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Work, Energy and Power

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Physics Part I

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Physics

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12

A 200 W sodium street lamp emits yellow light of wavelength 0.6 um. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

  • 1.5 x 1020

  • 6 x 1018

  • 62 x 1020

  • 3 x1019


A.

1.5 x 1020

 Efficient power P= 
P equals space straight N over straight t space equals space fraction numerator h c over denominator lambda end fraction space equals space 200 space x 0.25
N over t space equals space 50 space x fraction numerator lambda over denominator h c end fraction space equals space 1.5 space x space 10 to the power of 20
space equals fraction numerator 50 space x 0.6 space x space 10 to the power of negative 6 end exponent over denominator 6.6 space x space 10 to the power of negative 34 end exponent space x space 3 x space 10 to the power of 8 end fraction
space equals space 1.5 space x space 10 to the power of 20

 Efficient power P= 
P equals space straight N over straight t space equals space fraction numerator h c over denominator lambda end fraction space equals space 200 space x 0.25
N over t space equals space 50 space x fraction numerator lambda over denominator h c end fraction space equals space 1.5 space x space 10 to the power of 20
space equals fraction numerator 50 space x 0.6 space x space 10 to the power of negative 6 end exponent over denominator 6.6 space x space 10 to the power of negative 34 end exponent space x space 3 x space 10 to the power of 8 end fraction
space equals space 1.5 space x space 10 to the power of 20

1028 Views

What are different units of energy?

The different units of energy are :

(i) Joule            
(ii) Erg
(iii) eV            
(iv) KWh
(v) Calorie.

804 Views

Define the unit joule.

Work done is said to be one joule if one newton of  force displaces the body through a distance of one meter in the direction of applied  force .
799 Views

Define watt.

Power is said to be one watt if one joule of work is done in one second. 
860 Views

Is work a scalar or a vector quantity?

Work is dot product of two vectors. i.e. F with rightwards arrow on top times S with rightwards arrow on top.

And dot product is a scalar quantity. Therefore, work is a scalar quantity. 
1450 Views

What is watt?

SI unit of power is Watt. 
773 Views