You are Here: Home >< Physics

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

1. (Original post by thedon96)
Hi,
So in the first scenario the magnetic force is acting downwards causing there to be a reading of 112g on the mass balance. When the current is reversed, the scale should become negative since the force is now acting upwards (the opposite direction - Fleming's left hand rule). Also, since the current has been doubled and F = BIl (so F is directly proportional to I), the force will also double so that's why the answer is A. I think hahah
This has been going on for so long. It just doesn't make sense does it!

Posted from TSR Mobile
2. Haha, it genuinely makes no sense at all - I mean, are there even any mass balances that record "negative" masses?! It's ridiculous
If current is doubled, then you can say the force is doubled.

As F= b i l

and as it is reversed the force is now in a different direction which is up.

So there's no need to talk about Newton's third law?
4. (Original post by Master Sam)
So there's no need to talk about Newton's third law?
Not that I'm aware of.

Its a question that requires literally no calculation. Just knowledge of magnetic fields. I'm not a fan of them either.
5. Hey guys,

Is there a significant difference between an AC electric motor and AC an electric generator? I thought they were the same thing?

thanks
6. In a perfectly elastic collision is there a change in momentum?
Thanks
7. (Original post by Master Sam)
Hey guys,

Is there a significant difference between an AC electric motor and AC an electric generator? I thought they were the same thing?

thanks
I think an electric motor is a type of electric generator so effectively hey are the same thing
8. Tips for the multiple choice? like with the rearranging ones?
9. (Original post by toddle1)
In a perfectly elastic collision is there a change in momentum?
Thanks
Yes because it's an "elastic" collision, it's like throwing a tennis ball at a wall, it will collide with the wall and change direction and so change momentum. Also kinetic energy is conserved so the change in momentum must be equal to 2mv (2xmassxvelocity).
At least I'm sure thats right.
10. (Original post by toddle1)
In a perfectly elastic collision is there a change in momentum?
Thanks
Yes, but total momentum is always constant provided no external forces act so momentum before=momentum after. (On the other hand If you're asking about change in magnitude of momentum then the answer is 'not always'.)
I'm not exactly confident. I've hit the acceptance phase where it is what it is. Just keep working like. I need a B overall.

I know you'll do awesome mate!

I have WJEC C4 friday and thats what I'm not looking forward to.
Thanks!

Don't worry about it man, it's already done. The grade boundaries won't be as hight so you will be fine. Try to compensate for your other exams.

Not confident enough but Im just hoping it will fine which in fact won't be.
With maths, oh dear, I've not seen my maths teacher since easter.
Cheers

Posted from TSR Mobile
12. (Original post by xela238)
Tips for the multiple choice? like with the rearranging ones?
There isn't a magic pill that will make you better -- it takes practice. Can you post a specific example of 'rearranging ones'?
13. Why does a transformer need alternating current?! (I know this is trivial, I could answer it easily just a few days ago before I crammed my brain with unit 5!)
14. (Original post by NEWT0N)
Why does a transformer need alternating current?! (I know this is trivial, I could answer it easily just a few days ago before I crammed my brain with unit 5!)
something to do with the voltage not being able to be stepped down/up if it was DC but Im not sure why either
15. (Original post by NEWT0N)
Why does a transformer need alternating current?! (I know this is trivial, I could answer it easily just a few days ago before I crammed my brain with unit 5!)
Isn't it because DC won't produce an induced EMF in the secondary coil, because there is no change in flux linkage with a DC current?
16. (Original post by AR_95)
something to do with the voltage not being able to be stepped down/up if it was DC but Im not sure why either
Well I definitely remember it having something to do with the flux through the secondary coil.
(Original post by jf1994)
Isn't it because DC won't produce an induced EMF in the secondary coil, because there is no change in flux linkage with a DC current?
Ah yes, it's coming back: A changing magnetic field is needed in the secondary coil because (by Faraday's law) the rate of change of flux linkage needs to be nonzero to induce an emf in the secondary coil. A direct current will produce a steady field so rate of change of flux linkage = 0 --> no induced emf, which is bad. Great, thanks!
17. What's the difference between AC and DC
18. (Original post by AR_95)
What's the difference between AC and DC
Alternating current flows in both directions, direct is one direction only.

Posted from TSR Mobile
19. Is the definition of time constant simply- the time take for the voltage to reduce to 37% or 1/e^1 its original value

Posted from TSR Mobile
20. Aqa are a nightmare when they make multiple choice questions with NO CORRECT ANSWER!! Or textbooks with the WRONG ANSWER!!! oh the precious time I have waisted!!!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 22, 2016
Today on TSR

### Congratulations to Harry and Meghan!

But did you bother to watch?

### What do you actually do at University?

Poll
Useful resources

Can you help? Study help unanswered threadsStudy Help rules and posting guidelinesLaTex guide for writing equations on TSR

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE