter}=I_{full}×R_{ammeter}$ [By Ohm's law]
$V_{ammeter}=5×20=100V$
Let the Shunt resistor be $R_{shunt}$. The voltage across the shunt resistor must also be 100V when 5A flows through.
$V_{shunt}=R_{shunt}×I_{shunt}$
$⇒R_{shunt}=\frac{100}{5}=20Ω$
Thus,
the shunt resistor must be connected in parallel with the ammeter $20Ω$.