The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan The electron D.K.

PDST Resources for Leaving Certificate Physics 2 Leaving Certificate Physics: Topics  Mechanics  Temperature  Heat  Waves  Vibrations and Sound  Light  Electricity  Modern Physics:The electron  Option 1: Particle Physics  Option 2: Applied Electricity

PDST Resources for Leaving Certificate Physics 3 Cathode rays  Cathode rays were first observed in 1869 by the German physicist Johann Hittorf.  They seemed to come from the negative terminal (the cathode) of an evacuated tube.  They cause some substances to fluoresce, including glass to a some extent.  They could be deflected by a magnetic field or an electric field.  In 1897 British physicist J. J. Thomson showed the rays were composed of a previously unknown negatively charged particle, which was later named the electron.  Cathode ray tubes (CRTs) were long used as the picture tubes of television sets, oscilloscopes, heart monitors etc.

PDST Resources for Leaving Certificate Physics 4 Thermionic emission  Around 1873 Frederick Guthrie discovered that a red hot conductor would readily lose a negative charge but would hold a positive charge.  Thomas Edison noticed the same effect in Using the effect he made a device that could conduct electricity in one direction only and filed a patent based on the idea— the first ever patent for an electronic device.  The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that ‘thermionic emission’ could be used to detect radio waves and developed a vacuum tube diode which he patented in Edison

PDST Resources for Leaving Certificate Physics 5 Learning outcomes  The discovery of thermionic emission and of cathode rays led to the discovery of the electron. –Applications: Cathode ray tube, radio and television  Photoelectric emission and how Einstein developed the quantum energy concept to explain the effect. –Application: photocell  X-rays were discovered (1895, Röntgen)and the method of producing them developed and refined. –Application: medical uses as well as research and industrial uses.  Spectra explained in terms of electron transitions between discrete energy levels (1913, Bohr). –Application: diagnostic research tool and laser applications.

PDST Resources for Leaving Certificate Physics 6 Discharge Tubes  Callan developed high voltages  Geissler developed effective vacuum pumps  Discharge tubes developed  Cathode rays discovered  Experiments revealed them to be particles...then named electrons

PDST Resources for Leaving Certificate Physics 7 Thermionic emission  The emission of electrons from a metal when it is heated.... discovered by Edison  This paved the way for a revolution in communication.... i.e. radio and television

PDST Resources for Leaving Certificate Physics 8 Cathode Ray Tube Electron gun - deflection plates - fluorescent screen  Similar to Thomson’s experiment of 1897 where he identified the electron,  Used in cathode ray oscilloscopes for measuring voltages and frequencies.  Adapted for use in first generation televisions

PDST Resources for Leaving Certificate Physics 9 Electron gun  Accelerating and focussing effect on the electron beam  Electron volt as a unit of energy

PDST Resources for Leaving Certificate Physics 10 Recap questions  What property of cathode rays suggested that they were charged particles?  What is meant by the term “ space charge”?  What is the purpose of the grid in a cathode ray tube?  How does the grid function?  How does a cylindrical anode focus the electron beam?  What happens when cathode rays strike the fluorescent screen  What is an “electron volt”?

PDST Resources for Leaving Certificate Physics 11 Photoelectric effect  It was found that no electrons were emitted unless the frequency of light was above a certain value, called the threshold frequency.  Electrons were emitted immediately … there was no delay after first exposure.

PDST Resources for Leaving Certificate Physics 12 Elements of Einstein’s interpretation

PDST Resources for Leaving Certificate Physics 13 Photocell

PDST Resources for Leaving Certificate Physics 14 How x-rays are produced  What is the role of the heater current?  What is the role of the anode voltage?  Beware! X-rays are ionising radiation.

PDST Resources for Leaving Certificate Physics 15 What happens at an atomic level to liberate an x-ray?  Rapid deceleration of an electron ……. or ….  Dislodge an inner electron, with subsequent replacement by an outer electron

PDST Resources for Leaving Certificate Physics 16 X-rays and crystals  X-rays have shorter wavelength than light  The tiny separation between the layers of atoms in crystals is required to cause diffraction of x-rays  Much has been learned about crystals using x- rays

PDST Resources for Leaving Certificate Physics 17 X-rays and medicine  Doctors diagnose broken bones using x-rays  Doctors treat certain cancers with x-rays  Dentists diagnose hidden tooth decay using x-rays

PDST Resources for Leaving Certificate Physics 18 Summary  Cathode rays were discovered when discharge tubes were developed  J.J. Thomson identified in 1897 that the cathode rays were particles (electrons).  Thermionic emission was found to be an efficient way to obtain electrons.  The photoelectric effect initially proved puzzling until Einstein explained it in  X-rays were discovered by Roentgen in  X-rays are very useful but can be dangerous because they are ionising.  X-ray production is the converse of the photoelectric effect.