Question

A voltmeter of range $3\;{\text{V}}$and resistance $200\;\Omega$ cannot be converted to an ammeter of range:
A. $10\;{\text{mA}}$
B. $100\;{\text{mA}}$
C. $1\;{\text{A}}$
D. $10\;{\text{A}}$

Answer 1

We need to determine the minimum amount of current that can be measured by the ammeter. To obtain that apply Ohm’s law with the given readings of voltage and resistance. Formula of Ohm’s law:
$V = IR$.

Complete step by step answer:
Given,
The voltmeter range,
$v = 3\;{\text{V}}$
Resistance,
$R = 200\;\Omega$

Now, according to ohm’s law,
$V = IR$
Here, $V$ is voltage
$R$ is resistance
And $I$ is current

Rewrite the above equation for current $I$.
Therefore,
$I = \dfrac{V}{R}$

Substitute the values of $v$ and $R$ in above equation,
I = $\dfrac{{3\;{\text{V}}}}{{200\;\Omega }} \\\ = 0.015\;{\text{A}} \\\ {\text{ = 15}}\;{\text{mA}}$

Hence, the required current that the ammeter can measure must be ${\text{15}}\;{\text{mA}}$ at least.
Therefore, we cannot obtain the current of ${\text{10}}\;{\text{mA}}$ from the given ammeter.

So, the correct answer is “Option A”.

Additional Information:
Ohm’s law: Ohm’s law states that the current across two points is directly proportional to the voltage across the two points by a conductor. The formula of Ohm’s law is $V = IR$. Here, $V,I,R$ are the voltage, current and resistance respectively.

Voltmeter: A voltmeter is a device used to calculate the difference in electrical potential in an electric circuit between two points. Analog voltmeters pass a pointer over a scale in accordance to the circuit voltage; digital voltmeters use an analog-to - digital converter to provide numerical representation of the voltage.

Ammeter: A measuring device used to measure the current in a circuit is an ammeter. Electrical currents are measured in amperes, and hence the name. Instruments used for calculating smaller currents, beyond the range of milliamps or microamperes, are designated as millimetres or micro amps.

Note:
In this question we are asked to calculate the minimum amount of current that can be measured by an ammeter. An ammeter measures the value of current in amps or milliamps. In the question the value of voltage and resistance are given and using these values the current is obtained using Ohm's law, which is given by $V = IR$. Calculate the value of current in milliamps.