# A Level Physics

## Kit Betts-Masters  Set Explanations - Example Answers from Explain Questions

These answers seem to come up in markshemes time and time again!  I've taken these from the last five sets of Edexcel A Level Papers, including the new specimen papers and Sample Assessment materials.

So that's right, learn them in this level of detail!

Polarisation

• a polarising filter restricts the (electric field) vibrations of the (transverse) light wave to a single plane

• Perpendicular to the direction of propagation of the light

• the light incident on the filter must be plane polarised

• when the angle of rotation is a multiple of π rad (including zero),

• the plane of polarisation of the incident light is perpendicular to the transmission axis of the polarising filter hence the intensity of the transmitted light is zero

• when the angle of rotation is an odd multiple of π/2 rad

• the plane of polarisation of the incident light is the same as that of the transmission axis of the polarising filter hence maximum light is transmitted

Quantum

• electrons/atoms move to higher energy levels Or electrons/atoms are excited

• they then move to lower energy levels (accept ground state) and the energy (from the change) is given out in the form of a photon

• the energy levels are discrete Or only certain energy levels are possible

• the energy of the photon is equal to the difference in energy levels Or hf = E2 - E1 Or hc/λ = E2 - E1

• there are only a limited number of energy differences and only a corresponding set of frequencies/wavelengths

• different elements have different energy level (differences), so they will produce different frequencies/wavelengths

• Only one photon can transfer energy to a single electron Or a photon transfers all of its energy to a single electron Photon energy depends on frequency Or photon energy = hf

• Therefore photons must have a frequency greater than or equal to the minimum frequency (threshold frequency) in order to provide sufficient energy Or photon energy must be greater than work function

• Light with a greater intensity supplies more photons per second so more electrons can be emitted per second, causing a greater current

Wave nature of electrons!

• the pattern on the screen is caused by diffraction

• diffraction is a wave property, so this shows waves

Standing Waves

• Superposition/interference between waves travelling in opposite directions (wave reflected)

• At node the waves are in antiphase, so there is destructive interference Or At the antinode they are in phase so there is constructive interference

• At an antinode there is maximum amplitude Or At a node there is zero amplitude

• Sound waves incident upon surfaces within the concert hall will be reflected.

• Some frequencies will arrive from different directions with a phase difference of (any odd multiple of) πradians (1) OR path difference is odd number of half wavelengths

• Destructive superposition/interference will occur, causing

Heating and Resistance

• As temperature increase the (lattice) ion/atom vibrations increase

• (for the same current) electrons will collide more frequently with the vibrating (lattice) ions/atoms

• More energy dissipated by collisions so (for constant I) greater V required Or (constant V gives) lower v and, since I = nAvq, I will be lower

• Since V increases and R = V/I, R will increase with temperature Or Since I decreases and R = V/I, R will increase with temperature

• A rise in temperature causes the amplitude of the vibrating ions to increase (1)

• This causes the number of collisions per second between the ions and the moving electrons to increase (1)

• So the rate of flow of electrons decreases (causing the resistance of the metal to increase) (1)

Doppler Shift

• Doppler effect causes change in wavelength / frequency Or States (relative) motion of source (and observer) causes change in wavelength / frequency

• If the frequency is increased (the bat can tell that) the prey is moving towards (it)

• If the frequency is decreased (the bat can tell that) the prey is moving away from

EM Induction

• There is a changing flux (linkage) Or magnetic field lines are cut by the coil

• Inducing an e.m.f. (across the coil)

• The supply creates a changing magnetic field in the iron core

• Rate of change of flux in toothbrush coil is equal to rate of change of flux in charger coil (for an ideal transformer)

• The changing flux linkage in the coil of the toothbrush induces an e.m.f. according to Faraday's law

Thermodynamics I

• Some (thermal) energy will be transferred to the surrounding

Defining SHM

Acceleration is:

• (directly) proportional to displacement from equilibrium position

• (always) acting towards the equilibrium position Or idea that acceleration is in the opposite direction (to displacement)

Or

Force is:

• (directly) proportional to displacement from equilibrium position

• (always) acting towards the equilibrium position Or idea that force is a restoring force e.g. "in the opposite direction"

[accept towards undisplaced point/fixed point/central point for

equilibrium position]

[An equation with symbols defined correctly is a valid response for

both marks]

Trigonometric Parallax

• The star is viewed from two positions at 6 month intervals Or opposite ends of its orbit diameter about the Sun

• The change in angular position of the star against background of fixed stars is measured

• Trigonometry is used to calculate the distance to the star [Do not accept Pythagoras]

• The diameter/radius of the Earth's orbit about the Sun must be known

Standard Candle

• An astronomical object of known luminosity

• Measured brightness

• Use inverse square law [Reference to I=L/4πd2 with symbols defined]

Gas Pressure

• (Average) kinetic energy of molecules/atoms is less Or molecules/atoms slower

• Collision rate with walls of container is smaller

• There is less momentum/impulse (exchanged) per collision Or the rate of change of momentum is less

• Therefore a smaller force on the container wall

Rutherford (Alpha Particle Scattering)

• Most alpha particles pass through undeflected (1) OR some deflected through a small angle (1)

• A very small number are deflected through an angle greater than 90˚ (1)

• This suggests that the alpha particles are deflected by a charged nucleus that has a very small diameter compared to that of the atom rather than the charge being distributed throughout the atom (1)

• and that most of the mass of the atom is concentrated in the nucleus rather than distributed throughout the atom (1)

Newton's Laws

• Zero resultant force so zero acceleration

• applies Newton’s 2nd law so that resultant force on balloon causes it to accelerate OR applies Newton's 2nd law so that resultant force on balloon causes its momentum to change

• The forces are different types (1)

• The forces act on the same object (1)

• the magnitude of the force on each sphere is the same

• Either as the two forces are an action-reaction pair [reference to N3] Or force on each sphere is proportional to the product of the two charges (reference to Coulomb's law)

• so both spheres are deflected from the vertical by the same angle

(Same line, Same time, different objects, opposite direction.)

F=Bqv + F=BIL (Circular in Particle Physics)

• The magnetic force on the electrons acts at right angles to the plane containing B and v (1)

• Hence the force is always towards the centre of the circle (1)

• So providing a centripetal force on the electron or a centripetal acceleration that maintains circular motion (1)

Nuclear Stability

• (binding energy is) the energy equivalent to the mass deficit (1)

• When nucleons bind together to form an atomic nucleus (1)

Specific Latent Heat

• the temperature is constant when the puree boils because the average kinetic energy of the molecules in the puree is constant. (1)

•  when boiling occurs, the thermal energy supplied increases the potential energy of the molecules causing the molecules to move further apart (producing steam) (1) OR when boiling occurs, the thermal energy supplied increases the potential energy of the molecules breaking molecular bonds. (1)

Red-Shift + Hubble

• The wavelength change is bigger the further away the galaxies are (1)

• The further away galaxies are the faster they are moving, so all distant galaxies are moving away from each other (and the universe is expanding) (1)

Suggestions to Improve Practical

• Light gates can record short times accurately (1) OR with smaller uncertainty (1)

• Because human reaction time is not involved (1)

A description that makes reference to two of the following points:

• use of ficidual mark (1)

• eye close to liquorice lace to avoid parallax errors (1)

• Fixed metre rule close to lace (1)

• Use of set square to ensure rule vertical (1)

• measure the length of the wire with a metre rule and the diameter of the wire with a micrometer screw gauge

• the diameter of spread-out oil drop should be taken a number of times across a number of different directions and a mean calculated (1)

• time a larger number of oscillations

• as the greater the total time the smaller the % uncertainty

Material Behaviours

• Elastic material returns to its original shape when the deforming force is removed (1)

• But a plastic material would suffer a permanent deformation (1)

Projectiles (theory vs reality)

• Air resistance will act on the popper… (1)

• As a decelerating force (1) OR… dissipating energy (1)

• So the initial speed will be lower than in the absence of air resistance, so the suggestion is not correct (1)

Terminal Velocity

• As parachute opens (at B) the upwards force increases Along BC the velocity is decreasing at a non-constant rate

• The drag is greater than weight (negative gradient)

• The drag is decreasing (curved line) Eventually the drag force balances the weight

• No acceleration so line is horizontal

Resonance

• Resonance is occurring… (1)

• …when the driving frequency/forced vibration (at walking frequency) matches the natural frequency … (1)

• …energy transfer is maximum (1)

• Supporting the observation that the amplitude rapidly increases (1)

Damping

• energy was transferred from the oscillating bridge to the dampers

• this energy was dissipated in the dampers (and not returned to the bridge)

• hence the amplitude of oscillation was kept small

Stellar Evolution

• (main sequence) stars are (primarily) converting hydrogen to helium in their core

• stars on main sequence maintain a constant luminosity (for most of their lifetime)

• until hydrogen in the core depleted and no more hydrogen fusion in the core

• star collapses until temperature increases and helium fusion occurs at a higher temperature than hydrogen fusion

• outer layers of star expand and cool, making a red giant

• outer layers disperse leaving the central part

• fusion ceases and remaining material hot enough to emit radiation as a white dwarf

Ray Diagrams

Correct rays are:

• Ray through the principal focus and parallel to the principal axis

• Ray parallel to principal axis then through the principal focus

• Ray through the optical centre of the lens

Particle Definitions

• (meson) must be a quark combined with its own antiquark so that overall charge is 0 OR it can only contain up or down quarks (as it is not a strange particle)

• mesons are made up of quarks, whereas leptons are fundamental particles

• (baryons) three quarks

Collisions

• elastic collisions conserve KE, so collision is elastic

• apply principle of conservation of momentum