# A Level Physics Model Answers

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Hadrons and leptons are two groups of particles. Write an account of how particles are placed into one or other of these two groups

• Examples of hadrons are baryons and mesons
• Hadrons are not fundemental particles, they are made up of quarks (Baryons have 3 quarks, Mesons have a quark and an anti-quark)
• Hadrons feel the strong force, the weak force, and the electromagnetic force if the hadron is charged

Leptons

• Example of leptons are electrons and muons
• Leptons are fundemental particles
• Leptons do not feel the strong force however they do feel the weak force and electromagnetic force if the lepton is charged
• Leptons have rest mass
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## Baryons, Mesons and Leptons and their interactions

Baryons, mesons and leptons are affected by particle interactions. Write an account of these interactions

The Strong Interaction

• The exchange particle is a gluon
• The strong interaction is how protons and neutrons are held together in the nucleus

The Weak Interaction

• Experienced by hadrons and leptons
• The exchange particle is a W(+/-) boson
• An example of the weak interaction is electron capture

The Electromagnetic Interaction

• Experienced by all charged particles
• The exchange particle is a virtual photon
• An example of the electromagnetic interaction is electrons repelling
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## Oscilliscopes

An alternating current supply provides an output voltage of 12 V rms at a frequency of 50 Hz. Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and the frequency of the supply

• The power supply is connected to the input of the oscilliscope

To check rms voltage value

• Time base is switched off
• Y-gain is adjusted so a complete vertical line can be seen on the screen
• Measure the peak to peak voltage and divide by 2 for peak voltage
• Peak voltage is divided by root 2 to find rms voltage
• Compare this to the stated rms voltage value

To check frequency value

• Time base is switched on and adjusted until at least one full cycle is visible on the screen
• Measure the period of one cycle
• Use the period to calculate the frequency
• Compare this to the stated frequency value
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## Line spectra

Line spectra were observed before they could be explained by theory. Explain how the bombarding electrons cause the atoms of the vapour to emit photons and the existence of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels.

• Electrons bombard atoms of vapour and give energy to electrons in atom
• Electrons move to a higher energy level and are excited
• Excited electrons move down to lower energy levels losing energy by emitting photons
• Emmited photons have the same amount of energy as the difference in energy between the two energy levels that the electron moved between
• Photons have energy E=hf
• Photons of characteristic frequencies emitted from atoms of a particular element
• This is because atoms have discrete energy levels which are associated with particular energy values
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## Electron ground state

Explain what is meant by the ground state of an atom and describe the process that is taking place in the atoms emitting photons

• Electrons can only occupy discrete energy levels
• The ground state is the lowest energy state an electron can occupy
• Electrons collide with orbital electrons
• The collisions gives the electrons energy to move to a higher energy level
• Electrons can return to a lower energy level by losing energy
• Electrons lose energy by emitting photons which have the same energy as the difference in energy between the energy levels (delta E=hf)
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## Threshold frequency

The photoelectric effect suggests that electromagnetic waves can exhibit particle - like behaviour. Explain what is meant by threshold frequency and why the existence of a threshold frequency supports the particle nature of electromagnetic waves

• Threshold frequency is the minimum frequency required for emission of electrons
• If frequency is below the threshold frequency, there are no emissions even if intensity increased
• This is because the energy of the photon is less than the work function
• Wave theory can not explain this as energy of wave increases with intensity
• Light travels as distrete packets of energy called photons
• Photons have energy that depends on frequency (E=hf)
• If frequency is above the threshold, photon has enough energy to make electrons be emitted
• There is no time delay between electrons being emitted and the frequency exceeding the threshold
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## The Photoelectric effect

When light of a certain frequency is shone on a particular metal surface, electrons are emitted with a range of kinetic energies. Explain in terms of photons why electrons are released from the metal surface, and why the kinetic energy of the emitted electrons varies upto a maximum value

• Energy is needed to remove an electron from the surface
• the Work function ϕ of the metal is the minimum energy needed by an electron to escape from the surface
• Light consists of photons each witih energy E = hf
• One photon is absorbed by one electron
• An electron can escape (from the surface) if hf > ϕ
• Kinetic energy of an emitted electron cannot be greater than hf - ϕ
• An electron below the surface needs to do work/uses energy to reach the surface
• Kinetic energy of such an electron will be less than hf - ϕ
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## Electricity Practicals

When explaining a method, outline:

• How to get varied values of I and V
• The range of measurements
• How often to take readings
• At least 6 values are needed to plot a graph
• Ways to minimise error
• Advantages of using equipment such as a datalogger to get reliable results
• Able to take many readings
• Results can be displayed on a computer
• Safety points
• Repeat the method 3 times in total so you can calculate a mean
• Consider specific needs of the circuit components
• forward bias of diode
• maximum current
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## Stationary waves

A stationary wave is formed on a stretched string. Discuss the formation of this wave. Your answer should include an explanation of how the stationary wave is formed, a description of the features of the stationary wave and a description of the processes that produce these features.

• A wave and its reflection travelling in opposite directions, meet, interact, causing superpostion
• The two waves have the same wavelength (or frequency)
• A node is a point of minimum disturbance or zero displacement
• An antinode is a point of maximum amplitude
• Nodes are formed when two waves cancel each other out - destructive interference
• This is due to the 180o phase difference of the two waves at the node
• Antinodes are formed by constructive interference when the two waves are in phase
• Energy is not transferred along a stationary wave
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## Describing a velocity-time graph

When describing a velocity-time graph, mention

• The acceleration is the gradient
• Straight line is constant acceleration ie acceleration due to gravity
• Curve line is variable acceleration ie due to air resistance
• Flat line is constant velocity
• Newton I says the velocity remains constant is there is no resultant force
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## Comparing Gravitational and Electric Fields

For both radial and uniform gravitational and electric fields, compare and contrast their common features and their differences. You should consider the force acting between particles or charges; gravitational field strength and electric field strength and gravitational potential and electric potential.

Similarities

• In a radial field, both gravitational and electric involve an inverse square relationship.
• In both cases the force is proportional to a product (masses/charges)
• In a uniform field the force (magnitude and direction) is constant at all points
• Gravitational and electric field strength are both defined as a force per unit mass or charge.
• In both cases the field strength in a radial field is proportional to 1/r 2 .
• Both definitions of potential involve the work done per unit mass or charge.
• Both types of potential are proportional to 1/r in a radial field.

Differences

• Gravitational potential is always a negative but electric potential is negative for negative charges and positive for positive charges
• Gravitational forces are always an attraction whilst electric forces may be attraction or repulsion.
• A gravitational field is always directed towards the mass producing it whereas an electric field is directed towards a negative charge but away from a positive charge.
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## High Voltage Transmission

Discuss the principles involved in high voltage transmission systems, explaining why a.c. is used in preference to d.c. and how the energy losses are minimised

• current in cables causes joule heating equal to I^2xR
• resistance of cables should be as low as possible
• energy losses are reduced if current in cables can be reduced
• current can be reduced (for same power=IV) if voltage is increased
• voltages are changed using transformers, which work with ac but not with dc
• therefore, ac generation and transmission is essential
• the higher the voltage, the smaller the proportion of the input power that is wasted
• high voltage introduces insulation problems and raises safety issues
• voltage must be reduced as the supply reaches its consumers
• this is done in stages as the supply is moved from overhead cables to underground wires
• transformers cause energy losses because they are not perfectly efficient
• features are incorporated in the design of transformers to reduce losses from them
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## Conservation of momentum and energy

Discuss how the principles of conservation of momentum and conservation of energy apply

Momentum

• momentum is conserved because there are no external forces acting on the overall system
• because momentum has to be conserved, and it is a vector, the capsule must move along the original line of movement after the explosion

Energy

• total energy is always conserved in any physical process because energy can be neither created nor destroyed
• however, energy may be converted from one form to another
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## Geosynchronous and polar orbits

Compare the principal features of the geosynchronous and polar orbits and explain the consequences for possible uses of satellites in these orbits

Geosynchronous Orbits

• Orbital period matches Earth’s rotational period exactly and travels in the same direction
• Satellite maintains same position relative to Earth so only one particular orbit radius is possible
• Orbit height is greater than polar orbit satellite but speed is less than that of polar orbiting satellite
• Scans a restricted (and fixed) area of the Earth’s surface only
• Used for telecommunications generally, cable and satellite TV, radio, digital information, etc.
• Satellite is in continuous contact with transmitting/receiving aerial so it can be in a fixed position

Polar Orbits

• Orbital period is a few hours with many different orbital radii and periods possible
• Speed is greater than that of geosynchronous satellite
• Satellite scans the whole surface of the Earth and gives access to every point on Earth’s surface every day
• Used for surveillance of conditions on Earth, mapping, weather observations, environmental monitoring
• Contact with transmitting/receiving aerial is intermittent
• Lower signal strength required than that for geosynchronous satellite
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## Electromagnetic Induction

Key points to mention when explaining Electromagnetic induction

• An emf is induced whenever there is a change in the magnetic flux passing through a conductor
• The magnitude of the emf is proportional to the rate of change of magnetic flux linkage
• The induced emf will cause a current to flow in any complete circuit
• The induced current flows in such a direction as to oppose the increase in magnetic flux when the current is switched on in the coil.
• The current produces a magnetic field in the opposite direction to that produced by the coil.
• These two (alternating) fields interact like the fields between two facing like magnetic poles, giving repulsion.
• The current-carrying conductor is in a magnetic field, and consequently it experiences a force upwards
• The magnetic field to which it is exposed becomes weaker as it moves away from the coil
• This reduces the induced current, reducing also the magnetic force on the conductor
• The conducutor may reach a stable height if the magnetic force has decreased to the point where it is equal to the weight
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## Binary Star Systems

Determining orbital speed and period of binary star systems

• When one star passes in front of the other the amount of light received changes
• The brightest (lowest value of) apparent magnitude occurs when both stars can be seen.
• The dips occur when one star is in front of the other.
• The similarity in the dips suggests that both stars have similar temperatures/sizes
• The variation in wavelength is due to the Doppler effect
• The peaks and troughs occur when the stars are moving at their greatest velocity away from / towards us
• By observing absorption lines in the spectrum over time, the orbital period can be calculated
• Using the Doppler shift equations, orbital speed can be found
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## The Big Bang Theory

• The universe has expanded from a single hot dense point
• This expansion started approximately 13 billion years ago.
• Evidence comes from the Hubble relationship and observations of the red shift of distant galaxies.
• This shows that the galaxies are moving outwards from a single common point.
• Conclusive evidence comes from the cosmological microwave background radiation (which disproved the steady state theory)
• This follows a black body radiation curve which corresponds to a temperature of 2.7 K
• This can be interpreted as the left over “heat” of the big bang
• Hydrogen and helium is present in the Universe in the ratio 3:1
• This supports the idea that a very brief period of fusion occurred when the Universe was very young, which is consistent with the Big Bang theory.
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## Charged couple devices

Describe the structure and operation of the CCD and discuss the advantages of using a CCD for astronomical observations

• The CCD is a silicon chip
• The chip is divided into picture elements called pixels
• Incident photons are focused on the CCD
• The photons cause the release of electrons within the semiconductor via the photoelectric effect causing a potential difference in each pixel
• The number of electrons emitted is proportional to the intensity of the light.
• An electron pattern is built up which is identical to the image formed on the CCD.
• When exposure is complete the charge is processed to form an image.
• High quantum efficiency > 70%
• Light integration – using long exposure times to capture faint images.
• Device can be directly linked to computer for capture and analysis
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Siting

• Apart from visible and some parts of the radio wave section, all the other parts of the em spectrum are significantly absorbed by the atmosphere.
• To reduce the effects of absorption, IR telescopes are often placed in dry areas and/or very high up.
• X-ray and UV telescopes are generally put into orbit.
• To avoid atmospheric distortion, and light pollution, visible telescopes are often placed high up.
• To avoid interference from terrestrial sources, radio telescopes may be situated away from centres of population.

Size

• Telescopes are often built as large as possible in order to increase the collecting power, which is proportional to the diameter2 .
• The diameter of the objective of telescopes is also often as large as possible in order to improve the resolving power, as minimum angle resolved is proportional to 1/diameter.
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## Hydrogen Balmer lines

Describe how Hydrogen Balmer absorption lines are produced in the spectrum of a star.

• the atmosphere of the star has hydrogen atoms with electrons in the n=2 state
• light from the star passes through the atmosphere of the star
• electrons (in the n=2) are excited into higher energy states
• they can only absorb certain amounts of energy
• these certain energies are related to specific frequencies (E=hf)
• the electrons then de-excite
• the electrons may de-excite through different energy level changes
• when the electrons de-excite the light is radiated in all directions
• this means that the intensity of the light at particular frequencies is reduced, resulting in absorption lines
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## Evidence for accelerating rate of expansion

Explain why observations of supernovae led to the conclusion that the Universe is expanding at an accelerating rate and discuss why this conclusion is controversial.

• the absolute magnitude of type 1a supernovae is known, this allows supernovae to be used as standard candles
• using the inverse square law (or from values of absolute magnitudes) allows the distance to be calculated
• supernovae are very bright so they can be seen in very distant galaxies
• it has taken billions of years for the light from the most distant galaxies to reach Earth; these supernovae were therefore produced when the Universe was young
• measurement of red shift (to measure velocity) and use of Hubbleís Law shows that these supernovae are fainter than expected
• this indicates that the Universe is expanding faster now than when the supernovae exploded as the light has had to travel further to reach us than expected by a constant rate of expansion
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Make comparisons between these two methods of estimating the radius of a nucleus

Rutherford Scattering

• alpha scattering involves electrostatic repulsion
• only measures the least distance of approach, not the radius
• alpha particles cannot be detected when scattered by exactly 180o
• alpha particles have a finite size which must be taken into account
• Alpha particles are affected by the strong nuclear force

Electron Diffraction

• electron diffraction treats the electron as a wave having a de Broglie wavelength - the angle of the first minimum is used
• to have a small wavelength or wavelength comparable to nuclear diameter, the wavelength determines the resolution
• the wavelength needs to be of the same order as the nuclear diameter for significant diffraction
• the first minimum of the electron diffraction is often difficult to determine as it superposes on other scattering events
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## Kinetic Theory

By considering the motion of the molecules explain how a gas exerts a pressure and why the volume of the container must change if the pressure is to remain constant as the temperature increases

• molecules are in rapid random motion/many molecules are involved
• molecules change their momentum or accelerate on collision with the walls
• reference to Newton’s 2nd law either F = ma or F = rate of change of momentum
• reference to Newton’s 3rd law between molecule and wall relate pressure to force P = F/A
• mean square speed of molecules is proportional to temperature
• as temperature increases so does change of momentum or change in velocity compensated for by longer time between collisions
• as the temperature increases as the volume increases the surface area increases which reduces the pressure
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