Review Physic knowledge points
（key and difficult points）
Vector and scalar
Vector: displacement, velocity, momentum, force and acceleration. (Both have
magnitude and direction)
Scalar: distance, speed, time and temperature. (Only have magnitude)
Force and motion（key and hard part）
Momentum: P=mv
Impulse: I=Ft=m(v-u) v: final velocity u: initial velocity
Conservation of momentum: total momentum before collision = total momentum after
Hooke’s law: F=kx (k is spring constant, x is extension, not length of spring !!!)
Boyle’s law: P1V1=P2V2 (take place when mass and temperature have no change)
Sky diver
Step 1: just jumps out of the plane, no air resistance, weight causes accelerates.
Step 2: as velocity increases, air resistance increases, but resistance <weight, so velocity
still increase.
Step 3: air resistance is equal to weight. resultant force is zero, falling at constant
velocity.
Step 4: opens the parachute; air resistance increases dramatically. Now the resultant
force is upward, (resistance > weight) so starts to decelerate.
Step 5: as velocity decreases, air resistance decreases, acceleration decrease.
Step 6: air resistance balances the weight again. smaller constant velocity.
Turning effect of forces
equilibrium
Resultant force is zero.
Resultant moment is zero.
Centre of mass/gravity
Mass concentrate on a point
Gravity force acts on a point
Brownian motion
Gas molecules move rapidly and randomly.
This is due to collisions with other lager molecules.
Momentum changed, do random motion.
Increase evaporation
-
Increase temperature.
-
Increase surface area, more particles close to the surface, easier to escape.
-
Draught the surface, escaped particles are blown away so that cannot fall back.
pressure
How does pressure exist
Particles are moving freely and bouncing off the wall of the container (each other) and
momentum is changed, exerting a force on the wall of the container (each other).
Affects pressure
-
Mass, in a fixed volume, more particles and dense, more collisions, bigger force.
-
Temperature
-
Volume, as V increases, distance between particles is bigger, less collisions. In
opposite, as V decreases, pressure increases.
Thermal energy transfer
Conduction
Lattice: as heated, particles vibrate, jostle their neighbors and pass the heat.
Metal: electrons and ions vibrate more, free electrons move freely, collide the ions and
transfer the heat.
Radiation
-
All objects emit radiation.
-
Black/dark matt surfaces are best absorber and emitters.
-
White/light and shiny surfaces are poorest absorber/ best reflectors and emitters.
Sound (longitudinal wave)
Speed
Material
Speed m/s
air
330
water
1500
glass
5000
Sound travel fastest in solid, slowest in air.
Light (transverse wave)
Refractive index
sin(lager angle)
n=sin (𝑠𝑚𝑎𝑙𝑙𝑒𝑟 𝑎𝑛𝑔𝑙𝑒)
𝑣1
n=𝑣2 (light speed: 3*108)
1
n=sin 𝑐 (c=critical angle)
incidence angle (入射角) inflection angle(反射角)
incidence ray (入射光线) inflection ray (反射光线)
normal line (法线)
waves
Formula
Wave speed =
𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
𝑝𝑒𝑟𝑖𝑜𝑑
= wavelength × frequency
Topic 15 spectra (黄本)
Magnetism
Methods of magnetization
Magnetized:
-
stroking them with a magnet,
-
hammering them in a magnetic field
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putting them inside a coil with a direct current through it.
Demagnetized:
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hammering them,
-
heating them
-
putting them inside a coil with an alternating current through it.
Electricity
Electron Charge (C) = -1.6×10-19C
Proton charge = 1.6×10-19C
Electricity quantity
I = Q/t
e.m.f = E/Q (emf=voltage)
p.d = E/Q
P=UI=W/t=U2/R=I2R
W=Pt=UIt=U2/R×t=I2Rt
U=IR R=U/I R=U/I
𝑅1×𝑅2×𝑅3….
R=𝑅1+𝑅2+𝑅3…. (in parallel current)
Topic 19 electrical circuits
Electromagnetic forces
“左力右电“
Identify the magnetic field in a wire.
-
As the coil rotates, the magnetic field through the coil changes, which induces an
e.m.f. in the coil.
-
The magnitude of the e.m.f. is maximum when the coil is horizontal as the field lines
are cut the fastest, and zero when vertical as no field lines are being cut.
-
The e.m.f. can be increased by increasing the number of turns on the coil, increasing
the area of the coil, using a stronger magnet or increasing the speed of rotation.
-
Slip ring changes the direction of current, but magnetic field does not change, so
the cause the situation as the diagram showed above.
How a transformer work
i) An a.c. in the primary coil produces a changing magnetic field in the core.
¡i) This changing magnetic field induces an a.c. in the secondary coil.
iii) d.c input doesn't work.
Calculation
𝑉𝑝 𝑁𝑝
=
𝑉𝑠 𝑁𝑠
When efficiency is 100% I1V1=I2V2
Energy lost on cables = I ×R（increase voltage to reduce the energy lose）
2
Atomic physics
name
mass
speed
charge
alpha
4
faster
+2
beta
0
slowest
-1
gamma
0
fastest
0
Alpha can be reflected by thin air or 5cm skin
Beta can be reflected by a sheet of aluminum of 2mm or 100cm in air
Gamma can be reflected by the lead of 2 cm or 2m concrete
Application of radiations
i) Alpha--Smoke detectors
ii) Medical treatment of beta radiation
ill) Beta radiation--Thickness Control Mill
iv) Medical Tracers (gamma).
) Gamma radiation---detecting cracks
Using cell damage:
i) gamma ray-- Radiotherapy (gamma knife)
il) gamma ray--food irradiation
iii gamma ray- - sterilization (medical equipment)
Identify the particles movement
1.手心指向是电流方向/正电荷移动方向（beta），电子（负电）和电流的移动方向相反。
Beta 带负电，alpha 带正电。
故手心指向为 alpha 的移动方向，相反方向为 beta 移动方向。
2.四指指向为磁场方向
3.大拇指指向为受力方向