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Question : 7
Total: 7
(a) Define power and state its SI unit.
(b) A torch bulb is rated5 V 500 mA
(i) Power
(ii) Resistances
(iii) Energy consumed when it is lighted for2
(b) A torch bulb is rated
(i) Power
(ii) Resistances
(iii) Energy consumed when it is lighted for
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 = 5 V
Current = I = 500 mA =
= 0.5 A
(i) Power= V × I = 5 × 0.5 = 5 ×
= 2.5 W
(ii) Resistance= R =
By Ohm's law:V = I R
R =
R =
=
= 10 ohms
(iii) Energy consumed when it is lighted for2
Energy= ×
t = 2.5 × 60 × 60 s e c .
So,
Energy = 2.5 × 2.5 × 60 × 60
= 22500 Joules ∕ s e c .
(b) Given : Potential difference
(i) Power
(ii) Resistance
By Ohm's law:
(iii) Energy consumed when it is lighted for
Energy
So,
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