A summary of 3 JC Physics topics: Laser Semiconductor, Electromagnetism & Elecromagnetic Induction. I made this and I hope it will be beneficial!

1. Laser and Semiconductors
Population inversion: metastable energy state  atoms stay excited longer  more
excited atoms than those in ground state  population inversion achieved
Stimulated emission: when photons of energy equal to E2-E1 passes through medium,
either (1): photons absorbed & excited from E1 to E2
or (2): photons de-excited from E2 to E1
Since p.inversion achieved, (2) more likely to occur. Coherent photons are produced;
reflected back and forth by mirrors to cause more stimulated emissions and thus
laser.
Band theory:
At 0K, valence band is completely filled with electrons which are immobile (unable
to move in solid)
Higher level is conduction band ; empty, no electrons.
 For insulators, thermal excitation insufficient for electron in valence band to
acquire 5 eV to jump across band gap,
 For semicons, thermal excitation sufficient for electron in valence band to
acquire 1 eV to jump across band.
 For conductors, bands overlap. Conduction band partially filled with electrons.
Intrinsic Semicon:
Thermal agitation e- in valence band jump band gap  equal no. of conduction e-
and holes to act as charge carriers  form e-hole pairs  holes move in dir. of
electric field, electrons move opposite  net current in same dir. current flows
N-Type Semicon:
4 e-
from donor atom covalently bonded to Silicon atom  1 additional e- available
to represent a donor level just below conduction band  smaller band gap than
intrinsic  e- are majority carriers, electrical conduction in the presence of electric
field

2. P-Type Semicon:
3 e-
from acceptor atom covalently bonded to Silicon atom  1 electron deficiency
 hole represented by energy level EA just above valence band  after thermal
agitation, e- in valence band jump band gap to fill EA  holes in valence band act as
majority charge carriers  current flows
P-N junction/depletion region:
The mobile electrons from the n-side and the mobile holes from the p-side flow
(diffuse) across the junction and combine  e- from n-side fill the holes on p-side 
leaves the n-side with a positive charge layer (since it has lost electrons), p-side with
a negative charge layer (since it has gained electrons) The positive and negative
charge layers set up an electric field (or depletion region) in the junction
Electromagnetism
 Force acting on charge = mg = BILsin𝜃
= Bqv
 Bqv = mv2
/r [centripetal force]
 Felectricfield = qE
Inserting ferrous core into solenoid
Current pass thru solenoid  ext. magnetic field set up  magnetic domains of core aligned
in same direction  produce a magnetic field that strengthens external magnetic field as ext.
magnetic field is vector sum of magnetic field produced by current in solenoid and by
ferrous core

3. Velocity selector
FB = FE  Bqv = qE  V= E/B
Mag.field and e.field arranged perpendicularly to each other. Produce FB and FE respectively
in opp. Direction. When FE =FB, FR = 0, electron pass thru undeflected. (only for cases where
charged particles have specific speed, v)
Mass spectrometer
Inside dome-like structure:
Bqv = mv2
/r
In v.selector:
v= E/B
Electromagnetic Induction
Flux ∅ = BAcos 𝜃 ; 𝐹𝑙𝑢𝑥 𝑙𝑖𝑛𝑘𝑎𝑔𝑒 = 𝑁𝛷 = 𝑁𝐵𝐴cos𝜃
Note: For ‘BAcos 𝜃’, the 𝜃 is between B and the normal to the coil or surface. If you use Basin𝜃(some
textbooks use this), then the 𝜃 is between B and plane of the area of the surface.
Uniform e field downwards, uniform
magnetic field into paper
v.selector
r

4. Faraday’s Law:
𝜀 = 𝑑𝑁∅/𝑑𝑡
Lenz’ Law:
Definition: direction of induced emf that drives induced current to flow in a direction to
produce an induced magnetic field that opposes the change in magnetic flux linkage
Note: This opposing magnetic effect can be used in damping (refer to Oscillations)
From lenz law eqn,
E = | -d∅/𝑑𝑡 |
𝜀 = | − 𝐵𝑙𝑣 |
Magnitude of emf induced across moving conductor -> E = Blv