A Level Edexcel Physics Paper 2

Electron Gun Model Answer

- Low voltage supply heats filament causes thermionic emission/ electrons have enough energy to leave

- Electrons are repulsed from the cathode (filament) and attracted towards the anode

- This electric field causes electrons to accelerate

- There is a hole in the anode which creates a beam for electrons that pass through

- Electrons are then fired through a vacuum (to stop interfering particles) onto a fluorescent screen

Photo Electric Effect Model Answer

- 1 Electron near the surface of a metal gains 1 photon and gains enough energy to be liberated

- The work function is the energy needed for the electron to escape the surface

- increasing intensity will increase the number of photons

- increasing frequency will increase the energy of the photons

- These electrons released are called photoelectrons

- This effect can be explained through E = hf

PhotoElectric Equation

Ekmax = hf - work fucntion

Ekmax = max kinetic energy of freed electrons

hf = energy of photon

work function = the minimum energy to release an electron

Why does the photoelectric effect equation refer to kinetic energy as maximum kinetic energy?

It will not always be an equal energy transfer:

- Some energy may have been transferred to electrons below the surface

- Therefore for these electrons to liberate to the surface there will be heat transfer through collision

- therefore the electrons leaves the surface will less than max

Energy Frequency equation

E = hf

E = energy

h = plank's constant

f = frequency

Work function equation

Work Function = h x threshold frequency

Work Function

the minimum energy needed to liberate an electron from a metal atom

Threshold Frequency

minimum light frequency necessary to liberate an electron from a given metal

Energy is proportional to frequency

Wave Particle Duality Model Answer

Nuclear Binding Energy

The energy needed to separate a nucleus into individual protons and neutrons

E = mc^2

Binding Energy = mass difference x the speed of light^2

How to calculate nuclear binding energy from a nuclear decay equation?

Th -> Ra + a

Mass of Th nuclear-(Mass of Ra nuclear + mass of alpha particle)

Convert u to kg

Multiply by speed of light^2 E = mc^2

Convert to Mev

How to convert u to kg

kg / 1.6605x10^-27

How to convert kg to u

kg / 1.6605x10^-27

What is 1 u?

1.6605x10^-27kg

Converting u to Mev

u x 931.5

Converting Mev to u

Mev / 931.5

Electron Volt

The energy given to an electron by accelerating it through 1 volt of electric potential difference. 1.6 x 10-19 joules.

Model answer for Fission

- Strong nuclear forces that hold the nucleus together only act a small distance

- When a big nucleus absorbs a neutron it deforms the nucleus

- The electrostatic repulsion between protons becomes greater than the strong nuclear force

- it then splits (random)

Chain Reaction

As a fission reaction splits it produces 3 neutrons which can go on to split 3 other neutrons creating an exponential chain reaction

Model answer for fusion

- two light nuclei join together to make a heavier nucleus

- This gives out more energy per nucleon than than fission as the increase in binding energy is much larger for fusion reactors

Advantages of Fusion over Fission

- Power output is greater

- Raw materials are cheap and available (hydrogen in water)

- No radioactive waster is produced directly

Why Fusion is hard to do?

The need for:

- enormous pressure

- enormous temperatures

We need these to get two nuclei close enough together to fuse

- Containment using magnetic fields uses more energy than the reaction yields

We need this because it is the only way to contain plasma from container walls

Binding energy per nucleon curve

All reaction through fission or fusion aim to get to iron 56 because it is the most stable element

Absorption Spectra

a spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption of specific wavelengths.

How is an absorption/ emission spectrum produced?

Shining white light through a sample of a gaseous element

The samples of the emission spectrum (the wavelengths missing from the absorption spectrum) are a result of the absorption of energy by electrons corresponding to the energy levels of the element by the gas

Emission Spectrum

a spectrum of the electromagnetic radiation emitted by a source, showing coloured lines due to the emission of the wavelengths

Atomic Line Spectra Model Answer

- All elements and their isotopes have a line spectrum

- The spectra are the wavelengths absorbed or emitted from an element with an electron gains or looses energy

- The energy is lost in the form of photons of certain wavelengths

backround radiation

nuclear radiation that occurs naturally in the environment

Examples of background radiation

- cosmic radiation

- radioactive elements

- radioactive substances

- rocks

- living things

- air

How to account of background radiation?

Measure background count

Subtract this from radioactive element count

Nucleon number

the total number of protons and neutrons in the nucleus of an atom

Mass number

the sum of the number of neutrons and protons in an atomic nucleus

Proton number

the number of protons in the nucleus of an atom

Atomic number

the number of protons in the nucleus of an atom

How to find mass number?

protons + neutrons

The bigger number of element table

How to find proton/atomic number?

The smaller number on element table

How to find number of electrons?

atomic/ proton number

How to find number of protons?

atomic/ proton number

How to find number of neutrons?

mass number - atomic number

Model Answer for Alpha Particle Scattering Experiment

- Rutherford fired fast moving alpha particles at thin gold foil

- Most alpha particles passed straight through the foil

- Some alpha particles were deflected

- Some alpha particles bounced back

- This meant all of an atoms positive charge was concentrated at the centre of the atom

- The atom was mainly empty space

- Electrons surround the nucleus but at relatively large distances from it

Nature of Alpha Radiation

2 protons and 2 neutrons

A helium nucleas

Nature of Beta Radiation

An electron

Nature of Gamma Radiation

An electromagnetic wave of very short wavelength and high frequency

Mass of radiations

- Alpha = 4u

- Beta = 0.00055 u

- Gamma = 0

Charge of radiations

- Alpha = +2

- Beta = -1

- Gamma = 0

Alpha ionising ability

Very Strong

Beta ionising ability

Medium Strong

Gamma Ionising ability

weak

Symbols of radiaiton

Alpha = a

Beta = e or B

Gamma = y

Speed of alpha radiation

5% of the speed of light

Speed of beta radiation

98% of the speed of light

Speed of gamma radiation

speed of light

Alpha penetration

Stopped by paper, skin or few cm of air

Beta penetration

Stoped by 3mm of aluminium of about 1 m or air

Gamma penetration

reduced significantly by several cm of lead

Electric and magnetic fields effect on radiation

Alpha: yes

Beta: yes

Gamma: no

Kg to Mev/c^2

/ 1.6x10-19

x 10^-6

Mev/c^2 to Kg

x 10^6

x 1.6x10^-19

/(3x10^8)^2

7 SI Units

Mass (kg)

Length (m)

Temp (K)

Illumination (Candela)

Amount of Substance (mole)

Electrical Current (amp)

Time (s)

Hertzsprung-Russell diagram

- Temperature Scale goes right to left

- Luminosity is a log scale

- The sun is on main sequence just under red giants

Life Cycle of a small star

nebula, protostar, main sequence star, red giant, white dwarf, black dwarf

Life cycle of a medium star

nebula-protostar, main sequence star, red giant-super nova, neutrons star

Life cycle of large star

nebula, protostar, main sequence star, red giant, super nova, black hole

Refraction

The bending of a wave as it passes at an angle from one medium to another

Snell's Law

sini/sinr = refractive index

sin i = angle of incidence

sin r = angle of refraction

Refractive Index Equation

n = c/v

refractive index = speed of light in a vaccum/speed of light in the medium

What is the refractive index of air?

1

Equation for light moving from one medium to another

n1sinangle1 = n2sinangle2

Refractive Index x angle i = Refractive index x angle r

Equation for light moving through several mediums

n1sinangle1 = n2sinangle2

n2sinangle2 = n3sinangle3

n3sinangle3 = n4sinangle4

Ray Diagram for refraction

- Angle of incidence must equal angle of refraction

Wavefront Diagram for refraction

- Lines must be equally spaced out

The nature of radioactive decay

- Completely random

- spontaneous as you cannot force a nuclei to decay more than another

- For a particular nuclei there is a probability that it will decay

How to calculate half life?

Half life = ln2/decay constant

Half Life

the time taken for the radioactivity of a specified isotope to fall to half its original value.

Exponential Decay Equations

N = N0 x e^-decay constant x t

A = A0 x e^-decay constant x t

C = C0 x e^-decay constant x t

N = number of nuclei

A = Activity

C = Count rate

Hubbles Graph

- Receding velocity on y

- Distance on x

- Constant = Hubbles constant

- 1 / H is the age of the universe

Hubble's Law

v = Hd

v = recessional velocity

H = hubbles constant

d = Distance to that galaxy

How to calculate the age of the universe

t = d/v

v / Hd

t = d/Hd

t = 1/H

Age of the universe

13.7 billion years

Hubbles Constant

70 km/s/Mpc

1 AU in metres

1.5 x10^11m

Distance from earth to sun

1 Light Year in metres

One year =3.15x10^7s

Speed of light = 3x10^8

Distance = speed x time

= 3x10^8 x 3.15x10^7

= 9.45 x10^15m

1 parsec in metres

3.09x10^16m

Distance in pc = 1/parallax angle in seconds of arc

Gravitational Field

the region of space surrounding a body in which another body experiences a force of gravitational attraction.

Gravitational Field Strength

The force acting on each kilogram of mass in the field

g = F/m

Newton's Law of Universal Gravitation

Every object in the universe attracts every other object

Focal Length

the distance from the optical centre of a lens to the focal point

Focal Length Equation

power = 1/f

Drawing diagrams for convex lenses where object is after principle focus

1. Draw principle axis and a lens and an arrow to represent the object

2. Mark the principle focuses

3. First ray goes from object to lens then down through the principle focus on the right side

4. second ray goes from top of object through principle focus

5. Draw image where lines meet

Drawing diagrams for convex lenses where object is before principle focus

1. Draw principle axis and a lens and an arrow to represent the object

2. Mark the principle focuses

3. First ray goes from object to lens then down through the principle focus on the right side

4. second ray goes from top of object through principle focus

5. Trace back rays

6. Draw image where lines meet

Drawing Lens diagram for a concave lens

1. Draw principle axis and a lens and an arrow to represent the object

2. Mark the principle focuses

3. first ray goes from top of object to lens and then goes upward in line with focus

4. Second ray goes from top of object through optical centre

5. Where rays cross is where image forms

Concave Lens

Diverging

Spreads light

Correct short sightedness

Convex

Converging

Focuses light

Correct far sightedness

Frequency

Number of oscillation in one second

Frequency Equation

f=1/T

Time Period

The time is takes an oscillation to repeat

Time Period Equation

T = 1/f

Wavelength

Horizontal distance between the crests or between the troughs of two adjacent waves

Power in lenses

Power = 1 / focal length

For multiple lenses P = P1 + P2 + P3...

Measured in dioptres D

A more powerful lens the focal length is shorter

Real Image

An image formed on the right side of a lens and can be projected onto a screen upside down