When not in use, the needles in a milliammeter and voltmeter are in the position depicted in Figure A. When a student employs these tools in his experiment, the needle readings are in the locations depicted in Figure B. Determine the correct values of current and voltage the student should use in his calculations.

 

                                                     (OR)               

 

A student obtained a graph while conducting an experiment to study the dependence of current (𝐼) on the potential difference (𝑉) across a resistor, 

(i) What is the relation between the current and the potential difference from the graph? 

(ii) When the potential difference across it is 2. 5𝑉 what is the current that flows through the resistor?

 

The figure given shows there is an error in the instrument 

To find the correct reading once the measurement is done, we need to add or subtract these variations 

Number of divisions on the scale of milliammeter = 5

Least count of the milliammeter = 10/5 𝑚𝐴 = 2 mA

Zero error = − 4 𝑚𝐴 

Reading on the main scale = 34 𝑚𝐴 

𝐶𝑜𝑟𝑟𝑒𝑐𝑡 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 = 𝑀𝑎𝑖𝑛 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 − 𝑍𝑒𝑟𝑜 𝑒𝑟𝑟𝑜r

= 34 𝑚𝐴 − (− 4 𝑚𝐴) 

= 38 𝑚𝐴

Number of divisions on the voltmeter scale = 5 

The least count of the voltmeter = 1/5 = 0. 2 V

Reading on the main scale = 3. 6 

𝐶𝑜𝑟𝑟𝑒𝑐𝑡 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 = 𝑀𝑎𝑖𝑛 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 − 𝑍𝑒𝑟𝑜 𝑒𝑟𝑟𝑜𝑟

= 3. 6 𝑉 − (+ 0. 2 𝑉) 

= 3. 4 V

                                       (OR)

(i) As the graph depicts, the resistance (slope) is constantly increasing, which explains that the potential difference and the current are proportionately increasing. 

So, 𝐼 ∝ 𝑉 (Based on Ohm's law)

(ii) According to Ohm's law,

R = V/I

$\Rightarrow \mathrm{R}=\frac{1}{0.1}$

⇒ 𝑅 = 10 Ω

Again, taking ohm's law,

R = V/I

⇒ I = V/R

⇒ I = 2.5/10 = 0.25 A

Hence, 0. 25 𝐴 current flows through the resistor when the potential difference across it is 2. 5 𝑉.