Q23 of 46 Page 458

An electron and a positron moving at small speeds collide and annihilate each other. Find the energy of the resulting gamma photon.

As we know electron and proton has same mass i.e.


As they annihilate and produces gamma particle with some energy



There is a physical significance of special theory of relativity that Energy and mass are interchangeable and that is given by Sir Einstein famous equation E=



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21

A 100W bulb together with its power supply is suspended from a sensitive balance. Find the change in the mass recorded after the bulb remains on for 1 year.

 22

The energy from the sun reaches just outside the earth’s atmosphere at a rate of 1400 W m–2. The distance between the sun and the earth is 1.5 × 1011 m.

(a) Calculate the rate at which the sun is losing its mass.


(b) How long will the sun last assuming a constant decay at this rate? The present mass of the sun is 2 × 1030 kg.


 24

Find the mass, the kinetic energy and the momentum of an electron moving at 0.8c.

 25

Through what potential difference should an electron be accelerated to give it a speed of

(a) 0.6c, (b) 0.9c and (c) 0.99 ?