Lecture 17: Atomic Physics

Outline Einstein’s theory of relativity

Replaced newtonian mechanics when dealing with particle speeds comparable to the speed of light

Outline quantum theory

Understanding behaviour of atoms in absorption and emission of radiation

What is blackbody radiation?

Blackbody: a body that emits all radiation when hot

• all objects emit radiation whose total intensity is proportional to the 4th power of their Kelvin temp

• blackbody would also absorb all radiation that are incident on it and will reflect none, thus appears black

• example: the sun is a blackbody

What is quantum physics?

What happens to the total power of the emitted radiation as T increases

Total power of emitted radiation increases

What happens to the peak of the spectra as T increases

The peak of the spectra shifts towards shorter wavelength

Outline the physics of an ear thermometer

• doctors often use an ear thermometer to measure temperature

• this type of thermometer measures the amount of infrared radiation emitted by the ear drum

• it then converts the amount of radiation into a temperature reading

• this thermometer is very sensitive as compared to ordinary mercury thermometer, this is because the power emitted by the ear drum is proportional to the fourth power of temperature (T^4)

Describe attempts to explain blackbody radiation

• early attempts to use classical ideas to explain the shape of the curves failed

• in 1900 max planck developed a theory of blackbody radiation that is in complete agreement with the observed curve

• planck assumed that energy of atomic oscillator is discrete (quantized) and not continuous as assumed in classical theory

Describe quantized vs not quantized energy

Quantized energy: energy that can only take on discrete values

eg. staircase, piano

Non-quantized energy: energy that can take on any value within a range

eg. slide, violin

Outline planck’s quantum hypothesis

1. the energy of an atomic oscillator can have only certain discrete values En (that is, energy of oscillator is quantized)

2. oscillator emit and absorb energy when making a transition from one quantum state to another in the form of a single quantum of energy (i.e. energy of radiation is quantized)

What is the equation derived from Planck’s quantum hypothesis

what is the photoelectric effect?

electrons are ejected when light strikes a metal surface

an increase in intensity of the light beam means more photons are incident, so more electrons will be ejected but energy of each photon is not changed

outline some features of the photoelectric effect

1. no electrons are emitted if f < fc

2. as f increases, KEmax of electrons also increases

3. KEmax can be measured by reversing the terminals of the battery

4. KEmax = eVs

what did einstein propose about the photoelectric effect?

argued that when light is emitted by a molecular oscillator, the molecule’s vibrational energy of nhf must decrease by an amount of hf (or 2hf, etc.) to another integer times hf

to conserve energy light must be emitted in packets, or quanta

what is work functions (Wo)?

• minimum energy required to eject an electron from a material

• eg. Na - 2.28 eV, Ba = 2.48 eV, Cu = 4.70 eV

what is stopping voltage (Vs)?

the voltage at which an electron reaches the collector

describe einstein’s photon theory

classical theory cannot explain the photoelectric effect, but quantum theory can

outline atomic spectra: the key to the structure of the atom

• a gas heated in a discharge tube emits light only at characteristic frequency

• this is called line spectrum

• the line spectrum works as a ‘fingerprint’ for identification of the gas

outline the energy level diagram for a hydrogen atom

what is ionisation energy?

the minimum energy required to ionise the atom in the ground state (in case of a hydrogen atom it is 13.6 eV)