modern physics| Electrons| complete notes| class 12|NEB|

Review &Summary 

• Electric discharge if electric current flows through a gas it is known as electrical conduction or discharge in the gas.
• Discharge tube A tube used to study the electrical conduction through gases at low pressure is known as discharge tube.
• Cathode rays Stream of electrons moving with high speed, emerging normally from the cathode of a discharge tube, kept at a pressure of 0.01mm of Hg and under a very high potential difference of the order of 10-15kV, supplied from an induction coil.
• Specific charge of an electron is the ratio of charge to mass of an electron.
• Nature of cathode rays they are the fast moving electrons.

Useful information for multiple choice questions.

• cathode rays are streams of fast moving electrons.
• Electromagnetic waves are not deflected by electric or magnetic field.
• e/m for canal ray is different for different types of gas
• Electric discharge takes place at different voltage for different gases.
• Magnetic field is used to deflect electron in T.V.
• Millikan's experiment leads to the quantum nature of electric charge.
• Due to the low pressure at high attitude electric discharge takes place i.e. lighting.
• Oil droplets are used in Millikan's experiment because of its high viscosity and does not easily evaporate.
• In Thomson experiment, we use cross field i.e. electric and magnetic field are perpendicular to each other.
• Electric discharge stops at very high and very low pressure .
• Stokes' relation cannot be strictly applicable to very small droplet i.e. the radius of the droplet should be much larger than the intermolecular separation.

cross field

Uniform electric field is produced by charged parallel plate capacitors and uniform magnetic field is produced by a current  carrying coil. If the two uniform fields are perpendicular to each other in a such ways that the deflections produced by one on charged particle is cancelled by the by the other , the fields are cross field . if the beam of charged particles each carrying charge q is moving with the velocity v is subjected to cross fields force on the charged particles due to the electric field ,f=qe and the force on the charged particles due to magnetic field, fm=Bev .These two forces act directions. If the fields are adjusted in such ways that the two forces are equal in the magnitude'

ie eE=bev

v=E/B

in such conditions the beam will not deflected.

Thomson's Experiment to Determine Specific Charge(e/m) of Electrons 

The ratio of charge of mass is called the specific charge of an electron.it was first measured by sir joseph jhon Thomson in 1887,who got noble prize for his initiate works .

principle: if a beam of electrons is subjected to electric and magnetic field it experiences forces.by adjusting the magnitude and directions of the two fields, the net forces on the electron is made zero.

MOTION OF ELECTRON BEAM IN ELECTRIC FIELD

suppose a horizontal beam of electrons, moving with the velocity v, passes between two parallel plates as shown in the figure. A potential difference V between the two plates is applied, under plate being positively charged, then the electric field intensity between the plate is given by

E=V/D where d is the distance between two plates.

 

the force on a electrons of charge e moving between the plates is given by F=e*v/d 

some useful information for tackling short questions.

• cathode rays are the streams of electrons which posses kinetic energy owing to their mass and motions. On striking with the metal piece this energy gets transfer to the molecules of the metal, increasing their internal energy resulting in the increase of the temperature of the metal
• neon tube is a discharge tube containing neon at low pressure. Due to the potential differences maintained between the cathode and the anode, electrons are accelerated towards the anode and in colliding with the neon atoms ionize them and excite the  positive ions when these decay to a lower energy level ,they emit the characteristics pink light, hence a glowing gas such as that in a neon tube , gives only certain wave length of light.
• in the Millikan's oil drops  experiments, the size of the droplet should be small and should not evapourate.due to the small droplets , it acquires small terminal velocity. in water we don't  find such types of conditions because water may evaporates due to its high vapor's pressure and the droplets of water are quite large.
• importance of Millikan's oil experiments are;
• it shows that electronic charge is a small charged on a charged particles.
• it provides the evidence of quantization's of charge.
• by combining the result of Millikan's experiments with the result of Thomson experiments the mass of electrons can be determined.   

IMPORTANCE NUMERICALS PROBLEMS

1. In a evacuated tube electrons are accelerated from the rest through a potential differences of 3600v and then travels in a narrow beam through a field free spaces before entering a uniform magnetic  field the flux lines of which are perpendicular to the beam. in the magnetic field the electrons describes a circular arc of radius 0.01m calculate i) the speed of the electrons II) magnetic flux
2. a charged particles  is suspended in a uniform field of 3000volts/sec that it.  neither falls nor rises .find the charge on the drops.
3. In experiments a drop of diameter 9cm with a density of 90 gram/cm3 is observed . the condenser plates are 4 cm apart. A potentials of 72v applied across the plates keeps the drops just in balance. How many electrons are there in a drop?
4. in one experiments a singly charged drop was found to fall under gravity at a terminal velocity 40 m/s and to rise 4 m/s when a field of 2*100000v/m was suitably applied. calculate the q radius=6*10-5 m and the viscosity is 1.60*10-6NS/M2.
5. electrons are accelerated from rest by a potential difference of 100 v. what is their final velocity ?the electrons beam now enter normally a uniform electric field intensity 100000v/m calculate the flux density B of a uniform  magnetic  field applied perpendicular to the electric field if the path of the beam  is unchanged from its original directions .

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