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AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] Watch

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    *** OFFICIAL AQA PHYA4 FIELDS AND FURTHER MECHANICS JUNE 2015 EXAM DISCUSSION THREAD ***

    UNOFFICIAL MARK SCHEME:

    SECTION A:
    1. Weight (D)
    2. Momentum of Y is equal and opposite to the momentum of the alpha particle (A)
    3. After collision T1 is at rest and T2 moves at 4.0ms^-1 (A)
    4. Maximum acceleration is directly proportional to the amplitude (D)
    5. Graph with constant negative gradient passing through the origin (D)
    6. Maximum kinetic energy of the oscillating system is 2.5*10^-3J (B)
    7. 2.33 seconds (C)
    8. 4R (B)
    9. Pi*pGMR/3 (A)
    10. Field strength of Mars = 13.4NKg^-1 (B)
    11. Field strength is zero at mid point (A)
    12. Incorrect that Gravitational potential is a vector quantity (C)
    13. GM/r^2, (GM/r)^1/2 (D)
    14. Speed increases, period decreases (C)
    15. 4.09*10^-7 Vm^-1, upwards (A)
    16. n = 2, X = g, a = G, b = mass (C)
    17. Number of electrons 4.4*10^10 (C)
    18. 1000s (D)
    19. Incorrect that total energy taken from the battery during the charging process is 2mJ. (B)
    20. -322g (A)
    21. Coil with 0.5L by 0.5L (B)
    22. Particle follows circular path at right angles to a uniform magnetic field (A)
    23. Fx/Fy = 1/(root 2) (C)
    24. Ip = 0.35A (B)
    25. To increase the mechanical strength of the cables (D)

    SECTION B
    1.
    (a) forced vibrations are when theres vibrations that are forced that aren't at its natural frequency with a phase difference and different amplitude
    resonance is when the driving frequency=natural frequency causes oscillations large amplitude pi/2 phase diff.

    Forced oscillations:
    When a periodic force (PF) is applied to a natural oscillator (NO) (Which has a natural frequency).
    If PF has frequency much lower than NO, then NO oscillates with roughly same amplitude as periodic force. Small/no phase difference.
    If PF has frequency much higher than NO, then NO oscillates with small amplitude. Variable phase difference and amplitude. Max amplitude when PF is pi/2 radians out of phase with NO and minimum amplitude when PF is in phase with NO.

    (b) Which one shows greater damping?
    The cone with the ring
    The cone without the ring
    The ring does not affect damping.
    4 marks.

    (1) Cone with ring shows less damping.
    3 marks from:
    Damping reduces the amplitude of oscillations/total energy of system (over time) (1)

    Air resistance is same for both (the cone with and without ring) (1)

    Reducing speed reduces kinetic energy (and therefore total energy) of cone (and therefore amplitude of oscillations) (1)

    The ring increases the mass of the cone:
    Application of Newton's second law to deduce that an equal force has less effect on the speed of an object with greater mass.

    OR

    Air resistance does work against cone. Use of E_k = (1/2)mv^2 to deduce that the same work done on a heavier object reduces its speed less.
    (1 maybe 2)

    2.
    (a) Coulomb's law- proportional to product of magnitudes of charges, inversely proportional to square of separation..

    (b) V is proportional to 1/r. V is negative for a negative point charge.
    A positive test charge will have to do work against the field to reach a point in this field from infinity.

    (c) Show that the magnitude of the charge is 27.8 nC (approx 30nC).

    (d) Work done to move from r=0.2m to r=0.5m
    = 4.5x10^-5 J

    (e) Electric field strength at r= 0.4m
    = 1560 V m^-1

    3.
    Area under the curve gives the initial charge on the capacitor before discharge (1)
    [Between time=0 and time=a large value, probably > 5RC (1) ]

    Draw curve of current for 300kOhm resistor below 150 ohm graph. Lower y intercept. Shallower gradient, longer time constant.

    (a) 1.4m (height gained by 3.5N weight)

    (b) Energy losses due to...
    some energy converted to chemical energy or sound energy
    not 100% efficient in transferring to potential energy

    4.
    (a) (i) When switch S is closed, the ammeter deflects and then returns to 0.
    A current passes through P so a magnetic field is (suddenly) created in iron bar.
    There is a change of flux through Q, inducing an current (or emf that causes a current) in Q.
    After, current through P is DC/magnetic field is constant so no change of flux through Q => no current in Q (any longer).

    (ii) When the variable resistance is suddenly increased,
    (The ammeter) deflects in the opposite direction (to when switch S was closed) and then returns to zero.
    Sudden increase of resistance causes sudden decrease of current through P. This causes a sudden decrease in the magnetic field strength so a sudden decrease in flux through Q. (When switch S was closed it was an increase so current is in opposite direction). This induces a current in Q.

    (b) 0.0605 Wb turns

    (c) 0.121V

    5.
    (a) 131 degrees

    (b) 12.3N

    (c) 2.01 revolutions

    (d) Newton's first:
    - An object will remain at rest or in uniform motion if not acted upon by an resultant force.
    - Can be demonstrated by cutting the string while the ball is being spun: There is no longer a horizontal force acting on the ball, so it moves off in a straight line (in the horizontal plane) with a constant horizontal velocity (albeit under freefall in the vertical plane).

    Newton's second:
    -Force is rate of change of momentum
    -String provides the centripetal force.
    -Which acts at right angles to the ball's velocity.
    -This causes a centripetal acceleration.
    -Velocity is constantly changing direction, even though speed is not.
    -Therefore momentum is always changing.
    -Therefore there must be a resultant force.
    -Resultant force is constant in magnitude since rate of change of momentum is constant in magnitude.

    Newton's third:
    -Every action has an equal and opposite reaction.
    -The tension in the string acts on the ball.
    -The ball exerts an equal and opposite force on the string.
    -In real life, the Earth exerts a gravitational force downwards on the ball.
    -The ball exerts a gravitational force upwards on the Earth.

    Not horizontal in real life:
    -The Earth exerts a gravitational force on the Earth downwards.
    -This is equal to the ball's weight, W=mg.
    -The ball has a non-zero mass.
    -The ball does not accelerate towards the ground, so the resultant vertical force on the ball is zero.
    -Therefore there is a force acting upwards on the ball equal to its weight.
    -This can only be provided by the tension in the string.
    -The tension only has an upwards vertical component if it is below the horizontal. (Use of T=m g sin(theta))
    -A higher tension means the string can be closer to the horizontal.
    -This can be achieved by spinning the ball faster, as then the string must provide a larger centripetal force and so has greater tension.

    Date: 11th June 2015
    Time: 09:00am
    Duration: 1h 45m
    _________________________

    PHYA5 EXAM DISCUSSION THREAD:
    http://www.thestudentroom.co.uk/showthread.php?t=3047351

    ______________________________

    RESOURCES:
    My personal revision guide for PHYA4!
    Spoiler:
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    Past grade boundaries (courtesy of Lau14):
    Spoiler:
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    Old past papers and mark schemes:Recommended revision guide for the A2 exams (as flagged up by Disney0702):Useful revision website for topic by topic questions on PHYA4 (as flagged up by Kennethm):
    Spoiler:
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    Previous 6 markers for PHYA4 (as flagged up by Lau14):
    Spoiler:
    Show
    January 2010 – Describe two causes of the energy losses in a transformer and discuss how these energy losses may be reduced by suitable design and choice of materials.

    June 2010 – Deep space probes often carry modules which may be ejected from them by an explosion. A space probe of total mass 500 kg is travelling in a straight line through free space at 160 ms–1 when it ejects a capsule of mass 150 kg explosively, releasing energy. Immediately after the explosion the probe, now of mass 350 kg, continues to travel in the original straight line but travels at 240 ms–1, as shown in Figure 2. Discuss how the principles of conservation of momentum and conservation of energy apply in this instance.

    January 2011 – A student was required to design an experiment to measure the acceleration of a heavy cylinder as it rolled down an inclined slope of constant gradient. He suggested an arrangement that would make use of a capacitor-resistor discharge circuit to measure the time taken for the cylinder to travel between two points on the slope. The principle of this arrangement is shown in Figure 2. [diagram needed!] Describe the procedure the student should follow, including the measurements he should make, when using this arrangement. Explain how he should use the measurements taken to calculate the acceleration of the cylinder down the slope. (I found this question easier doing part b first).

    June 2011 – 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.

    January 2012 – Figure 5 shows an experimental arrangement that can be used to demonstrate magnetic levitation. The iron rod is fixed vertically inside a large coil of wire. When the alternating current supply to the coil is switched on, the aluminium ring moves up the rod until it reaches a stable position ‘floating’ above the coil. By reference to the laws of electromagnetic induction explain
    •why a current will be induced in the ring,
    •why the ring experiences a force that moves it upwards,
    •why the ring reaches a stable position.

    June 2012 – The amount of energy required to move a manned spacecraft from the Earth to the Moon is much greater than that required to return it to the Earth. By reference to the forces involved, to gravitational field strength and gravitational potential, and to the point X, explain why this is so.

    January 2013 – These geosynchronous and polar satellites have different applications because of their different orbits in relation to the rotation of the Earth. Compare the principal features of the geosynchronous and polar orbits and explain the consequences for possible uses of satellites in these orbits. In your answer you should explain why:
    •a low polar orbit is suitable for a satellite used to monitor conditions on the Earth.
    •a geosynchronous circular orbit above the Equator is especially suitable for satellite used in communications.

    June 2013 – Gravitational fields and electric fields have many features in common but also have several differences. For both radial and uniform gravitational and electric fields, compare and contrast their common features and their differences. In your answer you should consider:
    •the force acting between particles or charges
    •gravitational field strength and electric field strength
    •gravitational potential and electric potential.

    June 2014 – The circuit in Figure 6 contains a cell, an uncharged capacitor, a fixed resistor and a two-way switch. [diagram needed]. The switch is moved to position 1 until the capacitor is fully charged. The switch is then moved to position 2. Describe what happens in this circuit after the switch is moved to position 1, and after it has been moved to position 2. In your answer you should refer to:
    •the direction in which electrons flow in the circuit, and how the flow of electrons changes with time,
    •how the potential differences across the resistor and the capacitor change with time,
    •the energy changes which take place in the circuit.
    Lau14's INCREDIBLY helpful guide to the A2 EMPA:
    Spoiler:
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    Physics Practical Work – A2 EMPA

    There are two assessed practical sessions (tasks 1 and 2), each one hour long and carried out in lessons under exam conditions, and a written section (duration: 1hr 15). The total mark for the EMPA is 55, with mark distribution varying very slightly between the three parts – usually 15-16 for each practical task and 23-24 or so for the written section. The EMPA is worth 60 UMS (half as much as the other papers, 20% of the year).

    There are four written papers available (June 2010-2013) on the AQA website, but the practical sections are only available from 2013.


    Measuring instruments – range and precision

    The range of a measuring instrument is the lowest and highest reading it can measure (e.g. a thermometer may measure -20 to 100°C).

    The precision of a measuring instrument is one scale division (this term may be used differently in other subjects, this is the definition for AQA Physics), e.g. the precision of the thermometer may be ±1°C. Usually given as a ± value, but the ± might be omitted sometimes.

    Significant figures

    Any reading you take must be given to the correct number of significant figures (e.g. when measuring with a ruler of precision ±0.001m, the reading of half a metre must be written as 0.500, not as 0.5. 50.0cm and 500mm is also acceptable, as long as your units are correct and consistent).


    Dependent and independent variables

    The independent variable readings are those chosen by you (or the ones you are told to take) before carrying out the experiment.
    The dependent variable readings are those that you measure. You must take a repeat reading of this.
    E.g. if you are looking at V/I characteristics you could set voltage to particular values and then measure the current. The voltage is the independent and the current is the dependent variable.


    Recording your results

    There are at least 5 marks available for this in total.

    Two marks are for drawing a results table with a ruler and having the independent variable in the left hand column.
    Include a separate column for any values calculated to plot a graph (e.g. if you have to plot a graph of 1/x but you’ve measured x, add a column for 1/x).

    There are two marks for using the correct number of significant figures for every reading (see above).
    There is another mark for including all units in the table headings. Use either the name of the unit in full or the accepted abbreviation.


    Graph plotting

    There are at least 9 marks available for this.
    The origin does not have to be included unless proportionality is to be shown. If the origin is included mark on both “0”s.

    Choose a scale so that the points cover more than 50% of each axis of A4 graph paper (28x20cm), so horizontal separation of first and last points must not be less than 10cm and vertical separation of first and last points must not be less than 14cm. There are 2 marks available for this. Also pick a scale that is easy to plot and take readings for (e.g. 1cm = 10 units, 2 units or 4 units. Not 1cm = 3 units or 7 units).

    Label both axes of the graph and include the units. There are 2 marks for doing this correctly.

    All points must be accurately plotted (within a distance of 1mm or less from the correct position) with “x” or “+”. There are 3 marks for this (check it!).

    Most graphs will be straight lines. There will be 1 mark for drawing a line of best fit correctly. It should have approximately equal numbers of points on either side of the line, and shouldn’t be influenced by obviously anomalous results. Anomalous results should be indicated if ignored. If the plotted points suggest a curve, a smooth curve should be drawn.

    You may have to find the gradient of graphs. Mark on the graph a large triangle that takes up most of the graph (each side should be minimum 8cm). Write on the triangle the value of the height and the value of the base with the units of each. The gradient = height/base, and the units of the gradients are units of height/units of base.

    Where an intercept is required it can either be read directly from the graph, or a suitable calculation may be required.


    Random errors

    Random errors are always present when an experiment is carried out. They can be reduced or minimised but never removed entirely. They are also described as the uncertainty in the measurement. They can be reduced by taking repeat readings. E.g. a measurement of length using a metre ruler can be given as (25.6 ± 0.1)cm. The units must be included.

    Uncertainty in the measurement from a set of results (e.g. a measurement is repeated 8 times to give 8 values for the length) can be found by first calculating the mean. However, this cannot be given to a higher number of significant figures than the original values. Then find the difference between the mean and the furthest away value and ± it.

    E.g. 8 readings of diameter of a wire are measured with a micrometer to the nearest 0.01mm
    0.30, 0.28, 0.29, 0.27, 0.28, 0.26, 0.31

    The values are all different, but not because a mistake has been made. The micrometer is only capable of giving readings to ±0.01mm, so this would be the error even if all readings were the same. There is also a small variation in the diameter of the wire.

    The mean of these values is 0.2825mm, 0.28mm to the correct number of significant figures. The smallest reading is 0.28mm, and the largest is 0.31mm. The differences between these and the mean are 0.02 and 0.03mm, so the diameter is (0.28 ± 0.03)mm.

    Minimum uncertainty – as mentioned above, there is always an uncertainty in each and every measurement and if there is no variation in readings then the uncertainty is the precision of the measuring instrument.


    Percentage uncertainty

    The most accurate reading is the one with the smallest percentage uncertainty. % uncertainty = (actual uncertainty/reading) x 100.


    Systematic errors

    Systematic errors affect all readings by the same amount and are usually “zero errors”, for example:

    Zero errors in meters. This is where the meter doesn’t read zero when disconnected. Corrections can be made by adding or subtracting the required amount. E.g. if an ammeter reads +0.01A, 0.01A must be subtracted from every reading.

    Zero errors in other measuring instruments. This is where the instrument does not read zero even when the reading should obviously be zero. E.g. a micrometer that has been “zeroed” properly should read 0.000 when the jaws are tightened with nothing in between them. If they do not read 0.000, there is a zero error which can be corrected in the same way.
    OTHER RESOURCES:

    Disney0702's INCREDIBLY helpful recommended online revision guide:Lau14's INCREDIBLY helpful scanned handwritten notes:
    Spoiler:
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  4. File Type: pdf AQA-PHYA4-1-QP-MultipleChoice-Jun14.pdf (88.9 KB, 2937 views)
  5. File Type: pdf AQA-PHYA4-2-QP-Written-Jun14.pdf (784.5 KB, 1088 views)
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    Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get more responses. Hopefully someone will be able to get back to you
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    How are we finding the multiple choice, people?
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    (Original post by JaySP)
    How are we finding the multiple choice, people?
    Hard! But not as hard as the six markers!


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    Unit 4 seems manageable the main issue I'm having at the moment is also multiple choice questions. The exam is 1.45 hour so it'll be difficult staying focus and attentive for such long length.

    Anyone got any good resources or tips for multiple choice ?
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    I'm concerned about the lack of past papers. I have mocks next week and am wary of just doing all the papers I can find. As far as I know there are only around 6 official papers to this specification and I'm concerned that I'll just learn them if I keep doing them which isn't really very good for revision. I know there's something called 'exampro' which might help but there isn't much information about it. It seems to be extra papers made by AQA but costs £80 which I'd have to try and get the school to pay for I guess.
    Does anyone have any advice/ resources they can share?
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    (Original post by wmavfc)
    I'm concerned about the lack of past papers. I have mocks next week and am wary of just doing all the papers I can find. As far as I know there are only around 6 official papers to this specification and I'm concerned that I'll just learn them if I keep doing them which isn't really very good for revision. I know there's something called 'exampro' which might help but there isn't much information about it. It seems to be extra papers made by AQA but costs £80 which I'd have to try and get the school to pay for I guess.
    Does anyone have any advice/ resources they can share?
    In in the same position. Slightly worrying. Also a plea for help haha.


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    (Original post by CD223)
    In in the same position. Slightly worrying. Also a plea for help haha.


    Posted from TSR Mobile
    I look at all these papers that there are for maths that have been made up and get so jealous. Lol
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    (Original post by wmavfc)
    I look at all these papers that there are for maths that have been made up and get so jealous. Lol
    Aha me too! What do you find hardest in this unit?


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    (Original post by CD223)
    Aha me too! What do you find hardest in this unit?


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    I don't know tbh. Magnetism gets pretty hard but there are some hard questions in all topics
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    I've never enjoyed the electricity sections, and so probably find it the hardest. Doesn't help that we haven't fully finished covering the content on the electricity section, so not looked at it as indeph as other areas.
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    (Original post by JaySP)
    I've never enjoyed the electricity sections, and so probably find it the hardest. Doesn't help that we haven't fully finished covering the content on the electricity section, so not looked at it as indeph as other areas.
    I quite like capitance but emf is difficult
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    (Original post by wmavfc)
    I quite like capitance but emf is difficult
    I can't comprehend it in my head as well as the mechanics. But I agree capicitance isn't bad.
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    (Original post by wmavfc)
    I don't know tbh. Magnetism gets pretty hard but there are some hard questions in all topics
    (Original post by JaySP)
    I've never enjoyed the electricity sections, and so probably find it the hardest. Doesn't help that we haven't fully finished covering the content on the electricity section, so not looked at it as indeph as other areas.
    (Original post by wmavfc)
    I quite like capitance but emf is difficult
    (Original post by JaySP)
    I can't comprehend it in my head as well as the mechanics. But I agree capicitance isn't bad.
    I don't like EMI as there's so many ways of expressing the same equation!!




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    Just did a mock of this paper and struggled with the timing. First paper I've ever not finished with time to spare. Worrying lack of time to properly think!!
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    (Original post by wmavfc)
    Just did a mock of this paper and struggled with the timing. First paper I've ever not finished with time to spare. Worrying lack of time to properly think!!
    Yeah it sounds a decent amount of time for the number of questions but the MC takes more time than you think!!


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    (Original post by CD223)
    Yeah it sounds a decent amount of time for the number of questions but the MC takes more time than you think!!


    Posted from TSR Mobile
    Yeah like you still have to work through every question (or most, at least) to get the answer. Had a look at some grade boundaries though - June 2014 it was 53/75 for an A, and it's usually around that in the past years. That makes the paper seem slightly less daunting, at least.
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    (Original post by wmavfc)
    Yeah like you still have to work through every question (or most, at least) to get the answer. Had a look at some grade boundaries though - June 2014 it was 53/75 for an A, and it's usually around that in the past years. That makes the paper seem slightly less daunting, at least.
    Hmm, I seem to be getting around 45ish which is a bit off-putting at the minute because I need an A for uni haha.
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    (Original post by CD223)
    Hmm, I seem to be getting around 45ish which is a bit off-putting at the minute because I need an A for uni haha.
    Its OK - you can just do loads of practise on all of the ..6.. past papers
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    (Original post by wmavfc)
    Its OK - you can just do loads of practise on all of the ..6.. past papers
    Were there more than 6 for the AS exams? I remember there being more than that. That, or I just kept repeating the papers.
 
 
 
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