A2物理paper4考点笔记，定义题和解答题复习看它就够了

A2可谓是入读大学前最忙碌的一年，所有的申请工作都需要在这之前完成，在准备PS、推荐信、各种申请资料之余还要挤出时间来学习。因此A2的学员一定要学会高效复习。本文就给大家分享一下A2物理paper4的定义题和解答题的考点笔记与复习方法，帮你稳稳过con。

A2物理考试介绍

ALEVEL物理分为AS和A2（International A2）两个阶段

AS的时候物理需要考Paper1、2、3。

A2物理需要考Paper 4、5。Paper 1是选择题，Paper2、4是大题，Paper 3、5是实验题。相对来说，paper2比较困难，因为题目的灵活性比较强，而且需要背的定义概念比较多。

整套卷子用时一个小时，满分是60分。每一道大题的第一问基本都是定义题，所以一定要背熟每一个定义。定义题之后一般是计算题或者是解释题。

A2物理paper4复习方法

Step 1: 浏览一遍下文笔记的内容，学校还未学到的章节可直接跳过。注意笔记中章-小节的结构。当遇到 ①“啊，原来这问应该这样写！” ②“这个得分点看完没懂 T_T” ③“咦？这里导师写错了吧？”的时刻，在笔记对应地方做出标记和comment。

Step 2: 建立一份 word 文档，或 ipad 笔记，或活页笔记本，大家可按自己习惯任选方式。这个笔记方式最好能够 ①方便地随处添加/删除内容 ②方便地调整章节分层结构。

Step 3: 拿出 paper 4 以前所有做过的练习题，单元测验，期中期末考卷。把每一道题再看一遍，特别是以前做错的题，再做一遍。遇到定义和解释题，用自己喜欢的方式，用自己组织的语言，添加到自己的笔记中。这个过程可能需要2-4小时甚至更长 (pp 是用了两个下午写完的 T_T)。遇到不确定的问题，可以参考①官方标答 ②学校导师的讲义 ③pp 的笔记。

Step 4: 以后在做完每一份题目后，一定要抽出 10 分钟左右的时间，把定义题解释题总结到自己的笔记中，让自己的笔记不断的完善。

这份搭配食用方法可能会花费一些时间和努力，但它在复习复习时会是一个非常好用，有效率的小帮手。希望大家都能够掌握知识的结构，默写题的要点。在考试中助力拿分。

A2物理paper4复习笔记

Section 1. Definitions

1 Gravitational field

Newton's law of gravitation

gravitational attraction force between two point or spherical massesis proportional to product of massesand inversely proportional to square of separation

Gravitational field strength

gravitational force per unit mass

Gravitational potential

work done per unit mass while moving a test mass from inifinity to a specific point without changing its velocity

What is meant by a field of force?

a region in space where an opbject experience a force

2 Circular motion

Angular frequency

rate of change of angular displacement

in SHM, angular frequency is 2pi*frequency

Radian

consider an anlge suited in a circle, radian is the ratio between the arc length subtended by the angle to the radius of circle

3 Thermodynamics

Specific latent heat

energy per unit mass required to change the stateof an object at constant temperature

Specific heat capacity

energy per unit mass required to increase the temperature of an object by 1 degree without changing its state

Internal energy

sum of potential and kinetic energydue to the random motion of molecules

Avogadro constant NA

number carbon-12 atoms in 12 g of carbon-12

Mole

is unit of amount of substance that contains the number of 6.02*10^23

Delta_U: increase in internal energy

q: heat transferred to system

w: work done on system

4 Electromagnetism

Magnetic field

region in space where a magnet, magnetic material and current-carrying conductorsexperience forces

Coulomb's law

electric force between two point or spherical charges is proportional to product of chargesand inversely proportional to square of separation

Capacitance

charge per unit potential difference across a capacitor

Tesla

unit of magnetic flux density that is normal to a long wire carrying a current of 1A that creates a force of 1N/m

Faraday's law

induced emf is proportional to the rate of change of magnetic flux linkage

5 Quantum physics

Photoelectric effect

electromagnetic radiation with sufficient frequency incident on the surface of metal

will cause emission of electrons from the metal surface

Threshold frequency

minimum frequency above which incident light will be able to excite photoelectrons emission from metal surface

Photon

is a packet of energy of electromagnetic radiation

de Broglie wavelength

wavelength associated with a moving particle realted to its momentum

6 Nuclear physics

Radioactive

unstable nucleuswill emit ionising radiation randomly and spontaneously

Decay constant

probability of a nucleus to decay per unit time

Half-life

time for number of nuclei or activity to become half

Section 2. Circular motion and gravitational field

Why the gravitational field strength may be considered to be constant near Earth's surface?

g=GM/R^2 --> g=GM/(R+h)^2

so in change in height is much less than radius of Earth then g remains constant

Why is gravitational potentiala negative quantity?

gravitational potential is defined to be zero at inifinity

in moving a mass from infinity to a point, work done is always negative because gravitational force is always attractive, by definition of gravitational potential, negative work means potential is negative

Feature of geostationary orbits

period of 24 hours

orbits right above the equator

at fixed orbital distance

orbiting earth from west to east

How satellites stay in circular orbits around planets?

planet exerts gravitational attraction force on satellite

this gravitational attraction has constant magnitude and is always perpendicualr to satellite velocity, so it provides the centripetal force

Section 3. Simple harmonic motion

Explain how an expression can show that object will undergo SHM

magnitude of resultant force (or acceleration) on the object is directly proportional to displacement with constant proportionality

direction of resultant force (or acceleration) is always opposite to direction of displacement

Forced oscillations

Difference between free and forced oscillations

Free oscillation

oscillations without any external forcesapplied

so no loss of energy or no damping

so no driven forceso amplitude does not increase due to resonance

Forced oscillation

oscillations with continuous energy input provided by external periodic driving force

Resonance

when driving force has frequency matches the natural frequency of oscillator

resonance happens and oscillator absorb external energy very fast and grow to maximum amplitude

Damping

when external resistance forces act on oscillator

results in continuous decrease in energy and amplitude of oscillation

Section 4. Thermodynamics

1 Temperature and internal energy changes

What may be deduced from the difference in temperatures of two objects?

temperature determines the direction and rate of transfer of thermal energy between objects

What is the basic principle by which temperature is measured?

a property of a substance that chages with temperature

What is meant by saying that two bodies are in thermal equilibrium?

two bodies are at the same temperature and ther is no net energy transfer between bodies

Describe what is observed when viewing Brownian motion under microscope

specks of light reflected from pollen grains due to the random vibration of the pollen grain

There are two types of thermometers

Thermocouple

robust

very wide range

small size so small thermal capacity so act fast to change in temperature

high sensitivity

non-linear so requires calibration

Thermistor or resistance wire thermometer

robust

narrower range for thermistor, wide range for resistance wire

bigger size so big thermal capacity so act slower to change in temperature

high sensitivity for thermistor (but narrow range), low sensitivity for resistance wire

linear for both thermistor and wire

2 Ideal gases

Assumptions of ideal gases

Forces between particles are negligible except during collisions

so that potential energy between molecules can be assumed to be zero and internal energy is entirely kinetic energy

Volume of particles is negligible compared to volume of gas

Frequency of collision is low so that time between collision is longer than time during collisions

Collisions are perfectly elastic so no kinetic energy is lost

Number of particles is large

so that statistics can be applied to analyse molecules population

Describe motion of molecules in a gas according to the kinetic theory

gas molecules move with randomly distributed speed and direction

gas molecules have zigzag trajectories due to collisions between molecules

Section 5. Communication

1 AM/FM

Amplitude modulation

amplitude of carrier wave varies with the displacement of the information signal

Frequency modulation

frequency of carrier wave varies with the displacement of the information signal

2 Analogue and digital signals

What is meant by digital signal?

discrete signal that consists of a series of 1 and 0

What is meant by analogue signal?

signal that varies continuously with time

Analogue to digital convertion

What is the effect of sample frequency?

increase sample frequency will reduce width of digital signal

so fast changes in input signal can be reproduced

What is the effect of step size?

decrease step size will reduce step height

so smaller changes in input signal can be reproduced

Advantage of digital signal compare to analogue

it is possible to regenerate digital signal

so that noise can be eliminated completely and original signal can be recovered

digital signals can be turned from parallel to serial

so that reduced line of channel

more secure due to encryption

possible to check error when extra bit is added

What is meant by regeneration?

noise is removed from the signal

so that the original signal can be recovered

Definition of attenuation

gradual loss of intensity or amplitude of signal

Why an analogue signal cannot be regenerated?

in analogue signal the superposed noise cannot be distinguished from the signal

so when signal is amplified, noise will be amplified too

Describe functions of ADC

taking samples of analogue signal at regular intervals

and converting the analogue number to a digital number

Advantages of optic fibre compared to metal wire

greater bandwidth

less interference and noise

less cross-talk

less attenuation

less weight and cheaper

Functions of copper braid

block noise and crosstalk

provide a channel for return signal

Why a wire pair is not used to connet aerial to receiver?

wire pair has small bandwidth

there is noise and crosstalk

large signal attenuation

Section 6. Operational amplifier

Structure of a metal-wire strain gauge

plastic insulating casingthat contains the wire

winding grid shape of wire so that deformation of wire is significant under external pressure

Assumptions of ideal operational amplifier

infinite open-loop voltage gain

(voltage output) / (voltage input) = infinity

infite input resistance

so that there is no lost volts due to internal resistance

zero output resistance

so that there is no internal resistance

infinite slew rate

so that output signal changes simultaneously with input signal

infinite bandwidth

zero noise contribution

Negative feedback

Feedback: part of output signal is sent back and combined with input signal

Negative: feedback signal always makes input singal smaller

Benefits of negative feedback

increase stability

increase bandwidth

less distortion of output

Why there is a virtual earth point in inverting amplifier?

gain of amplifier is large

V+ is earthed

amplifier is not satureddue to the negative feedback

so V- must be very close to V+ so V- is considered as virtual Earth point

Why a relay is required?

the appliance circuit requires high power and high current

but the control circuit can only provide a small current

Section 7. Electromagnetism

1 Coulomb fields

Kinetic energy of charges determines the closest distance particles can get due to energy conservation (17 MJ 43 Q5)

Why is the electric field strength zero inside conducting spheres?

charges in a conductor will move if there is an electric field

charges will move until E=0

Why charges on a sphere can be considered to act as a point charge at its centre?

electric field lines created by a sphere are radial and perpendicular to sphere's surface. These lines appear to come from the centre of sphere

Gravitational forces are not considered when calculating force between charged particles because gravitational force is about 30 orders of magnitudes smaller

2 Capacitor

Uses of capacitors in electrical circuits

smoothing of rapid changing current

store electrical energy

block direct current

used in timing circuits

3 Magnetic fields

Why is the speed of particle not affected by magnetic field?

froce from magnetic field on the particle is perpendicular to velocity

so no work is doneby the force on particle and speed will not change

Why the path of electron in the magnetic field is arc of a circle?

when electron is moving perpendicular to magnetic field, the force from magnetic field on charge is always perpendicular to its velocity

magnitdue of force is constant

so it act ascentripetal force

4 Velocity selector

Explain how velocity selector using a uniform magnetic and electric field work

uniform electric field E and uniform magnetic field B are applied in the same region in space, and are normal to each other

charged particles enter the region of fields with velocity v perpendicular to both B and E

B exerts a force on charge due to F_B = Bqv

E exerts a force on charge due to F_E = Eq

B and E are setted so they exert forces in opposite directions (need to use fleming's left hand rule here)

F_B = -F_E for particles with a particular selected speed

these particles will move in straight lines across region of field and leave the selector

particles withdifferent velocitywill be deflectedand absorbed by the selector before leaving region of field

5 Hall effect

Hall voltage is developed across faces that are perpendicular to current and magnetic field

Describe the process how Hall voltage is setted up

magnetic field Bthat is perpendicular to the current I is applied across the material

charge carriers in material will experience a force that is perpendicular to both I and B

charge carriers will deflect and build-up so results in apotential difference V_Hacross the material

charge carriers stop building up and V_H becomes constant when F_B = F_E

V_H = BI/ntq

here t is the thickness of the side that is parallel to magnetic field

Why the Hall voltage is difficult to detect in a thin slice of copper and easier in semiconductor?

in copper (or any other metal) number density of electron is very large

so hall voltage will be very small

semiconductor has smaller values of n

Hall voltage can work on semiconductors with negatively charge carriers (electrons) and positively charged carriers (holes). What is difference of VH developed in the two different types?

using fleming's left hand rule, we can find magnetic field will deflect holes and electrons in the same direction

but holes and electrons have opposite charge

so Hall voltage will have opposite polarity

6 Eddy current

Why eddy currentdissipates energy from moving conductor in uniform magnetic field?

as conductor moves into or out of the uniform magnetic field, it experiences changing magnetic flux linkage

due to Faraday's law there will be eddy currents induced on the conductor

eddy currents produce heatingaccording to P=I^2*R

kinetic energy of moving conductorthus is converted into thermal energy and electromagnetic energy

so moving block will slow down

7 Domestic use of AC

AC: current or voltage that reverses polarityperiodically

period T, frequency f, peak voltage V0, voltage as a function of time

Why does heating occur when an AC is applied across a resistor while the average current is zero?

average current is zero because AC current hasnegative and positive currents that will cancel each other

heating power depends on I^2*R and I^2 is always positive, so heating effect will not cancel out

Root-mean-square (rms) voltage: the DC voltage that produces the same power is a resistor as an AC voltage with peak value V0 is V0/sqrt(2)

Why rms current in an air-cored solenoid is reduced when an iron core is inserted?

inserted core increases rate of change of flux linkage

so induced emf in solenoid is increased

the induced emf is in opposite direction to the applied emf so current will be reduced

8 Transformers

V1/V2 = N1/N2

V1*I1 = V2*I2

How does transformer work?

alternating currentin primary coil produces changing magnetic flux

the changing magnetic flux linkage is linked to secondary coil by the iron core

due to Faraday's law, there will beinduced emf and current in secondary coil

AC must be applied in primary coil for transformer to work, because DC cannot produce the changing magnetic fluxrequired for electromagnetic induction

Function of iron core

reduce flux lossbetween the primary and secondary coils

What is an ideal transformer?

no energy is lost so that input power equals output power

Sources of power loss within transformers

eddy currents in iron core dissipates electrical energy to thermal energy

resistance of coilsdissipates electrical energy to thermal energy

Why iron core should be laminated?

if laminated iron core has thickness less than the radius of eddy current, then eddy current will be reduced and less energy is dissipated to heat

What is advantage of using AC comparing to DC?

AC voltage can be changed easily using transformers

generators produce AC directly

High voltage is used in electricity transmission so current is lower, and less energy is dissipated to heat due to P=I^2*R

Section 8. Quantum

1 photoelectric effect

Physical processes that provide evidence for wave nature of a particle

electron diffraction

electron microscope

elecron double slit interference

Physical processes that provide evidence for particulate nature of EM radiation

photoelectric effect

atomic spectrum

Why emitted photoelectrons have arange of kinetic energy?

energy required to remove electron from metal surface varies for different electrons

so electrons loss different amount of energy while make their escape from metal

electrons originally sitting close to metal surface require the least amount of energy to escape and will have highest kinetic energy

If intensity of incident light is kept constant but wavelength is reduced, what's effect on photoelectric current?

reduced wavelength means energy for single photon is higher

constant intensity means the number of photon will be reduced

so fewer electrons will be released from metal and result in smaller current

2 Electron energy levels in gases and absorption and emission spectra

Why do transmission spectra contain a number of dark lines?

incident light consists of photons and interact with electrons in gas molecules

electrons absorb energy from photons and are excited to higher energy level

only photons withspecific energiesthat match the energy difference between energy levels will be absorbed

when high energy electrons de-excite, photons with the specific frequency are emitted in all directions

so along the original path of incident light, lines with specific wavelengths appear to be dark

3 Electron energy bands in solids and the band theory

Why there are energy bands rather than energy levels in solid?

in solids atoms are close together and have strong interactions

electron energy are determined by the effects of a number of atomsin its neighbourhood

different atoms change electron energy by different extent and electron will have big range of energy

so we say the electron energy levels are spreaded to form energy bands

Use band theory to explain why resistance of metal increases with temperature?

in metal, conduction and valence band overlap so there is no band gap and there are a population of conduction electrons

as temperature increases, lattice vibrations increaseso rate of collisionbetween electrons and lattice increases

higher rate of collision means electrons experience greater resistance while conducting so resistance of metal increases

as temperature increases, number of conduction electron will also increase by small amount, which decreases resistance. But this effect is much less sigfinicant compared to the increases in collision rate

Use band theory to explain why resistance of an intrinsic semiconductordecreases with temperature?

there are conduction and valence bands of electrons in semiconductor. Electrons need to absorb energy to be excited to conduction band

this excitation leaves positive holes in valence band

both excited electrons and holes are charge carriers

increase in temperature will excite more electrons to conduction bandsso increase the number of charge carrier

higher number of charge carriers means less resistance

increase in temperature also increases extent of vibration of lattice

increased vibration will increase collision rate between electrons and lattice and will increase resistance. But this effect is much less obvious than increase in charge carriers

Use band theory to explain why resistance of light-dependent-resistor change with light intensity?

there are conduction and valence bands of electrons in LDR. Electrons need to absorb energy to be excited to conduction band

this excitation leaves potivie holes in valence band

both excited electrons and holes are charge carriers

increase in light intensity will excite more electrons to conduction bandsso increase the number of charge carrier

higher number of charge carriers means less resistance

Section 9. Nuclear

1 Mass-energy equivalence

E=mc^2

Mass defect (delta_m)

sum of mass of individual separated protons and neutrons is greater than the mass of nucleus

this difference in mass is called mass defect

this difference in mass is because nucleus is more stable than individual nucleons, and part of mass of nucleus is used to form bonding between neutrons and protons

Binding energy

energy required to split a nucleus to individual protons and neutrons that are infinite far away

energy releasedwhen individual protons and neutrons are joined together to form a nucleus

binding energy per nucleon divides binding energy by nucleon number

the higher the binding energy per nucleon, the more stable the nucleus

2 Radioactive decay

Random nature of radioactive decay

it is impossible to predict exactly when a certain nucleus will undergo radioactive decay

behavior of apopulation of nuclei is predictablebecause probality of a nucleus to decay in 1 second is constant and the same for all nucleus

Spontaneous nature of radioactive decay

probability of a nucleus to decay in 1 s does not depend on physical conditionssuch as temperature and pressure

Activity (A)

number of radioactive decay per second, of a radioactive sample

has unit of becquerel

decaysexponentially with time

Decay constant (lambda)

probability of a nucleus to decay per unit time

spontaneous nature of radioactive decay means lambda is a constant

decay constant is the property of nucleus

Half-life

time taken for total number of nuclei in a sample to half

time taken for activity of a sample to half

half-life of a sample depends only on its composite nucleus

Radioactive decay experiment

Geiger-muller tube(Geiger counter, GM tube) is used to measure activity of a source

distance between GM tube and source must bekept constant

because detected activity decreases with increasing distance ot source

alpha particles will be absorbed by different thickness of air or other medium

Measure background count rate and subtract it from the measured count rate

Record the counts for a longer period of time or repeat and calculate mean can reduce random error in activity

Other sources of errors:

emission from radioactive daughter products

self-absorption in source

absorption in air before reaching detector

radiations are emitted in all directions, some direction might not be detected by GM tube

Section 10. Medical imaging

1 Ultrasound

Generation of ultrasound

AC signal is applied acrosspiezo-electric transducer

the electric signalwill cause layers of molecules in the transducer to distort (compression or streching)

AC signal thus applied current causes piezo-electric transducer to vibrate

when applied AC signal has frequency matches natural frequency of transducer, resonance will take place and transducer will vibrate with large amplitude

this resonance frequency is inultrasound range(frequency greater than 20 kHz) so it will produce ultrasound wave

Why ultrasound from transducer is pulsed?

transducer is used as both the emitter and receiver of ultrasound so reflected ultrasound should be able to be distinguished from emitted sound

arrival time of reflected wave is required to determine depths of body structure

intensity of reflected wave is required to determine nature of body structure boundaries

What is specific acoustic impedance?

product of speed of ultrasound in medium and density of medium

What is advantage of using high-frequency ultrasound compare to lower-frequency?

higher frequency means smaller wavelength so smaller structure can be resolved

2 X-ray

Intensity spectrum of x-ray

Why there is a continuous distributionof wavelengths?

this is called the braking radiation

these x-ray photons are produced by decelerating electrons when they hit metal target

electrons have different accelerations and loss different amount of energyto photons

hence there is arange of wavelength

Why there is a sharp cut-off at short wavlength?

the cut-off correponds to electrons that are stopped completely by a single collision so loss all of their energy to one photon

so it sets the limit of shortest wavelength

Why there is a series of peaks?

these peaks are the characteristic radiation

when incident electrons hit metal target, they might excite the metal electrons to higher energy level

when the excited metal electrons de-excite, they emit x-rays with specific wavelengths correspond to the energy difference between electron energy levels

Long wavelength photons are filterred out

How this filtering is achieved?

placing an aluminium foilbetween x-ray window and patient

Reason for filtering

long wavelength x-rays have low energy and so cannot pass through body

so these photons cause ionizing damage to bodybut do not contribute in constructing the x-ray image

Hardness of x-rays

What is hardness of x-ray beam?

hardness refers to the penetrating ability of an x-ray beam, which is related to the x-ray frequency

the higher the frequency, the harder the x-ray

how to make x-ray beam harder?

apply higher voltage in the accelerating circuit

Intensity of x-rays

proportional to the number of electronsthat incident on metal target per unit time

can be increased by increasing current in the heater circuit

high intensity x-ray can produce x-ray image quicker

X-ray images

Sharpness

clarity of edges of image

Contract

difference in degree of blackness in different body parts

3 Computed tomography

Principle of CT scanning

x-ray images of one slice of body are taken from different angles

then the images are processed by computerand combined to give 2D image

this process is repeatedfor many slices of body so that a 3D image of the whole body can be constructed

Comparing CT scan to standard x-ray scan

advantage

image is 3D so body can be viewed from any angle

disadvantage

higher amount of exposure to ionizing radiationso higher risk

4 Nuclear magnetic resonance

Working principle

nuclei spin about an axis so that each nucleus can be treated as a small magnet

a strong and uniform magnetic field is applied so nuclei precessabout the field with larmor frequency

a radio frequencypulse is applied that matches the larmor frequencyso it resonanceswith the precession of nuclei

nuclei therefore absorb energy efficiently from radio wave and are excited to unstable higher energy levels

when nuclei de-excite, radio frequency radiation are emitted

these radio wave radiations are detected and processed

relaxation timegives information on the different material of body

Function of non-uniform magnetic field

non-uniform field superposed on the uniform field is also applied

so that Larmor frequency can be changed in different parts of body

different parts of body can be tunned to resonant with incident radio wave

this allows tocontrol larmor frequency of nuclei in different part of body

changing field enables position of detection to be changed

希望以上内容可以帮助大家更好地复习A2物理paper4。A2成绩不达标，到手的也能飞。因为大学会根据你的AS给你一个，但更多的是依赖A2成绩判断你最终是否值得他们的，因此A2成绩一定要超级重视啊。如果你担心自己的6月考试不能稳定达标，或是害怕导师评估分出分过低，点击预约试听【ALEVELwinter vacation培训班】，全球海归授课，紧贴考试局新政策，根据不同考试局整理高频考察点，提前攻克薄弱环节，无论考试规则怎样变化，都能助力稳定A*实力。

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