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

*** 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]

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

Past grade boundaries (courtesy of Lau14):

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Old past papers and mark schemes:

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Recommended revision guide for the A2 exams (as flagged up by Disney0702):

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Useful revision website for topic by topic questions on PHYA4 (as flagged up by Kennethm):

Spoiler

Previous 6 markers for PHYA4 (as flagged up by Lau14):

Spoiler

Lau14's INCREDIBLY helpful guide to the A2 EMPA:

Spoiler

OTHER RESOURCES:

Disney0702's INCREDIBLY helpful recommended online revision guide:

Spoiler

Lau14's INCREDIBLY helpful scanned handwritten notes:

Spoiler

(edited 8 years ago)

Scroll to see replies

Sorry you've not had any responses about this. :frown: Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get more responses. :redface: Hopefully someone will be able to get back to you :h:
Reply 2
How are we finding the multiple choice, people? :s-smilie:
Reply 3
Original post by JaySP
How are we finding the multiple choice, people? :s-smilie:


Hard! But not as hard as the six markers!


Posted from TSR Mobile
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 ?
Reply 5
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?
Reply 6
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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Reply 7
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
Reply 8
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?


Posted from TSR Mobile
Reply 9
Original post by CD223
Aha me too! What do you find hardest in this unit?


Posted from TSR Mobile


I don't know tbh. Magnetism gets pretty hard but there are some hard questions in all topics
Reply 10
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.
Reply 11
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
Reply 12
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.
Reply 13
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!!




Posted from TSR Mobile
Reply 14
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!!
Reply 15
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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Reply 16
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.
Reply 17
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.
Reply 18
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 :biggrin:
Reply 19
Original post by wmavfc
Its OK - you can just do loads of practise on all of the ..6.. past papers :biggrin:


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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