NCEA Level 3 Physics MODERN PHYSICS90522
The Photoelectric effect - Experiment - Quantum theory & work function - Wave particle duality • Atomic spectra - Hydrogen line spectrum - Bohr model • Nuclear Physics - Level 2 Radioactivity revision - Nuclear fission & fusion - Conservation laws for nuclear reactions - Binding energy • Nuclear weapons • Exercise 10 (AC Circuits): MODERN PHYSICS Page 175 - 176 Page 177 - 180 Page 181 - 188 Page 189 - 198
INTRODUCTION By the end of 19th Century, most physicists felt that the major discoveries in physics had all been made. But a few ideas of physics were suddenly revolutionised. But what led up to this? 1887 Michelson & Morley found the speed of light is always the same. 1896 Radioactivity was discovered by Henri Becquerel. 1897 JJ Thompson ‘discovered’ the electron. 1898 Two new radioactive elements were isolated by the Curies – radium & polonium. 1900 Rutherford identified alpha & beta radiation & Villard discovered gamma rays. 1905 Albert Einstein published the special theory of relativity. To explain these discoveries two great theories of modern physics were born.
The structure of the atom • Nuclear reactions • The origins of the universe
THE PHOTOELECTRIC EFFECT If enough energy is supplied to a metal electrons can be emitted e.g. a TV tube. Place a positive terminal in the path of the energised electrons and they will move towards it. A current will flow. Often the energy comes in the form of heat however an experiment was done where light was used to cause electrons to be emitted. This was done by : Albert Einstein Max Planck
How did this revolutionise our Physics thinking? UV light hits the cathode and emits electrons. These move into the space and are attracted to the positive anode. The flow of electrons constitutes a small ‘photocurrent’ which is measured by an ammeter.
http://www.youtube.com/watch?v=RR4T1IlM2Jw Photoelectric effect demo
Why is the photocurrent so quick? Why is the frequency so important? Einstein and Planck argued that the light was made up of particles, which were quantised with a specific amount of energy. If this were the case then this would explain why the photocurrent was so immediate. They named the ‘light particle’ the photon. As the photon hits the electron it dislodges it.
Photoelectric effect findings • Low f: no electrons released, even if Intensity high. • High f: some electrons were released. • If Intensity was kept constant, and the f was increased, the same number of electrons were released however each electron had a higher kinetic energy. • If f was kept constant (above threshold value), and Intensity was increased, more electrons were released but each electron had the same amount of kinetic energy-so the intensity didn’t affect the energy per electron.
QUANTUM THEORY Einstein and Planck stated that all photons are quantised so that only a certain amount of energy is delivered, no more no less. There is a certain amount of energy associated with each wavelength of light: Red Yellow Green Blue Each wavelength is quantised RED YELLOW GREEN BLUE Ered Eyellow Egreen Eblue WHITE
As different frequencies of light have different energies this would explain why low intensity UV light emits electrons whereas high intensity red light does not. Thus: E f A constant is required and this is Planck’s constant ‘h’ measured in ‘Js’ E = hf Where h = 6.63 x 10-34 So when a high energy photon hits an electron, energy is used to release the electron from the force of attraction by the nucleus. If there is more energy than required to release the electron the extra energy comes off in the form of kinetic energy. This equation relates the frequency of the photon to the energy it carries. It tells us how the energy is quantised amongst the protons.
f = c / = 3.0 x 10 8 / 4.0 x 10-7 f = 7.5 x 1014 Hz E = hf = 6.63 x 10-34 x 7.5 x 1014 = 5.0 x 10 -19 J
WORK FUNCTION The kinetic energy of electrons given out by a metal surface, due to the photoelectric effect, can be calculated from the voltage of the photoelectric cell. Voltage = energy / charge kinetic energy = voltage x charge Ek = eV Where ‘e’ represents the charge on one electron. We call this kinetic energy so many electron volts. We know that at certain frequencies the electrons will not be liberated from the metal as there is not enough energy in the photons. We termed this f o . Anything above this frequency will be enough to dislodge the electron and increase its kinetic energy.
Here fo is where the frequency is such that the photons have enough energy for the electron to be liberated from the metal. Work has been done to achieve this. This work function is termed ‘’. V f fo Here the vertical intercept‘’, is the energy required to liberate the electron. At the fundamental frequency fo the electron is released and above this the energy is converted into the kinetic energy of the electron. Ek Gradient = h f fo Vertical intercept =
As a result of this we are thus able to calculate the kinetic energy and electron will have when hit with photons of a specific frequency. Ek= hf - Where : Ek = the maximum kinetic energy of emitted electrons hf = the energy of the photons = the work function (the minimum energy needed for electrons to escape from the surface). sodium Ek Different metals will require differing work functions to release their electrons depending on how tightly they hold them. lithium f 1 2
SOLUTION: E = hf = 6.63 x 10-34 x 7.0 x 1014 = 4.6 x 10-19 J E k = eV = 1.6 x 10-19 x 1.63 = 2.6 x 10-19 J Ek = hf - = hf – Ek = 4.641 x 10-19 – 2.608 x 10-19 = 2.0 x 10-19 J d. = hfo fo = / h = 2.033 x 10-19 / 6.63 x 10-34 = 3.0 x 1014 Hz
WAVE PARTICLE DUALITY Prior to Einstein and Planck most Physicist thought that light travelled in the form of waves. Young’s experiment helped to reaffirm this. Light thus: Travelled in waves Travelled at high speeds Had very small wavelengths. This allowed the phenomena of refraction, diffraction and interference to be explained. And then came Albert and Max. Their photoelectric effect put a question mark against all of the above. Physicists now think of photons as a combination of wave and particle properties, called a wave packet. The wave packet has a fixed amount of energy (like a particle, but has a frequency and wavelength (like a wave).