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

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Hey guys,

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

thanks

Reply 3061

toddle1

In a perfectly elastic collision is there a change in momentum?
Thanks :smile:

Reply 3062

toddle1

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? :s-smilie:

thanks

I think an electric motor is a type of electric generator so effectively hey are the same thing

Reply 3063

xela238

Tips for the multiple choice? like with the rearranging ones?

Reply 3064

Plasmapause

Original post by toddle1

In a perfectly elastic collision is there a change in momentum?
Thanks :smile:

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.

(edited 8 years ago)

Reply 3065

CD223

Original post by Adangu

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!

Original post by Mehrdad jafari

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

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Reply 3066

AR_95

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

Reply 3067

jf1994

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?

Reply 3068

AR_95

What's the difference between AC and DC

Reply 3069

CD223

Original post by AR_95

What's the difference between AC and DC

Alternating current flows in both directions, direct is one direction only.

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Reply 3070

gcsestuff

Is the definition of time constant simply- the time take for the voltage to reduce to 37% or 1/e^1 its original value

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Reply 3071

Sbarron

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!!!

Reply 3072

Sbarron

Original post by gcsestuff

Is the definition of time constant simply- the time take for the voltage to reduce to 37% or 1/e^1 its original value

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Yep or just T=CR

Reply 3073

Sbarron

Original post by Sbarron

Yep or just T=CR

Could also replace voltage with current or charge!!
Remember if there is a variable resistor keeping current constant then the circuit is NOT EXPONENTIAL! As I got caught out with this calculating something on a past question!

Reply 3074

Sbarron

Original post by Plasmapause

Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please

Reply 3075

aprocrastinator

Hey guys could anyone explain question 17 on this paper please: http://filestore.aqa.org.uk/subjects/AQA-PHYA41-QP-JUN14.PDF the answer is A but I'm just confused as to why the potential energy is positive...

(edited 8 years ago)

Reply 3076

Adangu

Original post by Sbarron

Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please

I think of it like change of momentum = mv-mu

because it is bouncing back and it is an elastic collision. One of the velocities can be said to be negative as it is in the opposite direction.

So the change in momentum would be mv-(-mv)
which is 2mv

Reply 3077

Plasmapause

Original post by Sbarron

Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please

Just prove it to yourself. Change in momentum = difference in velocity x mass. If you had a tennis ball that had a mass of M and when you throw it against the wall, it's velocity before colliding with the wall is V. Because it's an elastic collision, and the mass doesn't change and it's moving in the opposite direction at the same speed (so it's moving at a velocity of -V ). Difference in velocity is the velocity before collision minus the velocity after collision so ( V - -V) so basically V+V, hence 2V and so the momentum is 2MV (mass of the tennis ball did not change).

Reply 3078

gcsestuff

ImageUploadedByStudent Room1433866084.783941.jpg

Why is 8 not c.

I'm imaging a ball on a turntable. When it slides of how does it come of I a vertical plane?

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Reply 3079

Plasmapause

Original post by gcsestuff

Why is 8 not c.

I'm imaging a ball on a turntable. When it slides of how does it come of I a vertical plane?

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It's not C, because when the string snaps there is no force acting on it in the horizontal plane, however because gravity is acting on the string, it would be D.

"When released it goes off at a tangent to the circle... as if it had been thrown horizontally. Gravity affects its vertical velocity - acceleration in the vertical direction; it goes at constant velocity in the horizontal direction. It therefore follows a parabolic path when viewed in a verical plane."