Question

Q. Two devices connected to the same voltage source of 220 are operated for 30 sec. each. The device P draws 2A current and the Q draws 1A current. in which of the devices heat produce is more?
​

Answer 1

Given, V=220 V
Power rating of each bulb, P=10 W
Total current, I
t
​
=5 A
Let number of lamps in parallel be n.
Let individual resistance of each lamp be R.

From Joule's law of heating,
P=V
2
/R
10=220
2
/R
R=4840.............(i)

R
1
​
=R
2
​
=..............=R
n
​
=R..............(ii)
For resistances in parallel,
R
eq
​

1
​
=
R
1
​

1
​
+
R
2
​

1
​
+......+
R
n
​

1
​
........(iii)
From (ii) and (iii),
R
eq
​
=R/n..........(iv)
Substituting (i) in (iv),
R
eq
​
=4840/n...............(v)

From Ohm's Law,
V=I
t
​
R
eq
​
............(vi)
Substituting (v) in (vi),
220=5×4840/n
n=110