Physics Unit 1 Revision

8/13/2019 Physics Unit 1 Revision

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Siobhan Parish


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Structure of the atom A nucleon is the name for a proton or

neutron in the nucleus Mass Number

Atomic Number

/C /Charge ofproton /kg /Mass ofprotonProton +1.6 x 10-19 1 1.67 x 10-27 1

Neutron 0 0 1.67 x 10-27 1

Electron -1.6 x 10-19 -1 9.11 x 10-31 0.0005


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Isotopes are atoms with the same number ofprotons and different numbers of neutrons

Specific Charge

S.C. = Charge (C) = Ckg-1Mass (kg)

Electron hasthe largest

specific charge

A copper atom loses two electrons to form Cu, what is the specific chargeof the ion formed?Charge = (2 x (1.6 x 10-19))=3. 2 x 10-19

Mass = (63 x (1.67 x 10-27))= 1.05 x 10-25

6329

3. 2 x 10-19

1.05 x 10-25

= 3. 04 x 106 Ckg-1

Charge dependson how manyelectrons and

protons are lost!


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The strong nuclear forceThe strong nuclear force (SNF) overcomes theelectrostatic force of repulsion in the nucleus to keepthe protons and neutrons together

Range of SNF is about 3-5fm; about the same asthe diameter of a small nucleus. Electrostatic forcebetween two charged particles has infinite range

Has same effect between two protons as it doesbetween two neutrons and a proton and a neutron

Attractive from 3-0.5fm. Smaller than 0.5 itbecomes repulsive to stop nucleons being pushedinto each other


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Radioactive decay1. Alpha radiation Nucleus emits an particle and forms a new

nucleus

2. Beta radiation particle created in the nucleus and instantlyemitted. Neutron in nucleus changes to a proton It is a fastmoving electron. An antineutrino with no charge is also emitted

3. Gamma radiation radiation is electromagnetic radiationemitted by an unstable nucleus. It can pass though thick metalplates, it has no mass and no charge. Its emitted by a nucleuswith too much energy following an alpha or beta emission

42

0-1

particle -particle

Because neutron in thenucleus changes into aproton the atomic numberreduces by 1 but nucleonnumber stays the same.This means product nucleusis a different element.


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Electromagnetic wavesAn e-m wave consists of an electric wave and a magnetic wavewhich travel together and vibrate:

At right angles to each other and to the direction they aretravelling

In phase with each other

Type Radio Micro Infrared Visible UV X-Rays Gamma

range>0.1m 0.1m-

1.mm1mm 700nm

700nm-400nm

400nm- 1nm


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E-m waves are emitted by a charged particlewhen it loses energy. This can happen when: A fast-moving electron is stopped, slows

down or changes direction

An electron in a shell of an atom moves to ashell of lower energy

E-m waves are emitted as short bursts ofwaves, each time in a different direction. These

are called photons.Einstein assumed photon energy is inaccordance with the above equation.

E= hf

h= 6.67 x 10-34Js


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Antimatter When antimatter and matter meet they

destroy each other and emit radiation

+

emission takes place when a protonchanges to a neutron in an unstable nucleus

X Y + + +

Positron-emitting isotopes dont occur

naturally


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Diracs theory of antiparticles predicted that forevery particle, there is a corresponding antiparticlethat:

Annihilates the particle and itself if they meet.

This converts total mass to photons Has exactly the same rest mass as the particle

Has exactly the opposite charge to the particle

He also predicted the opposite process of PAIR

PRODUCTION. Photon with sufficient energy couldchange into a particle-antiparticle pair whichwould then separate from each other


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Annihilation:

Pair production:

Photon

Photon

antiparticle particle

particle

antiparticle

Photon


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One electron volt is the energy transferredwhen an electron is moved through apotential difference of 1 volt

1 MeV = 1.60 x 10-13J

Annihilation- when a particle and antiparticlemeet and their mass is converted to into

radiation energy. TWO photons produced (1photon would only take momentum away, notallowed because no outside forces act


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E0= hfmin

Minimum energy of each photon is given byequating the energy of the two photons tothe rest energy of the positron and of theelectron.

In pair production a photon creates a particleand an antiparticle and vanishes in theprocess. We can calculate the minimumenergy and frequency that a photon musthave to produce a particle-antiparticle pair

2E0= hfmin


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Ionising particles leave a visible trail of liquiddroplets when they pass through a cloudchamber

With a magnetic field applied to the chamber,

the trail of a charged particle would bend inthe field

- A positive particle would be deflected by thefield in the opposite direction to a negative

particle- The slower it went the more it would bend This is how the positron was discovered


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The electromagnetic force When two objects interact they exert and

equal and opposite force on each other.Momentum is transferred by these forces ifno other forces act on them

The electromagnetic force between twocharged objects is due to the exchange of

virtual photonspp

p p

Electromagneticforce betweentwo protons

Photon transfersmomentum so theyrepel each other


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Strong nuclear force holds the neutrons andprotons in a nucleus together.

Doesnt cause a neutron to change into aproton in -decay, or a proton into a neutron

in +decay These changes cant be due to the

electromagnetic force because the neutron isuncharged

The force must be weaker than the SNFbecause otherwise it would affect stablenuclei. Referred to as the weak nuclear force


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In both beta decays a new particle andantiparticle are created

These ARENT corresponding. One is apositron/electron and one is aneutrino/antineutrino

A neutrino can interact with a neutron tomake it change into a proton. Electron is

emitted as a result Antineutrinos can cause a proton to change

into a neutron, positron is also emitted


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The interactions are due to the exchange ofparticles called W bosons. Unlike photons theyexchange particles

Have a non-zero rest mass

Have a very short range, no more than about0.001fm

Are positively or negatively charged

-p

n

n

p

W- W+

+

Neutron-neutrinointeraction

Proton-antineutrinointeraction


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Beta decay The W-boson decays into a beta- particle and

an antineutrino

The W+boson decays into a beta+ particleand a neutrino

W-W+

n

-p

n

p

+

-decay+decay


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Electron capture A proton in a proton-rich nucleus can turn

into a neutron as a result of interactingthrough a weak interaction with an inner-shell electron

The same change can happen when a protonand an electron collide at high speed

n

p

W+e-

Electron capture


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Matter andantimatter

Hadrons Leptons

MesonsBaryons

, , , k

, k

, k0

,

, 0

e+, e-

e+, e

-

+,

-

+,

Interactions

Strong Weak

Weak

Baryons

K Mesons

mesons

Muons

Electrons

Decay

Muons have arest mass over

200 times greaterthan an electron


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Effects of cosmic rays Protons or small nuclei ejected from the sun

When they collide with atoms in ouratmosphere lots of new particles are created

2 mesons created by cosmic rays are the pimeson or the k meson

Mesons dont have antiparticles


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Electron scattering experiments indicated thatprotons are made up of 3 smaller particles

Quark AntiquarkUP

Charge, QStrangeness, S

u

+2/30

u

-2/30

DOWNCharge, QStrangeness, S

d-1/3

0

d+1/3

0

STRANGECharge, QStrangeness, S

s-1/3-1

s+1/3+1

Baryons have3 quarksMesons have2 quarks


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Baryonsp uud

n udd

Antibaryonsp uud

n udd

Hadrons

Mesons

Pions Pi mesons)+

ud uuddss du

Kaons K mesons)k+ k k

us sdds su

Kaons havestrangenessbecause theyare created by

stronginteractions anddecay throughweakinteractions


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Energy Includes rest mass where E= .Applies to all interactionsChargeApplies to all interactionsLepton numberApplies to all interactionsBaryon numberAll interactionsStrangeness

Only in strong interactions


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Photo electric effect occurs when em radiationfalls on a metal plate and electrons are emittedfrom the metal surface

Ultravioletradiation

Zinc plate

Gold leafelectroscope

-

-

- -

-


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Wave theory should also predict that it wouldtake longer for electrons to be released at lowintensity radiation than it would at highintensity: regardless of the wavelength

However, electrons are released immediatelywhatever the intensity


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Energy from photons must equal energy torelease electron plus any extra KE the electronpossesses.

Work function, energy required forelectron to escape from the surface of a metal,different for different metals

Threshold frequencyminimum frequency ofthe radiation required to release electrons fromthe surface

E= + KE

= hfmin


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A The photocathode will emit electrons oncethe energy/frequency of incident light issufficient

Electrons travel towards the anode

Electrons are attracted towards the anode andflow back round to the cathode establishing acurrent

Size of current is a measure of the number ofelectrons released by the photocathode


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Once f-min for incident light/radiation is met:- Number of electrons released per second =

I/e

- Current is proportion to intensity of light

- KE of electrons is independent of intensityPhotocell can measure the KE by putting a battery in theopposite direction and stopping the current. KE increases asfrequency increases

KEmax= hf -

KE

f

fmin

Gradient = h


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Ionisationthe creation of a charged atom(an ion)

Measuring ionisation energy

V

A Variable resistor

Voltmeter

Ammeter

Anode (+)

Filamentcathode (-)heated

Gas at lowpressure

Electrons


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V

A

Electrons boiled off in the heated cathode Attracted to + anode Accelerate through the gas At a certain speed they will knock electrons

out of the atom- ionise These electrons attracted to anode Current increases


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One volt is the potential difference that willgive one coulomb of charge one joule ofenergy when it is accelerated between twopoints through IV

Energy given to each electron is its charge xpotential difference between the electrodes

When the current increases this is theionisation energy, eV


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If gas atoms do not receive enough energy tobecome ionised they may, for certainenergies of colliding electrons, absorb theirenergy

Energies are called excitation energy and theelectrons in the atom move to a higherenergy level (further from the nucleus)

E i 100


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Ground state electrons have minimum energyfor frequency of photons

Electrons can emit photons and photons canexcite the electrons in an atom

Fluorescent tube

Mercury atoms collide ionisation &excitation

De-excitationUV photons emitted Photons absorbed by fluorescent coating

Coating de-excited Visible photonsemitted

E in x 100E out

hf E E


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Infrared lowest frequency UV highest frequency

A hot filament produces a set of discrete linesin different colours

Each line is only one colour and therefore onefrequency

Photons are produced by de-excitingelectrons as they move to lower energy levels

hf = E1E2


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The difference in energy between two levels isthe energy of the photon emitted during de-excitation

This assumes that n2is lower in energy than

n1 Helium was identified in sunlight from the

absorption line spectrum & as the alpha

particles from their line spectra


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Light as a wave (diffraction) When the gap is about a wavelength straight

lines become curved. The wavelength doesntchange

Light as a particle (photoelectric effect)

E=hf

Electrons as particles

Can be deflected by a magnetic flux


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Electrons as waves (diffraction) Thin metal foil acts like a diffraction grating

and the narrow beam of electrons willproduce a pattern like waves through a small

gap

Electronbeam

Screen

Patten of ringson the screen

NOT evenlyspaced

h


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p is momentum (mass x velocity) of theelectron, the electron appears to have awavelength.

If the electrons are given more energy, f is

larger and is smaller: diffraction rings aresmaller

Electrons energy determines which shell itoccupies because circumference must be awhole number of wavelengths

This is the de Broglie wavelength

= hp


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To make a current pass round a circuit theremust be a source of potential difference

Electric current is the rate of flow of charge in thewire or component

Current is due to the passage of chargedparticles, charge carriers

- In metals these are conduction electrons. Moveabout inside the metal repeating colliding witheach other and the fixed positive ions in the

metal- When a current passes through a salt solution the

charge is carried by ions; chargedatoms/molecules

Q = It


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Electrons flow from positive to negative Unit of current is the ampdefined in terms

of the magnetic force between two parallelwires when they carry the same current

Unit of charge is the coulombequal to thecharge flow in one second when the current isone amp

Q = It


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In an insulator electrons are attached an atomand cannot move away from the atom. When avoltage is applied across an insulator no currentpasses because no electrons move

In a metallic conductor most electrons areattached to atoms but some are not; these arethe charge carriers. When voltage is appliedconduction electrons are attracted towards

positive terminal In a semiconductor number of charge carriers

increases with an increase of temperature.Resistance decreases with increased temperature


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Potential difference is the work done (energytransferred) per unit charge. 1 volt is 1 jouleper coulomb

V = W/Q The emf of a source of electricity is the

electrical energy produced per unit chargepassing through the source


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Potential difference is the work done (energytransferred) per unit charge. 1 volt is 1 jouleper coulomb

V = W/Q The emf of a source of electricity is the

electrical energy produced per unit chargepassing through the source

Energy is transferred to other parts of thecircuit and some may be dissipated in thesource due to the internal resistance


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An electric current has a heating effect when itpasses through a component with resistance. Italso has a magnetic effect, this is used inloudspeakers and electric motors

Electric Heaters Work done is transferred as thermal energy

Charge carriers repeatedly collide with atoms

in the device and transfer energy to them Atoms vibrate more and resistor becomes

hotter


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Electric Motor Work done on the motor is transferred as

kinetic energy of the motor

The charge carriers are electrons that need tobe forced through the wires of the spinningmotor coil against the opposing force on theelectrons

Due to the motors magnetic field


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Loudspeaker Work done is transferred as sound energy

Electrons have to be forced through the wiresof the vibrating loudspeaker coil against theforce on them

Due to the loudspeaker magnet


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A component or device that has a potentialdifference across its terminals and a currentpassing through it. In time t:

The charge flow through it, Q = It

The work done by the charge carriers,W = QV = (It)V = IVt

The energy transfer, E in the component isequal to the work done, W

W = IVt


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Power = energy/time

The unit of power is the watt, W

One volt is equal to one watt per ampere

P = IV


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A circuit to measure the resistance of a resistor Ammeter must be in series with resistor so

the same current flows through them

Voltmeter must be parallel to the resistor

Variable resistor adjusts the pd and thecurrent

A Resistor

V

Variableresistor


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A resistor graph, pd against current, is astraight line straight through the origin

Resistance is equal to the gradient

Ohms law:The pd across a metallic conductor isproportional to the current through it,provided the physical conditions do notchange An ohmic conductor graph looks like this:

Current, A

Potential dif, V


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If the current and pd measurements for an

ohmic conductor are plotted with current onthe y-axis and pd on the x-axis then thegradient of the graph will be 1/R

RA


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ResistivityFor a conductor of length, L and cross-sectional area A, the resistance R is:

Proportional to L

Inversely proportional to A

The unit of resistivity is the ohm metre, m

Resistivity is resistance per unit length x cross-sectional area

=RAL


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Superconductivity A superconductor is a wire/device made of a

material that has no resistivity or resistanceat, and below, a critical temperature

When a current passes through it there is nopd across it. The current has no heatingeffect

Superconductors are used to make high-power electromagnets. They can transferelectrical energy without wasting energy


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Cellsource of electrical energy The symbol for an indicator or any light

source is the same

A diode allows current in one direction only.

A light emitting diode emits light when itconducts; the forward direction

The resistance of a thermistor decreased withincreasing temperature

The resistance of an LDR decreases withincreasing light intensity


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Advantage of a potential divider Current through the component and the pd

across it can be reduced to zero

Graphs

A wire gives a straight line of gradient 1/R

Lamp gives curve of decreasing gradient

Thermistor of constant temp gives straight

line. Higher temp = greater gradientI

V

I

V

I

V

Wire Lamp Thermistor

High temp

Low temp


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The diode To measure characteristics of the diode

measurements are made in the forward andreverse direction

I

V0.6


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Resistance and temperature Resistance of a metal increases with increase

of temperature

Positive ions in the conductor vibrate more

when its temperature is increased

Charge carriers cannot pass through themetal as easily when a pd is applied

A metal has a positive temperature coefficient


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Resistance of an intrinsic semiconductordecreases with increase of temperature

Number of charge carriers (conductionelectrons) increases with temperature

Therefore intrinsic semiconductors have anegative temperature coefficient

% change of resistance per kelvin change oftemperature is greater than a metal

Thermistors are often used as thetemperature-sensitive component in a sensor


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1. At any junction in a circuit, the total currentleaving the junction is equal to the total currententering the junction

2. The current:

- Entering a component is the same as thecurrent leaving a componentCOMPONENTS DONT USE UP CURRENT- Passing through two or more components in

series is the same through each componentRATE OF FLOW OF CHARGE THROUGH EACHCOMPONENT IS THE SAME AT ANY INSTANT


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The pd between two points in a circuit is theenergy transfer per coulomb of charge thatflows from one point to the other

If the charge carriers lose energy, the pd is a

potential drop If the charge carriers gain energy (by passing

through a battery/cell), the pd is a potentialrise equal to the pd across the battery orcells terminals


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1. For 2 or more components in series, thetotal pd is equal to the sum of pds acrosseach component

2. The pd across components in parallel is the

same.3. For a complete loop of a circuit, the sum of

the emfs round the loop is equal to the sumof the potential drops round the loop


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P= I2R


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Resistance heating Power supplied to a component = IV The energy per second transferred to the

component as thermal energy = I2R

If the component heats up, the temperaturerise depends on- Power supplied- Rate of heat transfer to surroundings

Energy transfer in time t = I2

Rt Energy transfer per second to the component

doesnt depend on the direction of thecurrent

=E


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Internal resistance of a source is the loss ofpotential difference per unit current in thesource when current passes through it Emf of the source is the electrical energy per

unit charge that the source produces The terminal pd is less than the emf. The

difference is due to the internal resistance ofthe source

When a cell of emf , internal resistance r, isconnected to an external resistor R the cell emfis:

=Q

= IR + Ir


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Power Power supplied by the cell in the equation

= IR + Ir becomes: I= I2R + I2r

The power delivered to R is:

2

R+r 2

Maximum power is delivered to the load whenthe load resistance is equal to the internalresistance of the source

r

Load

resistance,R

Powerdelivered


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The pd across the terminals of a cell can bemeasured by connecting a high-resistancevoltmeter directly across the terminals

The current is changed by adjusting a

variable resistor A lamp or fixed resistor limits the maximum

current that can pass through the cell

Ammeter is used to measure the cell current

Graph and circuit on next slide


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r

A

VCell terminal

Pd, V

Current, A

emf

Gradient =internalresistance

V= - Ir


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As terminal pd decreases the currentincreases

This is due to the lost pd increases as thecurrent increases

- Terminal pd is equal to the cell emf at zerocurrent because lost pd is zero at zerocurrent- Graph is straight line with negative gradient

r can be calculated by the equation:

r =V1 V2I1

I2


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Circuits with cells in series Current through the cells is calculated by

dividing the net emf by the total resistance

If cells are connected in the opposite

direction then the net emf is the differencebetween the two

Total internal resistance is the sum of all theindividual internal resistances


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Circuits with identical cells in parallel When there are n identical cells in parallel,

the current through cell is: I/n (I is the totalcurrent supplied by the cells)

The lost pd in each cell = Irn

(this is the same

as the terminal pd)

Cells act as a source of emf and internal

resistance


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Diodes in circuits Diodes have an infinite resistance in the

reverse direction or at pds less than 0.6V inthe forward direction


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V1=

R1


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If two resistors are in series connected to asource of fixed pd V0

Total resistance of the combination = R1+ R2 The current, I, through the resistors is given

by pd across the resistors/total resistance The ratio of the pds across each resistor isequal to the resistance ratio of the tworesistors

V2=

R2


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Supplying a variable pd Source pd is connected to a fixed length of

uniform resistance wire

A sliding contact on the wire can then be

moved along the wire giving a variable pdbetween the contact and one end of the wire

Variable potential divider can be used to:

Change loudness of sound from aloudspeaker

Vary brightness of a light bulb


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Sensor circuits Produce an output pd which changes as a

result of a physical variable (temperature,light)

1. Temperature sensors consist of a potentialdivider made using a thermistor and avariable resistor

2. Light sensors use a LDR and a variableresistor. If light intensity increases, theresistance and the pd fall


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An alternating current is one that repeatedlyreverses its direction

The frequency of an ac is the number ofcycles it passes through each second

The peak value of an alternating current isthe maximum current which is the same ineither direction

The peak-to-peak value is twice the peakvalue Voltage

Time


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Observing alternating current1. Use an oscilloscope to display the waveform

- Increasing the output pd from the signalgenerator makes the oscilloscope taller

(larger ac pd)- Increasing f increases the number ofcycles on the screen

2. Connect signal generator to a torch lamp

and make f low enough so you can see thebrightness of the lamp vary


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Heating effect of an alternating current The heating effect of an electric current varies

according to the square of the current

Electrical power, P, supplied to the heater for

a current I is given by IV or I2R At peak current I0maximum power is

supplied

At zero current zero power is supplied


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For a sinusoidal current, the mean power overa full cycle is half the peak power

The equation for this isI02R The direct current that would give the same

power, and therefore heating effect, as themean power is the root mean square value ofthe ac in the same resistor

Irms=1

2I0


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+-

X1

Anode

FilamentY1

Y2 Spot

Screen

X2 behind X1


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X

An

odeFilament Y1 Screen


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To display waveform: X-plates are connected to the time base of

the circuit which makes the spot move at aconstant speed left to right, it can be

calibrated in milli or microseconds The pd displayed is connected to the y-plates

so the spot moves up and down; this is whenthe waveform is traced

Y-input calibrated in volts per centimetre

+-

X1

Y2 Spot

X2 behind X1


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1. To measure peak pd: observe the waveformfrom top to bottom- the amplitude is half ofthis. Look at what the y-gain or y-sensitivityis set at and then times the y-gain by the

amplitude2. To measure frequency: Measure the timeperiod, T, of the waveform. Calculate thefrequency as 1/T

3. To measure T: Measure the distance of onefull cycle and times by the time base


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Vertical displacement does not change forconstant pd

The spot traces a horizontal line above orbelow the 0 pd line depending on whether

the applied pd is positive or negative Direct pd can be calculated by measuring the

vertical displacement of the line and the y-gain

Oscilloscopes have a high input resistance sothe current taken from the circuit is negligible


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Measuring the speed of ultrasound Time base of an oscilloscope can be used to

trigger ultrasonic transmitter so it sends out ashort pulse

If the receiver is applied to the y-input of theoscilloscope the waveform can be seen on thescreen

By measuring the horizontal on the screen from

the leading edge of the pulse to the start of thespots sweep the travel time of the pulse fromthe transmitter to the receiver can be

Documents

Physics Unit 1 Revision