Unit 1 Physics

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  • Created by: Aman
  • Created on: 10-12-11 14:48

inside the atom

Isotopes- atoms with the same number of protons and a differnet number of  neutrons.

Specific Charge- charge/mass. The electron has the largest specific charge of any atom.

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Stable and Unstable Nuclei

The Stong Nuclear Force

-Keeps the protons and neutrons together in the nucleus

-Strong attractive force from 0.5 fm to 3-4 fm

-repulsive before 0.5 fm(http://www.boredofstudies.org/wiki/images/b/bb/Sci_phys_quanta_strong_force.png)

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Stable and unstable nuclei

Radioactive decay

Alpha radiation

Unstable nucleus emitts a alpha particle decreasing its nucleon number by 4 and atomic number by 2.


Beta radiation

neutron in the nucleus changes into a proton. a beta particle is emitted as well as a antineutrino. Atomic number increases by 1, nucleon number remains the same.


Gamma radiation

emitted by a nucles with too much energy after alpha or beta decay.

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Electromagnetic waves

-electric wave and magnetic wave travel together and vibrate at right angles to each other

-they are also in phase with each other (reach a peak together, are in step with each other)



-electromagnetic waves are emitted as short bursts of waves. each burst is a Photon


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Particles and antiparticles

-all particles have antiparticles

-they have the same rest mass

- they have opposite charges

-When they meet they  annihilate each other creating 2 photons equal to the mass of the two particles

Pair production-  photon with sufficient energy changes into a particle-anitparticle pair and then seperate from each other.

 minimum energy of a photon needed=hf(min)=  2E₀                     1MeV=1.60 x 10⁻₁₃J

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How Particles interact

Momentum = mass x velocity

-when two objects interact they exert equal and opposite forces on each other

-feynman said that the electromagnetic force between two charged objects is due to exchange particles- W bosons (non zero mass, very short range 0.001fm, are charged)

Weak Nuclear force

causes neutrons to change into protons through beta +/- decay

-Neutrons interact with neutrinos giving a proton and beta-

-antineutrino and proton interact and give neutron and beta +


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How particles Interact cont...

Beta Decay

-w- boson decays into beta- and antineutrino

-w+ boson decays into beta+ and neutrino


Electon capture

-proton in a proton rich nucleus can turn into a neutron through the weak force with an inner shell electron. W+ boson turns electron into neutrino

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Particle zoo

-muon (μ) heavy electron. Negatively charged particle. rest mass=200x electron

-pion (π meson) +/-/0. greater mass than the muon but less than proton

-Kaon (K meson) +/-/0. greater mass that pion but less than proton

-kaon and pion produced by strong interaction.

-kaons decay through the weak interaction. decay products= pion, muon, antineutrino and antimuon neutrtino. they are also strange particles

-charged pions decay into muon, antineutrino/antimuon, neutrino. neutral pion decays into high energy photons.

-muons/antimuons decay into electrons and antineutrinos/ positron, neutrino

-obey conservation rules.

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Particle sorting

Hadron- Interact through the strong interaction and through electromagnetic interaction if charged. decay through the weak interaction apart from the proton as it is stable

Lepton- Interact through weak interaction and electromagnetic interaction if charged (muon, electron, neutrinos)

rest energy of products=total energy before-kinetic energy of products.

1GeV=1000MeV             Hadrons = ;

-Baryons-protons and hadrons that decay into protons

-Mesons- Do not decay into protons. (kaons and pions)


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Leptons at work

-leptons and antileptons can interact to produce hadrons

-they are fundamental elements

-leptons change into other leptons through the weak interaction. can be produced or annihilated in particle-antipartical interactions

-in interactions between leptons and hadrons a neutron/neutrino can change into/form a corresponding charged lepton

-lepton number is conserved in any change

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Quarks and antiquarks


-all strange particles decay through the weak interaction

-strangeness is always conserved in strong interactions but is not conserved in the weak interaction


Charge- up+ 2/3, down -1/3, strange -1/3   Strangeness- u 0, d 0, s -1

(signs are reversed for antiquarks, + changes to -)

-mesons=quark,antiquark pair


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quarks cont...

Baryons= 3 quarks    Antibaryon=3 antiquarks

-Proton =uud       Neutron= udd       Antiproton= antiup,antiup,antidown

Quarks and beta decay

-in beta- decay a neutron changes into a proton releasing electron and electron antineutrino, down quark changes into an up quark

-in beta+ decay proton changes into a neutron releasing positron and electron antineutrino, up quark changes into a down quark


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Conservation rules

Particles and properties

-conservation of energy and charge- applies to all changes in science. includes rest energies of particles

Rules only used for particle antiparticle interaction and decay

-lepton number must be conserved

-strangeness is conserved in the strong interaction but not in the weak interaction

-baryon number is conserved


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-electrons are emitted from the surface of a metal when electromagnetic radiation of a certain frequency is directed at the metal

-threshold frequency depends on the type of metal. emission does not take place below this.wavelength of incident light must be less than the maximum value which is equal to the speed of light/threshold frequency

-number of electrons emitted is proportional to intensity of radiation

-emission occurs without delay as soon as radiation is directed at the surface no matter the intensity



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Photoelectricity- Einstein's explanation


-when light is incident on a surface an electron on the surface of the metal absorbs a single photon from the incident light and gains energy equal to hf (energy of a light photon)

-electron can leave the surface of the metal if the energy gained exceeds the work function of the metal. Minimum energy required by an electron to escape from the metal surface.

kinetic energy of emitted electron E(kmax)=hf-Φ       hf=E(kmax)+Φ

threshold frquency-> f(min)=Φ/h

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Collisions of electrons with atoms


-number of electrons is not equal to protons

-formed by adding or removing electrons from an uncharged atom

-electrons passing through a fluorescent tube make ions when the collide with atoms of gas/vapour in the tube

eV=work done

Excitation by collision

Excitation-gas atoms absorb energy from colliding electrons without beingionised. happens at certain energies which are characterised by the gas

-if colliding electron looses all its kinetic, current due to the flow of electrons through the gas is reduced.

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excitation by collision cont...

-if the colliding electron doesn't have enough energy it will be deflected by the atom with no overall loss of kinetic energy

-Excitation energies- values at which atoms absorb energy.

-in gas filled tubes the excitation energies can be determined by increasing the pd between the anode and filament and measuring the current when the anode current falls.

-colliding electron makes an electron inside the atom move from an inner shell to an outer shell

-excitation energy<ionisation energy because it is not completely removed from the atom

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Energy levels in atoms

Ground state- lowest energy state of an atom. when it absorbs energy one of the electrons moves to a shell at a higher energy and is now in a excited state


-gases at low pressure emit light when they conduct electricity. can be used to measure excitation levels. happens because atoms absord energy due to excitation but do not retain the energy.

-when electrons move back down to a lower energy level they emit a photon which is equal to the energy lost by the electron/atom

-can de-excite to the energy level indirectly

energy of emittedd photon hf= E(energy level)1-E2

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Excitation cont...

Excitation using photons

-photon energy must be equal to the difference between the final and initial energy levels of the atom

- if it doesn't it won't be absorbed by the atom


-an atom can absorb photons and then emit photonsof the same or lesser energies

-atoms absorb ultraviolet photons and become excited. when they de-excite they emit visible photons. when the source of ultraviolet light is removed they stop glowing

Fluorescent tube- glass tube, fluorescent coating on inner surface with low pressure mercury vapour

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Fluorescent tube cont...

-When it is switched on it emits visible light because;

  • ionisation and excitation of mercury atoms happens as the collide with each other and other electrons
  • mercury emits ultraviolet and visible photons as well as photons with much less energy when they de-excite
  • ultraviolet atoms are absorbed by atoms in the fluorescent coating making the atoms excite
  • the coating atoms de-excite causing visible photons to be emitted
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Energy levels and spectra

-Visible light=400nm(deep violet) to 650nm(deep red)

-by measuring wavelenghts of line spectra you can determine the element that prodduces that light because enery levels of each type of atom are unique to that atom.

-each line in the spectrum is a different wavelength

-photons that produce each line have the same energy which is different to photons produced by any other line


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Wave Particle duality

diffraction of light- shows wave-like nature

-light emerging from slit spreads out like water wave. narrower the gap/longer wavelength=greater amount of defraction

Particle like nature

-photoelectric effect. light directed at the metal. electron absorbs and can be emitted if the energy it gains exceeds the work function.

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Matter Waves

-electrons in a beam can be deflected by a magnetic field- evidence of particle like nature

-matter particles have a dual wave-partical nature

-wave-like behaviour characterised by wavelength

λ=h/p      p=mv      p(momentum)

evidence of hypothesis - beam of electrons defracted into rings

- beam of electrons in vaccum fired at metal foil made of regions. region consitst of positive ions arranged in rows of regular pattern. causes electrons to be defracted like being shone through a slit

-electrons defracted in certain directions forming rings

-increasing speed of electrons makes rings smaller


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Electric Current

Current=Ampere (A) symbol-I

Charge=Coulomb(c) symbol-Q


Work done per unit charge is the potential difference or voltage across the component

Potential differnce is the work done (or energy transfer) per unit charge  pd=volt


emf of a source of electricty electrical energy produced per unit charge passing through the source

Work done W=IVΔt

Electrical Power P=IV

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Resistance of any component is equal to the pd across it divided by the current through it. unit=ohm Ω


Ohm's law states that the pd across a metallic conductor is proportional to the current through it , provided physical conditions do not change


Resistivity ρ=RA/L

unit is ohm metre( Ωm)

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Components and their Characteristics


test cicuit


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IV Charecterstics

(http://jcsu.jesus.cam.ac.uk/~rpc25/notes/physics/electricity/charac2.gif)lamp   (http://pvcdrom.pveducation.org/CHARACT/Images/Darklin.gif)diode - easily conducts after 0.6 volts

- in a thermistor a straight line is given. At a higher tempurature there is a greater gradient because as the tempurature increases the resistance decreases. When there is a low tempurature there is a smaller gradient

-The gradient of the light bulb decreases because the resistance increases as it becomes hotter.

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circut rules

-at a junction the total current leaving is equal to the total current entering

-the current entering a component is equal to the current leaving the component

-the current passing through components in a series circuit is the sme in each component

potential difference rules

-total pd across all components in series is equal to the sum of the potential differences across each component

-pd across components in parallel is the same

-the sum of the emfs around the loop is equal to the sum of the potential drops around the loop.

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More about resistance

-sum of the resistor  in series is equal to the total resistance


-resistors in parallel have the same pd. the sum of individualcurrents is equal to the current through a parellel combination of resistors

1/R=1/R1 +1/R2 + 1/R3 +...

Resistance heating

--charge carriers repeatedly collides with positive ions and energy is transferred to them

Rate of heat transfer=I²R


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emf and internal resistance

internal resistance

-caused by opposition to the flow of charge through the source causing electrical energy to be dissipated inside the source when charge flows through it.

emf-electromotive force electrical energy per unit charge produced


internal resistance of a source is the loss of potential difference per unit current in the sourcewhen current passes through the source.

ε=IR+Ir     r=internal resistance

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-power supplied by the cell=power delivered + power wastes deue to internal resistance

power supplied by the cell Iε=I²R +I²r

 maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source.

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More Circuit Calculations

cell current= cell emf/ total circuit resistance

-if the cells are connected in series in the same direction the net emf is equal to the sum of the individual emfs

-if they are connected in opposite directions the net emf is the difference between the emfs in each direction

-the overall internal resistance is the sum of the individual internal resistances

identical cells in parallel     V=ε- Ir/n

Diodes in circuits

-reverse based has infinite resistance. pd of 0.6V exists across a forward facing diode that is passing a current

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Potential Divider

-supplies a pd which is between zero ans source pd

-supplies variable pd

-supplies pd that varies with physical conditions

the ratio of the pds across each resistor is equal to the resistance ratio of the two resistors

sensor circuit

-produces output pd which changes as a result of a change of a physical variable

-Temperature sensor= potential divider,thermistor, variable resistor. temperature  of thermistor changes, changing the pd across the circuit

-light sensor= LDR, variable resistor. light intensity increases resistance of LDR falls.

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Alternating current

-current repeatedly reverses its direction

frequency- number of cycles that pass each second. unit-Hz, 1 cycle per minute

time period T=1/f

-peak value- maximum current which is the same in either direction

-mains pd varies with time-sinusoidal (makes sine wave)

-Oscilloscopes can be used to observe waveform of the alternating pd

-increasing output pd from the signle generator makes the trace taller (peak value made larger)

-Increasing frequency increases the number of cycles per second.

-if the signle generator is plugged into a torch lamp and set the frequency low enough you can see the light flashing on and off.

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heating effect of an alternating current

-heater supplied with alternating current would keep turning off and on.

-heating effect varies depending on the square of the current

P=IV=I²R    (R=resistance of the heater element)

-peak current I₀, maximum power supplied= I₀²R

-mean power over a full cycle is half the peak power

-Root mean squared value of alternating current- value of direct current that would give the same heating effect as an alternating current in the same resistor

=1/√2 x the peak value

mean power supplied can be found using rms value


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Using an Oscilloscope

-electron gun emits electrons in a beam at a fluorescent screen.

-position of the light on the screen is affected by the pd across either pair of deflecting plates.

-if pd is applied over x plates it moves horizontally. Plates connected to a time base circuit making the spot move at a constant speed. Calibrated in milliseconds or microseconds per centimeter

-if pd is applied across the Y plates it moves vertically. Makes the spot move up and down so that the wave form can be traced on the screen. Calibrated in volts per centimeter. Value referred to as y-sensitivity/y-gain

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Ajay Dhorajiwala


that really helps, thanks

jodie goffe


really helpful but on page 12 i think it is supposed to say 

-in beta+ decay proton changes into a neutron releasing positron and electron NEUTRINO, up quark changes into a down quark

not antineutrino

apart from that its great !



what is A on page 28 


resistivity = RA/L

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