Question

A 1000 w heating unit is designed to operate on a 120 V line the line volt drops to 110 V find the percentage drop in his eat output

16%

30%

9%

27%

Answer 1

We know P=IV
so I= P/V
​​​​​in first case
i1= p1/v1=1000/120=8.34
i2=p2/v2=1000/110=9.09


now H=(i^2)rt
H1= i1^2 *r1*t1
H2=i2^2*r2*t2
​​​​​​resistance and time are constant so r1=r2
t1=t2
now percentage change = (h2-h1)/h2 *100
=(i2^2-i1^2)/i2^2 *100
=(82.62-69.55)/82.62 *100 =15.8 = 16 percentage