Level 1: Stretch a 1200 W Heater's Wire to Double Length: New Power?
Stretch it by just ten percent.
A heater coil is stretched so that its length increases by 10% (volume unchanged) and is used on the same supply. Its power decreases by about:
At constant volume the area falls as the length rises, so the resistance R = ρL/A grows as the square of the length.
The power falls to about 82.6% of its old value, a decrease of about 17%. The resistance rises by 21%, but power on a fixed voltage falls by the smaller fraction 0.21/1.21, so neither 10% nor 21% is right.
Every Level 2 starts from a quick question, Level 1, and changes one thing. Seeing what that change does is the skill JEE Main tests. Try it here, free and with no signup, then read the worked answer.
