ib physics 12 atomic physics 2 – photoelectric effect mr. jean
TRANSCRIPT
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IB Physics 12
Atomic Physics 2 – Photoelectric Effect
Mr. Jean
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The plan:
• Video clip of the day.
• IA - Time
• Quantum Theory
• Energy Levels
• Photo Electric Effect
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Review of last day:
• States of energy
• Excited particles & Quantum Jumps
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• What Planck was essentially saying was that energy emitted or absorbed by atoms is QUANTIZED or exists in packages (bundles) of energy in specific sizes.
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Photoelectric Effect:
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Photoelectric Effect:
• The photoelectric effect confirms the theory of the quantization of energy. Hertz was attempting to verify Maxwell's theories. (≈ 1887) He assembled a circuit that generated an oscillating current that caused sparks to jump back and forth across a gap.
• He showed that the sparks were generating electromagnetic waves by seeing sparks forming in the gap of the receiver.
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• With this, he verified Maxwell's theories that when the metal electrodes were exposed to ultraviolet light, the sparks were enhanced.
• 10 years later, the electron was discovered. Physicists suggested that the UV light had ejected electrons from the electrodes.
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• The ejection of electrons by UV light is the photoelectric effect.
• Lenard was the first to perform detailed experiments on the photoelectric effect. (1902)– He designed an apparatus where electrodes
were sealed in a vacuum tube with a quartz window. (UV light does not penetrate glass.)
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• Lenard wanted to determine the kinetic energy of the photoelectrons. When the emitter was - and the collector was +, he found that when the intensity increased, the current increased.
• When he switched the polarity of the emitter and the collector, he found that when he increased the potential difference (voltage), the current would eventually stop.
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Results from Testing:
• i) As the intensity of the light increases, energy absorbed by the surface increases and the number of photoelectrons increases. (classical physics) – more intense light more energy absorbed
by metal more electrons ejected from metal.
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More Results:
• ii) Classical physics also indicates that as the kinetic energy increases, the intensity increases. – However; Lenard found that the kinetic energy
is not affected by the intensity. • Kinetic energy is determined ONLY by the
frequency of the light.
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Important realization:
• KE dependent on frequency not intensity.
• Einstein proposed that 'photoelectrons' are only ejected from the surface of the metal terminal if the frequency of the EMR was above a certain minimum level called the threshold frequency, fo.
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Einstein’s results:
• Brightness or intensity of EMR had nothing to do with it, as EMR wave theory had predicted.
• In 1905, Einstein explained the photoelectric effect by saying EMR consisted of discrete bundles of energy which he called photons and that atoms would only absorb energy in these bundles.
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Photo Electric Effect:
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Photo Electric Effect:
• http://www.youtube.com/watch?v=lTxs1usXSmk
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Check out this website:Photoelectric Effect
• http://phet.colorado.edu/en/simulation/photoelectric
• High resolution screen needed….
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• Photons have an energy, KE. They either give up all their energy or none of it.
• The energy possessed by these photons depended on the frequency of the EMR. – E = hf
• Einstein was the first one to indicate that light and other EMR acted like particles.
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• Kinetic energy (Ek) of electron freed by photoelectric effect is given by:
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Electron Volts:
• Expressing extremely small amounts of energy at the atomic level is usually done in units called electron volts (eV).
• 1 eV = 1.6x10-19 J
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Example:
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Planck’s Constant:
• http://www.youtube.com/watch?v=lTnpb9YPrQs
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To do:
• Work on questions from Chapter 18 & 19
• Study for Quiz Chapter 16 & 17