Modern Physics NCEA AS 3.5 Text Chapters:20,22

The Photoelectric Effect The photoelectric effect occurs when shining light (usually UV) onto a piece of metal causes electrons to be given off. The photoelectric effect occurs when shining light (usually UV) onto a piece of metal causes electrons to be given off. This effect can be used in a photoelectric cell to produce small electric currents. This effect can be used in a photoelectric cell to produce small electric currents. Photoelectric cells are used in Photoelectric cells are used in Lightmeters Lightmeters Burglar alarms Burglar alarms TV cameras etc TV cameras etc

Photoelectric Cells Thin glass tube (evacuated) Emitter – curved metal plate Collector A LIGHT

Photoelectric Experiments When the photoelectric effect was studied in detail, the experimental results were very different to what was expected. A new theory about the nature of light was needed to explain what happened. When the photoelectric effect was studied in detail, the experimental results were very different to what was expected. A new theory about the nature of light was needed to explain what happened. Scientists at the time considered light to behave like a wave…… Scientists at the time considered light to behave like a wave……

Photoelectric Experiments What was expected: What was expected: Brighter light would cause electrons with more kinetic energy to be emitted Brighter light would cause electrons with more kinetic energy to be emitted What actually happened: What actually happened: Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had

Photoelectric Experiments What was expected: What was expected: If very dim light was used, it would take some time before any electrons had absorbed enough energy to escape from the metal If very dim light was used, it would take some time before any electrons had absorbed enough energy to escape from the metal What actually happened: What actually happened: With UV light, even the faintest light caused some electrons to be emitted instantly With UV light, even the faintest light caused some electrons to be emitted instantly

Photoelectric Experiments What was expected: What was expected: The frequency (or colour) of the light used would not affect the energy of the emitted electrons. The frequency (or colour) of the light used would not affect the energy of the emitted electrons. What actually happened: What actually happened: The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted. The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted.

Photoelectric Experiments Einstein explained these results, using an idea suggested by Max Planck, that said electromagnetic radiation comes in fixed “packets” or quanta of energy called photons Einstein explained these results, using an idea suggested by Max Planck, that said electromagnetic radiation comes in fixed “packets” or quanta of energy called photons The amount of energy each photon has depends on the frequency of the radiation. The amount of energy each photon has depends on the frequency of the radiation.

Photoelectric Experiments Each photon has a fixed amount of energy given by: Each photon has a fixed amount of energy given by: h=Planck’s Constant = 6.63x Js h=Planck’s Constant = 6.63x Js This suggested that light behaved like a moving particle, rather than a wave This suggested that light behaved like a moving particle, rather than a wave

Photoelectric Experiments The power supply provides an opposing voltage to the p.e. cell. The power supply provides an opposing voltage to the p.e. cell. The variable voltage is adjusted until the current in the circuit is zero The variable voltage is adjusted until the current in the circuit is zero LIGHT V

Photoelectric Experiments When the current was zero, the supply voltage was equal to the cut-off voltage of the cell When the current was zero, the supply voltage was equal to the cut-off voltage of the cell Different frequencies of light were tried, and the cut-off voltages measured: Different frequencies of light were tried, and the cut-off voltages measured:

Photoelectric Experiments These were the results: These were the results: Below a certain threshold frequency f 0 no electrons were emitted Below a certain threshold frequency f 0 no electrons were emitted f V f0f0

Photoelectric Experiments The maximum E k of the electrons can be found from the voltage: The maximum E k of the electrons can be found from the voltage: Where e= electron charge = -1.6x Where e= electron charge = -1.6x10 -19

Photoelectric Experiments Another way of looking at that last graph: Another way of looking at that last graph: f EkEk f0f0 Intercept= Work function Φ (or B) Gradient = h

Photoelectric Experiments By equating to y=mx+c: By equating to y=mx+c: E k = max kinetic energy of emitted electrons E k = max kinetic energy of emitted electrons hf = energy of incoming photons hf = energy of incoming photons Φ = The work function of the metal – the minimum amount of energy required for the electron to escape from the metal surface. Φ = The work function of the metal – the minimum amount of energy required for the electron to escape from the metal surface.

Photoelectric Experiments Different metals have different f 0 ’s and work functions depending on how tightly they hold onto their electrons Different metals have different f 0 ’s and work functions depending on how tightly they hold onto their electrons f EkEk f0f0 f0f0 Cu Pb

The Conclusion So the photoelectric effect could be explained by thinking of light as a stream of incoming particles that collided with electrons in the metal. If the photon had enough energy, it could knock the electron free of the metal and send it across the cell to the collector. So the photoelectric effect could be explained by thinking of light as a stream of incoming particles that collided with electrons in the metal. If the photon had enough energy, it could knock the electron free of the metal and send it across the cell to the collector. If photon was too small, it couldn’t hit electrons hard enough (overcome work function) so no electrons emitted. If photon was too small, it couldn’t hit electrons hard enough (overcome work function) so no electrons emitted.

Atomic Spectra 2 types 2 types Emission – certain frequencies of light given off by low pressure gases excited by heat or electricity Emission – certain frequencies of light given off by low pressure gases excited by heat or electricity Absorption – certain frequencies absorbed from a continuous spectrum by low pressure gases Absorption – certain frequencies absorbed from a continuous spectrum by low pressure gases Spectra are unique to each element and can be used to identify unknown elements Spectra are unique to each element and can be used to identify unknown elements

The Hydrogen Spectrum Balmer studied the emission spectrum lines of Hydrogen, as it is the simplest atom. Balmer studied the emission spectrum lines of Hydrogen, as it is the simplest atom. He was limited by the fact that he could only observe visible frequencies – we now know there are UV and IR spectral lines He was limited by the fact that he could only observe visible frequencies – we now know there are UV and IR spectral lines The spectral lines are caused by the movement of electrons between different energy shells in the atom The spectral lines are caused by the movement of electrons between different energy shells in the atom

The Hydrogen Spectrum In Balmer’s case he was looking at spectral lines caused by electrons jumping from higher energy level (shells) down into the 2 nd shell. In Balmer’s case he was looking at spectral lines caused by electrons jumping from higher energy level (shells) down into the 2 nd shell. They would release their extra energy as a photon of light. They would release their extra energy as a photon of light. Other Scientists later found series of spectral lines corresponding to jumps into the 1 st, 3 rd, 4 th, 5 th etc Other Scientists later found series of spectral lines corresponding to jumps into the 1 st, 3 rd, 4 th, 5 th etc

The Hydrogen Spectrum ∞ Lyman Series S=1 (UV) Balmer Series S=2 (visible) Paschen Series S=3 (IR) Bracket Series S=4 (IR) Pfund Series S=5 (IR) Nucleus Shell no. / Energy level

The Hydrogen Spectrum A formula was worked out to calculate the wavelengths of these lines: A formula was worked out to calculate the wavelengths of these lines: R=Rydberg’s Constant=1.097x10 -7 R=Rydberg’s Constant=1.097x10 -7 S=Series no. (the shell jumped into) S=Series no. (the shell jumped into) L=Line no. (the shell jumped from) L=Line no. (the shell jumped from)

The Hydrogen Spectrum The formula worked perfectly for Hydrogen, but started to get more inaccurate the bigger and more complex the atom got The formula worked perfectly for Hydrogen, but started to get more inaccurate the bigger and more complex the atom got Absorption spectra are produced by electrons absorbing photons of energy which allows them to jump up energy levels Absorption spectra are produced by electrons absorbing photons of energy which allows them to jump up energy levels

Bohr’s Model of the Atom Rutherford’s student Niels Bohr proposed that: Rutherford’s student Niels Bohr proposed that: Electrons in H could only exist in stable orbits with certain fixed amounts of energy, called energy levels Electrons in H could only exist in stable orbits with certain fixed amounts of energy, called energy levels An electron moves from one energy level to another by either emitting or absorbing a photon of light equal in energy to the difference between the two energy levels An electron moves from one energy level to another by either emitting or absorbing a photon of light equal in energy to the difference between the two energy levels

Bohr’s Model of the Atom The energy levels in the Hydrogen atom are given by : The energy levels in the Hydrogen atom are given by : h=Plancks constant = 6.63x h=Plancks constant = 6.63x c=speed of light = 3x10 8 c=speed of light = 3x10 8 R=Rydbergs constant = 1.097x10 7 R=Rydbergs constant = 1.097x10 7 n=energy level = 1,2,3,4…… (quantum number) n=energy level = 1,2,3,4…… (quantum number)

Bohr’s Model of the Atom All energy values are negative – this represents the fact that it is an energy which binds the electron to the nucleus All energy values are negative – this represents the fact that it is an energy which binds the electron to the nucleus The lowest energy state n=1 is called the ground state The lowest energy state n=1 is called the ground state As n  ∞, E  0. This represents the energy required to ionise the atom by removing the electron completely. As n  ∞, E  0. This represents the energy required to ionise the atom by removing the electron completely.

Bohr’s Model of the Atom -2 Energy (x J) 0 n=1 n=2 n=3 n=4 n=∞

Electron Volts Sometimes an alternative unit for energy is used called the electron volt Sometimes an alternative unit for energy is used called the electron volt 1eV is the energy gained by 1 electron when accelerated by a potential of 1 Volt 1eV is the energy gained by 1 electron when accelerated by a potential of 1 Volt 1eV=1.6x J 1eV=1.6x J Using this unit: Using this unit:

Nuclear Reactions 3 types: Radioactive Decay – the spontaneous emission of particles from the nucleus of an atom Nuclear Fission – splitting one large nuclei into two smaller ones Nuclear Fusion – combining two small nuclei into one large one.

Radioactivity 3 types: 3 types: Alpha  Alpha  Beta  Beta  Gamma  Gamma  Named in order of their discovery. Named in order of their discovery. Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them. Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them.

The Nucleus In small atoms, the number of protons and neutrons are usually the same (roughly) In small atoms, the number of protons and neutrons are usually the same (roughly) In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation

Alpha Particles Helium nucleus Helium nucleus Charge of +2 Charge of +2 Mass of 4 (a.m.u) Mass of 4 (a.m.u) Travel slowly ie. 10% of light speed Travel slowly ie. 10% of light speed Don’t travel very far ie. A few cms in air Don’t travel very far ie. A few cms in air Low penetration power – can be stopped by a piece of paper Low penetration power – can be stopped by a piece of paper Very good ionising power – because they’re big and slow. Very good ionising power – because they’re big and slow.

Beta Particles An electron from the nucleus An electron from the nucleus Charge of -1 Charge of -1 Same mass as an electron (effectively 0) Same mass as an electron (effectively 0) Travel relatively fast – up to 95% of light speed Travel relatively fast – up to 95% of light speed Travel about 30 cms in air Travel about 30 cms in air Average penetration power – can be stopped by a few mm of Aluminium Average penetration power – can be stopped by a few mm of Aluminium Average ionising power Average ionising power

Gamma Radiation A wave of electromagnetic radiation (energy) A wave of electromagnetic radiation (energy) No charge No charge No mass No mass Travels at light speed Travels at light speed Travels several metres in air Travels several metres in air High penetration power – Several cms of lead needed to stop it High penetration power – Several cms of lead needed to stop it Low ionising power – because no mass Low ionising power – because no mass

Radiation One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field: One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field:   

The Nucleus Writing nuclei Writing nuclei X = element symbol A = mass number or nucleon number (the number of p+n) Z = atomic number (the number of protons)

Isotopes Atoms with the same atomic number but different mass numbers Atoms with the same atomic number but different mass numbers Eg: Eg:

Alpha Decay Example: Radium 226 decays to Radon 222 by alpha decay: Example: Radium 226 decays to Radon 222 by alpha decay: Note: Both mass and charge must be conserved Note: Both mass and charge must be conserved (ie 226=222+4, 88=86+2

Beta Decay Cobalt 60 decays by beta decay to Nickel 60 Cobalt 60 decays by beta decay to Nickel 60 Again, mass and charge are conserved Again, mass and charge are conserved NB. the  or  symbols can be used instead of He or e NB. the  or  symbols can be used instead of He or e

Half-life The time it take for the decay rate to have halved, or…. The time it take for the decay rate to have halved, or…. The time taken for half of the original atoms to have decayed The time taken for half of the original atoms to have decayed Usually shown on a graph Usually shown on a graph

Half-life

Detecting Radioactivity Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas 400V DC Supply Counter or speaker - Anode: central wire +Cathode: metal cylinder Geiger-Muller tube filled with low pressure Ar End: thin mica window

Uses of Radioactivity Radiation therapy to treat cancer Radiation therapy to treat cancer Sterilisation Sterilisation Carbon dating Carbon dating Nuclear medicine eg tracers Nuclear medicine eg tracers Smoke detectors Smoke detectors

Binding Energy If we put together a nucleus from individual protons and neutrons, we would find that the mass of the resulting nucleus is less than the total mass of the individual nucleons. If we put together a nucleus from individual protons and neutrons, we would find that the mass of the resulting nucleus is less than the total mass of the individual nucleons. This reduction in mass is called a mass deficit This reduction in mass is called a mass deficit

Binding Energy In order to break up a nucleus into separate nucleons the mass deficit must be restored by adding extra energy. In order to break up a nucleus into separate nucleons the mass deficit must be restored by adding extra energy. This energy changes into mass according to Einstein’s famous equation: This energy changes into mass according to Einstein’s famous equation:

Binding Energy This energy shortage has the effect of holding the nucleus together so it is called the binding energy. This energy shortage has the effect of holding the nucleus together so it is called the binding energy. Binding energy represents the amount of “glue” holding the nucleus together. Binding energy represents the amount of “glue” holding the nucleus together. The more binding energy per nucleon, the more stable an atom will be The more binding energy per nucleon, the more stable an atom will be

Binding Energy Mass number B.E per nucleon (MeV) Fe 238 U 4 He 7 Li Fusion Fission

Nuclear Fission Breaking large unstable nuclei into smaller ones. Breaking large unstable nuclei into smaller ones. Lots of possible combinations of fragments from one initial nucleus Lots of possible combinations of fragments from one initial nucleus Eg: Eg:

Nuclear Fission When a large nucleus is split into smaller fragments, the fragments have less mass per nucleon When a large nucleus is split into smaller fragments, the fragments have less mass per nucleon The lost mass is released as energy in the form of kinetic energy of neutrons and gamma rays The lost mass is released as energy in the form of kinetic energy of neutrons and gamma rays

Nuclear Fission Only one neutron is needed to start the reaction, but several are produced Only one neutron is needed to start the reaction, but several are produced This starts a “chain reaction” This starts a “chain reaction” n U Ba Kr n n n U Ba Kr n n n U Ba Kr n n n U Ba Kr n n n

Nuclear Fission If the chain reaction is controlled it can be used in a nuclear reactor If the chain reaction is controlled it can be used in a nuclear reactor If it is uncontrolled it explodes as a nuclear or atomic bomb If it is uncontrolled it explodes as a nuclear or atomic bomb

Nuclear Fusion The joining of two small nuclei to form one larger one The joining of two small nuclei to form one larger one This is the process that powers the sun This is the process that powers the sun Eg: Eg:

Nuclear Fusion Fusing two light atoms together results in a nucleus with less mass per nucleon Fusing two light atoms together results in a nucleus with less mass per nucleon The lost mass results in a release of energy The lost mass results in a release of energy

Nuclear Fusion Fusion requires extreme temperature (eg millions of degrees) to occur, and has not practically and economically been used in power generation (yet….) Fusion requires extreme temperature (eg millions of degrees) to occur, and has not practically and economically been used in power generation (yet….) Hydrogen bombs have been successfully made, but require a fission reaction to provide the necessary temp. Hydrogen bombs have been successfully made, but require a fission reaction to provide the necessary temp.