Q.One lamp of 90 watt and 30 volt has to be used in a line of 120 volt. Lamp must be used with a residence-
1. 10 ohm in series
2. 20 ohm in parallel
3. 20 ohm in series
4. 10 ohm in parallel
Answers
Answer:
Resistance of lamp
R
0
=
P
V
2
=
90
(30)
2
=10Ω
Current in the lamp
I=
R
0
V
=
10
30
=3A
As the lamp is operated on 120V DC, then resistance becomes
R
′
=
i
V
′
=
3
120
=40Ω
For proper glow, a resistance R is joined in series with the bul
R
′
=R+R
0
⇒R
α
=R
′
−R
0
=40−10=30Ω
Answer:
30V- 90w is operated in 120v line
Explanation:
Resostance of the lamp
R•= V^2/P
= (30)^2/90
= 900/ 90
= 10 ohm
Current in the lamp
I = V/R•
= 30/10 A
= 3A
As the lamp opereted at 120V DC
R' = V'/i
=120/3
= 40 ohm
There fore R' =R^ + R•
There fore R^ = R' - R•
= 40- 10
= 30 ohm
30 ohm is the correct answer, but it is not given. in the option
Thank you