NCEA Level 3 Physics

 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): Page Page Page Page

By the end of 19 th 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? 1887Michelson & Morley found the speed of light is always the same. 1896Radioactivity was discovered by Henri Becquerel. 1897JJ Thompson ‘discovered’ the electron. 1898Two new radioactive elements were isolated by the Curies – radium & polonium. 1900Rutherford identified alpha & beta radiation & Villard discovered gamma rays. 1905Albert Einstein published the special theory of relativity. To explain these discoveries two great theories of modern physics were born.

Relativity– which deals with objects moving at very high speeds.Quantum theory– which deals with very small particles Newtons laws help to explain objects travelling at ‘slow’ speeds, but they do not work for particles travelling at extremely high speeds. Modern physics has helped to explain such phenomena as:  The structure of the atom  Nuclear reactions  The origins of the universe

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 : 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.

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The photocurrent is proportional to the light intensity. The brighter the light, the more electrons emitted. The electrons were ejected immediately. The weakest measurable UV light caused an immediate photocurrent. This is hard to explain if light is a wave. Surely the atom would need a much longer time to absorb the energy when the light is weak? The photocurrent also depends on the frequency of the light (i.e. its colour). Really bright red light would not eject electrons at all. But even a weak UV radiation causes some photocurrent to flow. Why? In fact, below a certain frequency, no electrons are emitted at all. Each metal has a certain threshold frequency. When the light frequency is below the threshold, then no current can be produced no matter how bright the light. Why? Four things were observed:

This revolutionised Physics thinking in that two things could not be explained if you followed the earlier theory that light acted as a wave, from Young’s experiment. The two things were: 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. 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.

 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.

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 REDYELLOWGREENBLUE WHITE Each wavelength is quantised E red E green E blue

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 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.

Calculate the energy of a photon of blue light of wavelength 4.0 x m The speed of light c = 3.0 x 10 8 ms -1. f = c / = 3.0 x 10 8 / 4.0 x f = 7.5 x Hz E = hf = 6.63 x x 7.5 x = 5.0 x J f = c / = 3.0 x 10 8 / 4.0 x f = 7.5 x Hz E = hf = 6.63 x x 7.5 x = 5.0 x J

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 E k = 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.

fofo f V Here f o 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 ‘  ’. fofo f EkEk Gradient = h Vertical intercept =  Here the vertical intercept‘  ’, is the energy required to liberate the electron. At the fundamental frequency f o the electron is released and above this the energy is converted into the kinetic energy of the electron.

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. E k = hf -  Where : E k = 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). 11 22 sodium lithium EkEk f Different metals will require differing work functions to release their electrons depending on how tightly they hold them.

In an experiment, blue light of frequency 7.0 x Hz shines on a photoelectric cell and produces a cut off voltage of 1.63V. What is the energy of photons of blue light? What is the maximum kinetic energy of the ejected electrons? What is the work function of the metal? What is the threshold frequency at which electrons are first emitted? SOLUTION: a.E = hf = 6.63 x x 7.0 x = 4.6 x J b.E k = eV = 1.6 x x 1.63 = 2.6 x J c.E k = hf -   = hf – E k = x – x = 2.0 x J d.  = hf o f o =  / h = x / 6.63 x = 3.0 x Hz

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).