Question

Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?​

Answer 1

Answer

We know that, P=

R

V

2

From the figure,

R

1

=

P

1

V

1

2

=

100

220×220

R

1

=484Ω

R

2

=

P

2

V

1

2

=

60

220×220

R

2

=806.7Ω

The equivalent resistance in parallel combination,

R

eq

1

=

R

1

1

+

R

2

1

R

eq

1

=

484

1

+

806.7

1

R

eq

=302.5Ω

The current flowing in the circuit,

I=

R

eq

V

=

302.5

220

I=0.7272A

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Answer 2

Answer:

From Joule's Law of heating,

P=V

2

/R

For first lamp,

R

1

=220

2

/100

For second lamp,

R

2

=220

2

/60

When they are connected in parallel,

R

eq

1

=

R

1

1

+

R

2

1

=

220

2

/100

1

+

220

2

/60

1

⟹R

eq

=220

2

/160

From Ohm's Law,

V=IR

eq

220=

160

220

2

I

I=0.73 A

Explanation:

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