A voltage source sends a current 2-5 A to a resistor of 20 `Omega ` connected across it for 5 minutes. Calculate : (i) the p.d. of the source, (ii) the electrical energy supplied by the source, and (iii) the heat in cal, produced in the resistor.

Explanation

Given : I = 2.5 A, R = 20 `Omega `
t = 5 minutes `= 5 xx 60 s = 300 s`
(i) p.d. of the source `V = 1 R = 2.5 xx 20 = 50 V`
(ii) Electrical energy supplied by the source
`= VIt = 50 xx 2.5 xx 300 = 3.75 xx 10^4 J `
(iii) Heat produced in the resistor `= 3.75 xx 10^4 J`
`= ( 3.75 xx 10^4 )/( 4.18 ) cal = 9 xx 10^3 ` cal (nearly)
Alternative : Heat produced `= 0.24 I^2 Rt` cal
`= 0.24 xx (2.5)^2 xx 20 xx 300 = 9 xx 10^3` cal