Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in series with each other across the two wires of 220 V line if the maximum current allowed is 5 A?
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Answer:
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
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