Question

A milliammeter of resistance of 5 ohms gives a full-scale deflection for a current of 15 mA. If the milliammeter is to be used to measure a current up to 1.5 A the size of the resistance that must be attached to the milliammeter is
A. 0.0505 ohm
B. 0.505 ohm
C. 0.05 ohm
D. 505 ohm

Answer 1

Hint: In this question we have been asked to calculate the resistance that must be attached to the given milliammeter to measure a current of 1.5A. We know that Ohm's law states the relation between the voltage, current and the resistance in a circuit. Therefore, using Ohm's law, we shall calculate the Voltage required for the milliammeter for full-scale deflection. Later using this we shall calculate the resistance required to measure 1.5A current with an ammeter of calculated voltage.

Complete step by step answer:
We have been given that a milliammeter gives full deflection for current of 15 mA i.e. 0.015 Amperes. The resistance of milliammeter is 5 ohms. Therefore, the voltage required for full deflection will be given by,
\[V=IR\]
\[V=0.015\times 5\]
Therefore,
\[V=0.075V\]……………… (1)
It is given that the milliammeter is used to measure the current of 1.5 Ampere. Therefore, the resistance required for the given milliammeter can be given by,
\[{{R}_{e}}=\dfrac{V}{{{I}_{e}}}\]
After substituting the values
\[{{R}_{e}}=\dfrac{0.075}{1.5}\]
Therefore,
\[{{R}_{e}}=0.05\Omega \]
Therefore, the correct answer is option C.

Note:
The Ohm's law states that the voltage across a conductor is directly proportional to the applied current. When introduced with the constant of proportionality resistance R, we get the famous Ohm's law equation. Milli ammeter is an instrument used to measure small currents. The current measured in this instrument is in milliamperes.