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# Edexcel Unit 4: Physics on the Move 6PH04 (11th June 2015) watch

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1. (Original post by BP_Tranquility)
In the formula E=V/d -
Is d always the separation between the plates or is d the distance from the point charge between the plates to the positive plate?

And is V the potential difference between the 2 parallel plates or the potential difference between point charge and positive plate?
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Between plates - the pd is the same no matter where between the plates the charge is placed.

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2. (Original post by BP_Tranquility)
In the formula E=V/d -
Is d always the separation between the plates or is d the distance from the point charge between the plates to the positive plate?

And is V the potential difference between the 2 parallel plates or the potential difference between point charge and positive plate?
Posted from TSR Mobile
The distance between the plates. Electric field strength is constant at any point (being the force per unit charge), so it couldn't ever be the distance between the point charge and the plates.

V is between the plates, too
3. Can anyone help me with this question? http://gyazo.com/becbe370d9a8a46ba14c0b1145439a1f
The markscheme says "must be accelerating due to circular motion, (speed constant but) direction/velocity changing'
I don't understand how that has anything to do with X-rays (I thought it was relativistic somehow :/)
So yeah, any help would be much appreciated

And also, this question: http://gyazo.com/9ed5fea18b599c1ef4afe5d760fa1cf3
I thought mg /was/ the centripetal force?

Oh and also this one haha http://gyazo.com/4f06fef95afb458101d16c4c127d3f08
Could someone explain what is happening in each of A-D? Thankyou!!!!
4. anyone got any notes on magnetic fields?
5. (Original post by cerlohee)
Can anyone help me with this question? http://gyazo.com/becbe370d9a8a46ba14c0b1145439a1f
The markscheme says "must be accelerating due to circular motion, (speed constant but) direction/velocity changing'
I don't understand how that has anything to do with X-rays (I thought it was relativistic somehow :/)
So yeah, any help would be much appreciated

And also, this question: http://gyazo.com/9ed5fea18b599c1ef4afe5d760fa1cf3
I thought mg /was/ the centripetal force?

Oh and also this one haha http://gyazo.com/4f06fef95afb458101d16c4c127d3f08
Could someone explain what is happening in each of A-D? Thankyou!!!!
1. Any charged particle being accelerated (in this case the acceleration is the change in direction due to circular motion) emits radiation called synchrotron radiation, which, in this case, are X-rays.

2. They are asking for the normal reaction force, not the centripetal force.
So the forces acting are centripetal force downwards (assume it's separate from the weight, obvious since there is circular motion at the start of the track even though the weight would have been acting perpendicularly), weight downwards and normal reaction upwards:

Fcentri = W - R
Rearrange to find R and you get answer D.

3. Think about when an emf is induced - according to Faraday's law when there is a change in magnetic flux linkage. And according to Lenz's law, due to conservation of energy, this will be in the opposite direction.

Note that electromagnetic induction only happens when there is a change in magnetic flux *linkage*, ie magnetic flux times number of coils, and not magnetic flux density, which, in practice, means that no emf is induced if the coil is stationary relative to the magnetic field, ie "when there is a change in number of magnetic field lines being cut" (thus A, C and D are eliminated). In A there is motion but it is parallel to the field since they are in the same plane so the same number of field lines are being cut - they are just closer together. (actually I'm not 100% sure about this, but even if there were an emf induced it would be in the opposite direction according to Lenz's law, since the increase in flux linkage in Y is in the direction of Y's current). In B you are going from a magnetic field to no magnetic field meaning that there is a change in magnetic flux linkage so an emf is induced. Since the change in Y is essentially the opposite of the current previously flowing, according to Lenz's law the current in X will be the opposite of the opposite, ie the same direction as in Y previously.

Please correct me if I said anything wrong - physics is not my strong point. Also for next time, it would be helpful could also post either a link to the paper/mark scheme or the correct answer.
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6. (Original post by bbyturtlexox)
anyone got any notes on magnetic fields?
This site is pretty good:
http://www.physicsandmathstutor.com/...dexcel-unit-4/
Magnetic fields notes:
http://ow.ly/NHpTd
Magnetic field questions:
1. http://ow.ly/NHq0R
2. http://ow.ly/NHq4y
7. How do I master Magnetic fields??? I know the basic content bit I always stumble when I come across application questions for this topic

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8. This is annoying me right now. Question 8 of Jan 2013 multi choice-

'Conductor of length 50mm carries currnt of 3A at 30° to a magnetic field of magnetic flux density 0.40T. What is the magnitude of magnetic force acting on the conductor?'

Mark scheme says it's 0.030N. But F=BIl sin0, and 0.40 x 50x10-3 x 3 x sin(30) = 0.059... what am I missing?!?
9. (Original post by jay_em)
This is annoying me right now. Question 8 of Jan 2013 multi choice-

'Conductor of length 50mm carries currnt of 3A at 30° to a magnetic field of magnetic flux density 0.40T. What is the magnitude of magnetic force acting on the conductor?'

Mark scheme says it's 0.030N. But F=BIl sin0, and 0.40 x 50x10-3 x 3 x sin(30) = 0.059... what am I missing?!?
You did the right method - I got the mark scheme answer with the same calculation. Check whether your calculator is in degree mode (radian mode gives your answer).

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10. (Original post by aersh8)

Note that electromagnetic induction only happens when there is a change in magnetic flux *linkage*, ie magnetic flux times number of coils, and not magnetic flux density, which, in practice, means that no emf is induced if the coil is stationary relative to the magnetic field, ie "when there is a change in number of magnetic field lines being cut" (thus A, C and D are eliminated). In A there is motion but it is parallel to the field since they are in the same plane so the same number of field lines are being cut - they are just closer together. (actually I'm not 100% sure about this, but even if there were an emf induced it would be in the opposite direction according to Lenz's law, since the increase in flux linkage in Y is in the direction of Y's current). In B you are going from a magnetic field to no magnetic field meaning that there is a change in magnetic flux linkage so an emf is induced. Since the change in Y is essentially the opposite of the current previously flowing, according to Lenz's law the current in X will be the opposite of the opposite, ie the same direction as in Y previously.

Please correct me if I said anything wrong - physics is not my strong point. Also for next time, it would be helpful could also post either a link to the paper/mark scheme or the correct answer.
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For the bolded part, wouldn't an EMF be induced if there was a change in the magnetic field strength? Eg if it was connected to an alternating current even if it wasn't moving? And wouldn't a change in magnetic flux density always result in a change in flux linkage?
Yes I think I agree with the first thing you said about A, as the magnetic field produced by the coil is in the direction away from X, with a constant number of lines of flux going through?
Your explanation for B makes a lot of sense thankyou!!
And for C, the decrease in resistance would lead to an increased EMF, so there an increase in flux lines through X leftways, so its going to produce a magnetic field rightwards and the corkscrew rule says that current is going to flow anticlockwise, so it's not C...
Thank god this makes sense, was struggling so much!!

If anyone else was confused by this question, this is the video I watched to help visualize what was going on https://www.youtube.com/watch?v=wXcg7GnwssU

11. (Original post by aersh8)
You did the right method - I got the mark scheme answer with the same calculation. Check whether your calculator is in degree mode (radian mode gives your answer).

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Thanks- that seems to have fixed it. Good job I found this out before the exam itself!
12. (Original post by cerlohee)
For the bolded part, wouldn't an EMF be induced if there was a change in the magnetic field strength? Eg if it was connected to an alternating current even if it wasn't moving? And wouldn't a change in magnetic flux density always result in a change in flux linkage?
Yes I think I agree with the first thing you said about A, as the magnetic field produced by the coil is in the direction away from X, with a constant number of lines of flux going through?
Your explanation for B makes a lot of sense thankyou!!
And for C, the decrease in resistance would lead to an increased EMF, so there an increase in flux lines through X leftways, so its going to produce a magnetic field rightwards and the corkscrew rule says that current is going to flow anticlockwise, so it's not C...
Thank god this makes sense, was struggling so much!!

If anyone else was confused by this question, this is the video I watched to help visualize what was going on https://www.youtube.com/watch?v=wXcg7GnwssU

Yeah it would, by the bolded bit I meant if it was constant
Yes, what you said makes sense, I didn't bother to try using the hand rules for this question - I find it easiest to think GCSE style in terms of field lines being cut, although for long answer questions more "sophisticated" terminology is needed.
No problem
13. magnetic field questions in exam papers absolutely infuriate me
14. June 2012 has to be the crappiest paper in the history of Edexcel Physics unit 4 papers. 60% of it is just magnetism and electricity.
Thank God I was only in Year 10 then.

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15. http://www.thestudentroom.co.uk/show...1#post56555831
Could someone please explain me this?! its about capacitance ive attached the question!
Ive attached a screen shot of a question from june 2014 where the switch is moved so that the capacitor is discharged through a resistor , isnt the current in the circuit doing to increase? In the markscheme its mentioned that the current will decrease exponentially , shouldn't it increase? Because charges will flow in the circuit?
16. (Original post by Fatima SJ)
http://www.thestudentroom.co.uk/show...1#post56555831
Could someone please explain me this?! its about capacitance ive attached the question!
Ive attached a screen shot of a question from june 2014 where the switch is moved so that the capacitor is discharged through a resistor , isnt the current in the circuit doing to increase? In the markscheme its mentioned that the current will decrease exponentially , shouldn't it increase? Because charges will flow in the circuit?
It starts off high (so an initial very sharp increase due to the high p.d across the capacitor encouraging lots of charge flow. However, as it discharges, the p.d across the capacitor decreases exponentially, V=Voe^-t/RC, which means that current is less "encouraged" to flow. You can tell this is an exponential decrease in current as V=IR, so V and I are proportional. This means that current is going to follow the same decay during discharging.

17. (Original post by cerlohee)
It starts off high (so an initial very sharp increase due to the high p.d across the capacitor encouraging lots of charge flow. However, as it discharges, the p.d across the capacitor decreases exponentially, V=Voe^-t/RC, which means that current is less "encouraged" to flow. You can tell this is an exponential decrease in current as V=IR, so V and I are proportional. This means that current is going to follow the same decay during discharging.

So the current will always decrease exponentially when a capacitor is discharged . Yep this makes sense. Thanks alot!
18. (Original post by Fatima SJ)
So the current will always decrease exponentially when a capacitor is discharged . Yep this makes sense. Thanks alot!
Exactly! Welcome
19. Does anyone know of any places to get a load of difficult unit 4 questions?

I found this custom made unit 4 paper on TSR but it doesn't have a mark scheme- would greatly appreciate it if someone made one though!!
http://www.thestudentroom.co.uk/show....php?t=1160855
20. Can anyone explain these 2 questions to me?

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