Question

The emf of a standard cell is 1.5V and is balanced by a length of 300 cm of a potentiometer with a 10 m long wire. Find the percentage error in a voltmeter that balances at 350 cm when its reading is 1.8 V.  

Answer 1

Given:

E₁ = 1.5 V, l₁ = 300 cm, l₂ = 350 cm

From individual cell method of potentiometer, 

`"E"_1/"E"_2 = "l"_1/"l"_2`

∴  E₂ = E₁ × `"l"_2/"l"_1`  

= `1.5 xx 350/300 = 1.75`V

But given reading is 1.8 V

∴ Error = 1.8 – 1.75 = 0.05 V

∴ Percentage error = `0.05/1.75 xx 100`

= 2.8571%

The percentage error in a voltmeter is 2.8571%.