IB Physics 12 Atomic Physics 2 – Photoelectric Effect Mr. Jean.

IB Physics 12

Atomic Physics 2 – Photoelectric Effect

Mr. Jean

The plan:

• Video clip of the day.

• IA - Time

• Quantum Theory

• Energy Levels

• Photo Electric Effect

Review of last day:

• States of energy

• Excited particles & Quantum Jumps

• 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.

Photoelectric Effect:

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.

• 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.

• 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.)

• 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.

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.

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.

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.

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.

Photo Electric Effect:

Photo Electric Effect:

• http://www.youtube.com/watch?v=lTxs1usXSmk

Check out this website:Photoelectric Effect

• http://phet.colorado.edu/en/simulation/photoelectric

• High resolution screen needed….

• 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.

• Kinetic energy (Ek) of electron freed by photoelectric effect is given by:

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

Example:

Planck’s Constant:

• http://www.youtube.com/watch?v=lTnpb9YPrQs

To do:

• Work on questions from Chapter 18 & 19

• Study for Quiz Chapter 16 & 17