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

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Reply 3600
Avatar for bwr19
bwr19
2
Original post by _Caz_
Is it A? If it is I can explain it to you :smile: just want to make sure it's right first...


It is, yeah!
when calculating rsultant electric fields, is it E1+E2 if the charges are negative and positive and E1-E2 if the charges are the same?? (like two positives)
Original post by saad97
Could you explain how the get the right answer please? Ican't seem to get it.
Also these types of questions are on every paper, is there a specific way to do them because I always get them wrong :frown:


Ok gonna be hard to explain.

Basically make a formula for F.

F = 1 / r^2 (ignore charges and constant because we are talking about factors)

and we know that

0.5F = 0.5 x 1/ (2x10^-3)
time this by 2 to give F = 1/2(20 x10^-3)

So we have 2 equations for F. Meaning the R value is the same so equate.

r^2 = 2(20x10^-3) and then solve
This is the hardest momentum question I've seen, any ideas??

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1433960816767.jpg31.4KB
Original post by thedon96
Hi, in the formula booklet, the theta is not the angle of the coil to the field lines, it's the angle the NORMAL to the coil makes to the field lines. That's why if a coil's normal is perpendicular to the field lines (hence the coil is parallel), emf is a maximum because E = BANsin(theta) and when if the coil's normal is perpendicular, theta is pi/2 making E a max. Hope this helps


What's confusing is that i thought Emf and flux linkage were proportional (Faraday's law) - but the equations show that flux linkage uses cos, while emf uses sign, which means they reach a maximum value and minimum value at different angles?
Original post by bwr19
It is, yeah!


Right okay, the formula for emf is BANwsin(wt). Sin functions vary between -1 and 1 so max emf will be when sin(wt)=1. Max emf=BANw. As you know, Flux linkage is BAN. So if you make BAN the subject of the formula you get maxemf/w=flux linkage (BAN) . From circular motion you know that w=2*pi*f so you substitute w with that and it gives you A :smile:
Original post by Dante991
I got A too, can you talk me through how you did it?


I just posted an explanation just before this post :smile:
Original post by lilygriffin
13) lets call Q1Q2/4piEo k so it's easier to work with

F=k/d^2 for the first distance
0.5F=k/0.002^2 for the second distance

Times second distance by 2 to get F=2k/0.002^2

Make the F's equal and cancel the k

1/d^2=2/0.002^2

Rearrange to get d=(0.002^2/20)^0.5 which is 0.014... so B


Original post by Dante991
Ok gonna be hard to explain.

Basically make a formula for F.

F = 1 / r^2 (ignore charges and constant because we are talking about factors)

and we know that

0.5F = 0.5 x 1/ (2x10^-3)
time this by 2 to give F = 1/2(20 x10^-3)

So we have 2 equations for F. Meaning the R value is the same so equate.

r^2 = 2(20x10^-3) and then solve


Thank you.
Original post by Kingnig
Does anyone has a list of past grade boundaries?


See the first post :smile:
Reply 3609
Avatar for Deddy
Deddy
1
Original post by saad97
Could you explain how the get the right answer please? Ican't seem to get it.
Also these types of questions are on every paper, is there a specific way to do them because I always get them wrong :frown:


Sorry, I missed out the squares for the distances. You should end up with
0.5/d^2 =1/(20x10-3)^2 , then d=14mm.

I think that the best approach is to look at what they give you and think, if the Force is F at one point, and 0.5F at another, then they are multiples of eachother so you can equate them.

Another question I saw like this was when they told you that gravitational potential at one point was 45V and another was 50V, again same approach the 45 is 0.9(50).
Original post by huniibehi
This is the hardest momentum question I've seen, any ideas??

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Mass is 0.5kg.

The acceleration on the moon is 10/6, hence its velocity will change by 30/6 from 3 seconds of falling = 5 metres/second.

Momentum = mv = 0.5*5 = 2.5?
Wow some of these questions have got me feeling like Sterling with a 1v1 against de gea
Original post by Lau14
See the first post :smile:


Oops, the nerves are already kicking in XD
Original post by utvctang
What's confusing is that i thought Emf and flux linkage were proportional (Faraday's law) - but the equations show that flux linkage uses cos, while emf uses sign, which means they reach a maximum value and minimum value at different angles?

Yeah that's exactly right but EMF is not proportional to flux linkage, it is proportional to the RATE OF CHANGE of flux linkage so the maxima and minima of flux linkage happen when the normal to the coil is parallel to the field lines and the maxima and minima of EMF happen when the normal is perpendicular to the field lines.
Original post by huniibehi
This is the hardest momentum question I've seen, any ideas??

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the velocity after falling for 3 seconds: use v=u+at
where a = g/6

mass = mg

momentum = mass * velocity
Original post by Kingnig
Oops, the nerves are already kicking in XD


Happens to all of us! Just got to stay calm in the exam
Reply 3616
Avatar for bwr19
bwr19
2
Original post by _Caz_
Right okay, the formula for emf is BANwsin(wt). Sin functions vary between -1 and 1 so max emf will be when sin(wt)=1. Max emf=BANw. As you know, Flux linkage is BAN. So if you make BAN the subject of the formula you get maxemf/w=flux linkage (BAN) . From circular motion you know that w=2*pi*f so you substitute w with that and it gives you A :smile:


Clever. Thanks a lot!
Reply 3617
Avatar for a.a.k
a.a.k
2
Emf=Blv
Emf=emf zero*sin( 2*pi*f*t)

These are not in the data sheet.

Anyother plz share

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(edited 8 years ago)
Original post by oonic0rn
when calculating rsultant electric fields, is it E1+E2 if the charges are negative and positive and E1-E2 if the charges are the same?? (like two positives)


Yes, that's right. It always helps to draw a little diagram though.

+ve E1 -------------------> charge E2 -----------------> -ve

+ve E1 ---------------------> charge E2 <--------------------- + ve
Reply 3619
Avatar for CD223
CD223
OP
10
Original post by Dante991
Wow some of these questions have got me feeling like Sterling with a 1v1 against de gea


Some have got me feeling like David Moyes' time as Utd manager.


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