Max P lanck proposed that an atom can absorb or emit energy only in chunks known as quanta . The energy, E ,of each quantum depends on the frequency of the radiation E= hf h= 6.626 x 10 -34 J·s (Planck’s constant)
Max Planck proposed that an atom can absorb or emit energy only in chunks known as quanta.
h= 6.626 x 10-34 J·s (Planck’s constant)
E = hf= hc/λModern Physics
Einstein used the photon hypothesis to explain the Photoelectric effect ( emission of electrons from a metal surface when a light is incident on it)
(Most energetic photons)
E = hn = hc/l
h = 6.6x10-34 [J*sec](Planck’s constant)
(Least energetic photons)
There is a threshhold negative potential, the stopping potential, V0 , below which no current will flow into the circuit
Each light intensity has a maximum current flow, but the stopping potential remains the same
When V is negative, only electrons with a kinetic energy greater than |eV0| can reach the anode, the maximum kinetic energy is given by eV0.
Special relativity - the laws of physics are same in all inertial frames of reference
For more information see http://www.fourmilab.ch/cship/lorentz.html
αAlpha particles are produced from the radioactive decay of heavy elements such as uranium. They are composed of two neutrons and two protons identical to the nucleus of a helium atom. Because of their relative size and electrical charge from the two protons, alpha particles can travel only a very short distance in any material. For example a normal sheet of paper can stop alpha particles.Alpha, Beta, & gamma radiation
βBeta particles are electrons that come from transformation of a neutron in the nucleus of an atom to a proton. They can travel up to about five meters in air and one centimeter in tissue.
gGamma rays are electromagnetic radiation similar to X-rays. Unlike the latter, which are produced by machines, gamma rays are emitted from the nucleus of a radioactive atom that is in an excited state. Gamma rays travel at the speed of light and can penetrate long distances in air and tissue. Several centimeters of lead or meters of water are needed to stop typical gamma rays
- involves strong and coloumbic forces
- alpha particle and daughter nucleus have equal and opposite momentums
(i.e. daughter experiences “recoil”)
1) - decay
- converts one neutron into a proton and electron
- no change of A, but different element
- release of anti-neutrino (no charge, no mass)
2) + decay
- converts one proton into a neutron and electron
- no change of A, but different element
- release of neutrino
3) Electron capture
- conversion of strong to coulombic E
- no change of A or Z (element)
- release of photon
- usually occurs in conjunction with other decay
- heavy nuclides split into two daughters
- U most common (fission-track dating)
Fission tracks from 238U fission in old zircon
- usually reported in dpm (disintegrations per minute),
example: 14C activity = 13.56 dpm / gram C
- because activity is linerarly proportional to number N,
then A can be substituted for N in the equation
How many 14C disintegrations have occurred in a 1g wood sample formed in 1804AD?
t1/2 = 5730y so l = 0.693/5730y = 1.209e-4 y-1
N0=A0/l so N0=(13.56dpm*60m/hr*24hr/day*365days/y) /1.209e-4= 5.90e10 atoms
N(14C)=N(14C)0*e-(1.209e-4/y)*200y = 5.76e10 atoms
# decays = N0-N = 2.4e9 decays
based on experimentation not on pure reason.
Atoms are solid and indivisible.
Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic particles were a fundamental part of all atoms.Adding Electrons to the Model
Thin gold foil
path of invisible -particles
Rutherford shot alpha () particles at gold foil.
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass.Ernest Rutherford
There are 2 types of spectra: continuous spectra & line spectra. It’s when electrons fall back down that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).
Electron Spin and the Pauli Exclusion Principle
u quark: electric charge = 2/3
d quark: electric charge = -1/3
Proton = uud electric charge = 1
Neutron = udd electric charge = 0
If each proton were 10 cm across, each quark would be .1 mm in size and the whole atom would be 10 km wide.
Another subatomic particle, the neutrino, plays a crucial role in radioactive decays like n -> p+ + e- + ve
The ve(electron-neutrino) is closely related to the electron but has strikingly different properties.