Specific charge.pptx

e/m- Determination of Specific charge of an electron

1. DETERMINATION OF SPECIFIC CHARGE (e/m) Of ELECTRON BY THOMSON’S METHOD.

2. Contents Introduction History Principle Equipment Description Cathode Ray Tube Formulae Conclusion

3. Introduction Objective: Determination of the specific charge of the electron (e/m) by Thomson’s method. Measuring separately the electric charge (e) and the rest mass (m) of an electron is a difficult task because both quantities are extremely small. Mullikan oil drop experiment enabled us to calculate electron charge but mass of the electron can’t be calculated directly. Mass of electron is calculated by determining the e/m ratio . In this experiment we observe the motion of free electrons in a vacuum tube. From their response to electric and magnetic fields , the ratio of charge to mass for the electron can be determined.

4. History During the late 19 th century, it was impossible to determine very small charge and mass of particle like electron. Even imagination of electrons as an individual part was not clear. J. J. Thomson first determined the specific charge (charge to mass ratio-e/m) of the electron in 1897. J. J. Thomson was able to determine the e/m ratio by using the cathode ray discharge tube , applying electric and magnetic fields perpendicular to each other as well as to the path of electrons. In 1896, J. J. Thomson performed a series of revolutionary experiments with cathode ray tubes and demonstrated some interesting things about the atom.

5. In the presence of electrical or magnetic field, the conduct of cathode rays are similar to that expected from negatively charged particles.   E lectric field produced by a short length of deflection plates and the magnetic field is applied from outside over the same length. Cathode rays originating from a heated cathode(electron gun) were actually a stream of small negatively charged particles which is evident from their behaviour as discussed above. This suggests that the cathode rays consist of negatively charged particles, called electrons He named these particles as “corpuscles” but we now know them as electrons. He Provided the first evidence that atoms were in fact not fundamental building blocks of matter. He measured the charge to mass ratio of the cathode rays by observ ing how much they were deflected by magnetic field and how much energy they carried.

6. According to Thomson ,the amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon: 1.The magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection. 2. The mass of the particle — heaver the particle, lesser the deflection. 3. The strength of the electrical or magnetic field — the deflection of electrons from its original path increases with the increase in the voltage across the electrodes ,or the strength of the magnetic field. Measurement of the specific charge of cathode rays for different metals made him conclude that the particles that constituted cathode rays form a part of all the atoms in the universe. For his work J. J. Thomson received the Nobel Prize in Physics in 1906, “in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases”.

7. PROPERTIES OF CATHODE RAYS In 1878,William Crookes discovered cathode ray by studying the conduction of electricity through gases in a discharge tube filled with a gas at very low pressure (0.01 mm Hg) and huge voltage of order 10^4 . 1.Cathode rays are beam of fast moving electrons . 2. These rays are composed of material particles and posses momentum as they are capable of rotating a light paddle when placed in their path. 3. These rays produce heating effect. 4. These rays ionize the gas through which they pass. 5. These also effect the photographic plates termed as fogging. 6. The characterstics of cathode rays are independent of the material of the cathode and nature of gas used in discharge tube.

8. Principle If an electron is subjected to electric and magnetic fields it experiences forces. By adjusting magnitude and directions of field the net force on electron is made zero that is beam of electrons travel without any deflection. I f both magnetic and electric field are applied and their magnitude and direction is adjusted so that the charged particles move without deviation. Here Electron goes undeflected in presence of both electric and magnetic field if resultant force on the electron is zero. i.e electric force= magnetic force. EQ=Q v B , V=E/B

9. EQUIPMENT DESCRIPTION Equipment used in the experiment include the following: Cathode ray tube (CRT),CRT power supply , wooden frame, deflection magnetometer, Wooden bench, bar magnets , stop watch. 1. CATHODE RAY TUBE :

10. Cathode ray tube is the main component of this experiment. It has three components namely 1.ElectronGun 2.Deflecting plates 3.Fluoroscent screen The cathode ray tube is filled with helium at very low pressure. You need the low pressure in the tube so that the passage of the beam of electrons is not impeded very much by Collision with air molecules

11. 1.ELECTRON GUN: The electron gun section of the cathode ray tube provides a sharply focused, electron beam directed towards the fluorescent-coated screen. This starts from thermally heated cathode, emitting the electrons which is called as thermo ionic emission. The whole assembly of cathode, control grid, focusing anode, and accelerating electrode are called the electron gun. The control grid controls the brightness of the spot on the screen. By controlling the number of electrons by the anode , hence the focusing anode ensures that electrons leaving the cathode in slightly different directions are focused down to a narrow beam and all gather at the same spot on the screen. The control grid is given negative potential with respect to cathode. This grid controls the number of electrons in beam, going to the screen. The momentum of the electrons determines the intensity, or brightness, of the light emitted from the fluorescent screen due to the electron bombardment . T he function of these anodes(focusing and accelerating) is to concentrate and focus the beam on the screen and to accelerate the speed of electrons.

12. DEFLECTION PLATES :The deflection system in CRT consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. Vertical plate moves electron beam upward and downward, and horizontal plates moves electron beam from one side to another. FLOUROSCENE : The light which is emitted through phosphorous excitation is called fluorescence. When the electron beam stop, the phosphorous crystal regain their original position and release a quantum of light energy which is called phosphorescence or persistence. AQUADAG : The Aquadag is the aqueous solution of graphite which is connected to the secondary of the anode. The Aquadag collects the secondary emitted electrons which are necessary for keeping the CRT screen in the state of electrical equilibrium.

13. FORMULA:- Formula deduced from this experiment to calculate e/m ratio is as follows                     e/m  =  V py ( l+ p/2 )/ t   A 2                     B      =  B (T 2 / T 2  )Sin φ    Where     e      –  charge of an electron                     m     –  mass of an electron                    V     –  voltage applied to deflection plates                   y     –   displacement of the spot                     t       –  separation between the deflecting plates                    p      –  length of deflecting plates                    l –  distance between the end of deflecting plates and screen                    B      –  magnetic induction                    B   –  horizontal component of earth’s magnetic field                     T     –  Time period of oscillation of deflection magnetometer needle in the presence of two bar magnets.                    T 0    –  period of oscillation of deflection magnetometer needle in the presence of earth’s magnetic field.                   Φ     – inclination of the magnetic needle to the earth’s magnetic field                    A    – area under the curve plotted between x and [ B ( L- x ) ]                    x     -    is the distance of the screen from the aperture                     L    – distance of electron gun exit to the screen.

14. IMPORTANT CONCEPTUAL QUESTIONS Why CRT tube is placed along north south direction? This is because the earths magnetic field is along north-south and when CRT tube is placed in the same direction then the motion of electron beam is not affected by earth’s magnetic field. How magnetic field is applied in this experiment? Magnetic field is applied using bar magnets or pair of Helmholtz coils. But in this experiment we use bar magnets to produce magnetic field. Why the magnetic field of bar magnets can’t be measured by gaussmeter? charged plates cannot be measured using a gaussmeter as the magnetic field produced by bar magnets is quite small. Therefore magnetic field is calculated using deflection magnetometer.

15. CONCLUSION: ERROR ANALYSIS : Therefore we can calculate mass of the electron by using above e/m value. By substituting all known values in the formula , the value Standard value of e/m was found to be JJ Thomson found that the value of e/m for these cathode rays remains the same for all cathodes ,anodes , gas present, and applied voltages.