Electrons Thermionic Emission Deflection of Electrons in Electric Field Deflection of Electrons in Magnetic Field Determination of e/m Cathode Ray Oscilloscope

Thermionic Emission (1) When a metal is heated sufficiently, its free electrons gain enough kinetic energy to leave the metal. This process is called thermionic emission.

Thermionic Emission (2) In practice, thermionic emission is caused by heating a filament of metal wire with an electric current.

Thermionic Emission (3) The work done on each electron from the filament is W = eV where V is the p.d. across the filament and the anode. Electron-volt The electron-volt is an amount of energy equal to the work done on an electron moved through a p.d. of 1V. 1 electron-volt = J

Work Function The work function  of a metal is the minimum energy that must be supplied to enable an electron to escape from its surface.  is conveniently expressed in eV. The smaller the work function the lower the temperature of thermionic emission.

Work Function for some metals

Properties of Electron Beams (Cathode rays) - 1 Cathode rays travel in straight lines. Cathode rays can cause fluorescence.

Properties of Electron Beams (Cathode rays) - 2 Cathode rays possess kinetic energy. Cathode rays can be deflected by electric field and magnetic field. Cathode rays may produce heat and X-rays. Cathode rays can affect photographic plates.

Deflection of Electrons in a Uniform Electric Field (1) Consider an electron beam directed between two oppositely charged parallel plates as shown below. With a constant potential difference between the two deflecting plates, the trace is curved towards the positive plate. + - d

Deflection of Electrons in a Uniform Electric Field (2) The force acting on each electron in the field is given by where E = electric field strength, V = p.d. between plates, d = plate spacing. p

Deflection of Electrons in a Uniform Electric Field (3) The vertical displacement y is given by This is the equation for a parabola.

Deflection of electrons in a uniform magnetic field

Deflection of Electrons in a Uniform Magnetic Field (1) The force F acting on an electron in a uniform magnetic field is given by Since the magnetic force F is at right angles to the velocity direction, the electron moves round a circular path.

Deflection of Electrons in a Uniform Magnetic Field (2) The centripetal acceleration of the electrons is Hence which gives

Determination of Specific Charge - e/m J. J. Thomson

Determination of Specific Charge Using a Fine Beam Tube (1) The principle of the experiment is illustrated by the diagram below. × × × × × × × × v F=Bev Electron gun r

Determination of Specific Charge Using a Fine Beam Tube (2) (For an electron moving in a uniform magnetic field) Since and the kinetic energy of the electron provided by the electron gun is Where V is the anode voltage.

Determination of Specific Charge Using a Fine Beam Tube (3) So Rearrange the equation gives The value of the specific charge of an electron is now known accurately to be C/kg

Thomson’s e/m Experiment (1) Thomson’s apparatus for measuring the ratio e/m × × × × × × × × × + - v

Thomson’s e/m Experiment (2) A beam of electron is produced by an electron gun with an accelerating voltage V. The electron beam is arranged to travel through an electric field and a magnetic field which are perpendicular to each other. The apparatus is set-up so that an electron from the gun is undeflected.

Thomson’s e/m experiment (3) As the electron from the gun is undeflected, this gives Bev eE v i.e. On the other hand, Combining the equations, we get

Deflection Tube (1) There are three possible arrangements: Electric deflection field only Magnetic deflection field only Crossed electric and magnetic deflection fields

Deflection Tube (2)

Magnetic Field strength due to Helmholtz Coil

Cathode Ray Oscilloscope (CRO) The structure of the cathode ray tube

Cathode Ray Tube All CRT's have three main elements: an electron gun, a deflection system, and a screen.

Electron Gun Indirectly heated cathode (C) To emit electrons by thermionic emission. Grid (-ve potential with respect to C) To control the number of electrons to reach the anode from the cathode. Anodes (+ve potential with respect to C) To accelerate the electrons and focus the electron beam.

Deflecting System X-plates Y-plates To cause horizontal deflections. To be connected via the X-amplifier to a build-in circuit called the time base. Y-plates To cause vertical deflections. External signals are applied via the Y-amplifier to the Y-plates.

Fluorescent Screen Coated with a phosphorous. To convert the kinetic energy of the fast-moving electrons into photons of visible light. The brightness is proportional to the number of electrons striking on the screen.

Time base It generates a sawtooth potential difference across the X-plates. It causes the bright spot to travel across the screen from left to right at steady speed.

Uses of CRO An oscilloscope can be used as http://www.teachnet.ie/dkeenahan/2006/page33.html An oscilloscope can be used as 1. an a.c. and d.c. voltmeter, 2. for time and frequency measurement, 3. as a display device,(Displaying waveform) 4. for phase relationships measurement, 5. for frequency comparison. (Lissajous’ figures) http://www.ee.usyd.edu.au/tutorials_online/topics/labintro/cro.html

Lissajous’ Figures (1) Lissajous’ figure can be displayed by applying two a.c. signals simultaneously to the X-plates and Y-plates of an oscilloscope. As the frequency, amplitude and phase difference are altered, different patterns are seen on the screen of the CRO. http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=31&CFID=447128&CFTOKEN=45263804

Lissajous’ Figures (2) Same amplitude but different frequencies

Same frequency but different phase Lissajous’ Figures (3) Same frequency but different phase In phase π/2 π 3π/2 In phase π /4 3π/4 5π/4 7π/2 http://surendranath.tripod.com/Applets.html

Indirectly heated cathode The cathode consists of a small diameter nickel cap. The closed end of the cap is coated with emitting material. Electrons can only emitted in one direction.

Grid The grid is in the form of a solid metal cap with small hole in the centre. The grid potential may be varied to control the number of electrons passing through the grid opening.

Anodes Focusing anode Accelerating anode It is charged a few hundred volts positive with respect to C. Accelerating anode It is charged several thousand volts positive in relation to C. It forces the electrons to travel down the centre of the tube.