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

High 5 lol hopefully ISA will pull it up

Are people in agreement with my unofficial mark scheme (see first post) for the multiple choice? Now just need to fill out the written.


Posted from TSR Mobile

Can I just ask how question 20 is -332g? I've seen the same thing on June 2011 Section A but I don't understand how a top pan balance can have a negative reading because the balance is zeroed before the switch is close, meaning the 0g weight is with the magnet on top and no force from the magnetic field. Surely when the current is reversed and doubled the reading on the balance is just zero? I obviously know that doubling current in opposite direction would double the force in the opposite direction (-116 x 2) but the balance surely has to still read zero? I'm so confused, just seems like common sense to me! Please help me to understand why the mark scheme opposes obvious logic.

Can I just ask how question 20 is -332g? I've seen the same thing on June 2011 Section A but I don't understand how a top pan balance can have a negative reading because the balance is zeroed before the switch is close, meaning the 0g weight is with the magnet on top and no force from the magnetic field. Surely when the current is reversed and doubled the reading on the balance is just zero? I obviously know that doubling current in opposite direction would double the force in the opposite direction (-116 x 2) but the balance surely has to still read zero? I'm so confused, just seems like common sense to me! Please help me to understand why the mark scheme opposes obvious logic.

Ok so the balance is first zeroed.
when a current passes through the wire it experiences a force, since the balance reading goes up the force must be upwards. This then produces an equal and opposite force on the magnet which acts to increase its weight.

Now when the current is reversed and doubled, the force acting on the wire acts downwards and the force is doubled.
This causes an equal and opposite force on the magnet upwards, Which produces the effect of less weight. (since force is doubled the weight is twice as less)
Hope that helps.

basically imagine pushing down on the magnet that would increase the reading of the scale.
Now imagine pulling up on the magnet but dont lift it off, its scale reading would go down.

It's simply that the balance allows to show a negative figure when pulled upwards above zero..... When the force pushes the balance down you get +166 but when the balance is raised up (by twice the force)....well you've done the maths! That's all there is to it!

It's simply that the balance allows to show a negative figure when pulled upwards above zero..... When the force pushes the balance down you get +166 but when the balance is raised up (by twice the force)....well you've done the maths! That's all there is to it!

But the balance cannot tell the size of this force? As in the force could be 10N upwards, 100000N upwards... The balance has no way of knowing this force. So logic dictates that the balance reading just returns to zero?

I know the maths obviously it's simple, but the logic doesn't add up.

Sorry to anyone if I appear to be arrogant or rude I am just annoyed at this!!!

But the balance cannot tell the size of this force? As in the force could be 10N upwards, 100000N upwards... The balance has no way of knowing this force. So logic dictates that the balance reading just returns to zero?

I know the maths obviously it's simple, but the logic doesn't add up.

Sorry to anyone if I appear to be arrogant or rude I am just annoyed at this!!!

Ok imagine some old fashioned hanging scales.... If a giant blows down on the scales the reading increases.... If he blows the scale up... The reading decreases..... The balance will show more of a magnitude in readings the harder he blows (the larger the force applied) even though this force is invisible!

This thread has been great for answering each other's questions and helping one another out. I personally feel like the MC was a lot easier due to this thread. Thanks for that everyone!

Let's smash PHYA5 now - perhaps the number of posts on that thread can't beat this in 6 days though 😂😂


Posted from TSR Mobile

That really doesn't help lol :P Are you saying that the force on the wire created by the magnetic field (upwards, after current is reversed) also creates a force in the opposite direction of equal and opposite size and direction?