CBSE Class 10 Science 2020 Outside Delhi Set 2

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Question : 7
Total: 7
(a) Define power and state its SI unit.
(b) A torch bulb is rated 5V and 500mA. Calculate its
(i) Power
(ii) Resistances
(iii) Energy consumed when it is lighted for 2
1
2
hours.
Solution:  
(a) Power is defined as the rate of doing work, it is the work done in unit time. The SI unit of power is Watt (W) which is joules per second (J\/ s).
(b) Given : Potential difference =V=5V
Current =I=500mA=
500
1000A
=0.5A

(i) Power =V×I=5×0.5=5×
5
10
=2.5W

(ii) Resistance =R= ?
By Ohm's law: V=IR
R=
V
I

R=
5
0.5

=
50
5
=10ohms

(iii) Energy consumed when it is lighted for 2
1
2
hours
Energy = Power × Time
t=2.5×60×60 sec.
So,
Energy =2.5×2.5×60×60
=22500Joules sec.
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