VISUAL  PHYSICS  ONLINE

8.1   FROM THE UNIVERSE TO THE ATOM

        THE ATOM

 

P81 009

In a modern cathode ray tube (CRO), the electrons are produced by thermionic emission from a hot filament forming the cathode. The heating of the filament by a low voltage DC power source causes electrons to be released from the hot filament with nearly zero kinetic energy. The electrons are then accelerated away from the filament by a large potential difference applied between the accelerating voltage plates. This forms a narrow electron beam with the electrons traveling at the same speed and impact onto the fluorescent screen to produce a bright spot in the centre.

 

The accelerating voltage was 5.00 kV. When a magnetic field was applied perpendicularly to the electron beam, the electrons (cathode rays) followed a circular path with a radius of 5.57 mm. Calculate the strength of the magnetic field.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

View solution below only after you have completed the answering the question.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution

Accelerating voltages increases KE of electrons

         

Electron in a uniform magnetic field experiences a magnetic force that is always directed towards the centre of a circle – centripetal force

           

         

           

           

qe = 1.602´10-19 C     me = 9.109´10-31  kg     V = 5.00 kV = 5.00´103 V  

 R = 557 mm = 5.57´10-3 m        B = ? T

            B = 0.043 T