Blackbody

Power emitted by an object is proportional to the surface area and the fourth power of the temperature.
As substances are heated they start to glow. The hotter the objects are, the more they glow. All objects emit radiation and they will cool to their surroundings, unless they gain energy from other energy sources, leading to thermal equilibrium. It is a perfect emitter and absorber of radiation. No reflection from blackbody.

Blackbody Spectrum from PHET simulation

Entrophy: Entropy always increases as heat transfers from hotter to colder object. It appreas how heat dissipates/spreads out. It is irreversaible process in any isolated system. Entropy of the entire universe has to increase towards the maxium. Entropy is a measure of disorder of things - Boltzman.

S = k log W




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Photoelectric effect
Effect
1. Below certain threshold frequency fo, no electrons are emitted.
2. Increase in intensity of light has no effect on Ek of electrons.
3. Above fo, Ek of electrons is proportional to the frequency.
Reasons
1. Light is quantized ( photons have E = hf )
2. Electrons trapped in metal (Zinc) need a certain energy in order to be ejected.
3. phi Work function ( measure in electron volt ) = energy needed to eject electrons.

You can make a circuit to stop photoelectrons.
Stopping voltage Vs is the terminal p.d. that is needed in order to stop photoelectrons.

hf =j phif + Emax ,
hf = hfo + eV

h: constant
f: frequency of incoming light (Hz)
fo: threshold frequency ( Hz )
j?: Work funtion (eV)
Emax: maxium kinetic energy of electrons (eV)
e: charge (C)
V: stopping voltage (V)


Heisenberg's uncertainty principle
xp>=h/4p

x: uncertainty on position
p: uncertainty on momentum(speed)

Et>= h/4p

Nuclear physics ( decay )
N = Noe -l t
No : Initial mass
N : Final mass
T: Time it takes to have half the original mass or number of particles (N)
l (decay constant) : probability of decay in a given time ( 1/s or 1/year)
t: time elapsed
A: activity (Bq, decays /s)

T = ln2/l
Derivation:

Define t = T1/2,
N = No/2 and N = Noe -l t
\ N = No/2 = Noe -l T1/2
Take log on each side of equation
ln(1/2) = lne -l T1/2
ln(1/2) = -l T1/2= ln 1 - ln 2 = 0 - ln 2
T1/2 = ln 2/

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