The Electron Flashcards
What are the characteristics of an electron?
-An electron orbits the atom in defined shells
-It is the fundamental and smallest unit of charge
-It is negatively charged
-It is indivisible
-The electron was named the Irish physicist G J Stoney
-Robert Millikan measured the charge of the electron in 1911 in his oil drop experiment
-Its mass is 1/1840 of an atomic mass unit (u)
What is an electronvolt (eV)?
A measure of energy that can be converted to joules. Calculated by multiplying the charge on one electron by the voltage accelerating it
Define thermionic emission.
The emission of electrons from the surface of a hot metal
Explain how a cathode ray tube (CRT) works.
-A low voltage is passed through a metal filament, which causes thermionic emission of a cathode plate
-The emitted electrons are attracted to a cylindrical anode by a very high voltage
-When the the electrons reach the hole in the cylindrical cathode, some pass through and out the other side at a high velocity
-The emerging electrons pass through two ses of parallel plates
-When they strike the desired point on the screen, the electrons transfer their kinetic energy into a bright flash (scintillation)
-All the scintillations construct a picture fast enough to convince the human eye that it’s a single image
Cathode rays…
-travel in straight lines
-cause certain substances to emit light
-produce X-rays when they strike metal with high velocity
-can be deflected in EM fields
What is an X-ray?
An electromagnetic radiation of high frequency, produced when high energy electrons strike a metal target
How is an X-ray produced?
-The cathode is heated by a low voltage
-Electrons are emitted through thermionic emission
-Extra high voltage attracts and accelerates electrons towards the anode
-The tungsten target is backed by an oil coolant to prevent melting and overheating
-When the electrons strike the target, their energy is converted
-The electrons produce approximately 1 per cent X-rays and 99 per cent heat
-Everything is covered in a lead-lined chamber to prevent leakage
-High energy produces hard (very penetrating) X-rays and low produces soft
State the properties of X-rays.
-They are high-frequency EM radiation
-They can penetrate certain materials
-They can ionise certain materials
-They can expose photographic film and cause certain materials to flouresce
What are some uses of X-rays?
Medicine and industrial (luggage)
What is the photoelectric effect?
The emission of electrons from the surface of a metal when EM radiation of a suitable frequency is incident on it
State Einstein’s photoelectric law.
The kinetic energy of the fastest moving electron emitted is calculated as the difference between the energy of the incident photon and the work function of the metal
What is Einstein’s explanation of the photoelectric effect?
-Light is made up of photons
-When light strikes a metal, each photon can interact and give its energy to one electron only
-If the energy of the photon is greater than or equal to the minimum energy, an electron can leave
-Any energy that exceeds the work function is given to the electron as kinetic energy
What is a photocell?
A photocell uses the photoelectric effect to operate devices
What are some applications of the photoelectric effect?
-Burglar alarms
-Smoke alarms
-Automatic doors
