IB Physics 12

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IB Physics 12

• Mr. Jean
• February 11th, 2014

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The plan

• Video clip of the day.
• IA - Time
• Quantum Theory
• Energy Levels
• Photo Electric Effect

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IA - Time

• Lets take the first 30 minutes to work on the
  IAs

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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

• 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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Photoelectric Effect
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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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Einsteins 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?vlTxs1usXSmk

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Check out this websitePhotoelectric Effect

• http//phet.colorado.edu/en/simulation/photoelectr
  ic
• 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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Plancks Constant

• http//www.youtube.com/watch?vlTnpb9YPrQs

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To do

• Work on questions from Chapter 18 19
• Study for Quiz Chapter 16 17