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Work, Energy and Power

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Physics Part I

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CBSE Gujarat Board Haryana Board

Previous Year Papers

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Class 10 Class 12
Show that the two-body decay of this type must necessarily give an electron of fixed energy and, therefore, cannot account for the observed continuous energy distribution in the β-decay of a neutron or a nucleus (Fig. 6.19).

[Note: The simple result of this exercise was one among the several arguments advanced by W. Pauli to predict the existence of a third particle in the decay products of β-decay. This particle is known as neutrino. We now know that it is a particle of intrinsic spin ½ (like eor n), but is neutral, and either massless or having an extremely small mass (compared to the mass of electron) and which interacts very weakly with matter. The correct decay process of neutron is:→ p + e+ ν] 

The decay process of free neutron at rest is given as, 

→ p + e– 

From Einstein’s mass-energy relation, we have

Energy of electron =  Δ


Δm = Mass defect = Mass of neutron – (Mass of proton + Mass of electron) 

where, c = Speed of light

Δm and c are constants.

Hence, the given two-body decay is unable to explain the continuous energy distribution in the 
β-decay of a neutron or a nucleus.

What is watt?

SI unit of power is Watt. 

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 .

What are different units of energy?

The different units of energy are :

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


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. 

Define watt.

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

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