AQA Physics A - PHYA4 (11/06/12) - Exam thread

I thought it says somewhere in the textbook that electric fields don't change the velocity of the particle or something along the lines of this:

''no work is done by the magnetic field on the particle as the force always acts at right angles to the velocity of the particle . Its direction of motion is changed by the force but not the its speed. The kinetic energy of the particle is unchanged by the magnetic field''

thats on page 113 on nelson thornes aqa textbook

that threw me off too, but know that the emf is at a maximum when the coil is cutting MOST of the field, as emf = deltaBAN/t , so when the face of the coil is parallel to the field. i.e the change in area or b field usually, as its' difficult to quickly wrap wires on to coils quickly!

i think that sentence meant instead of the coil turning clockwise/anticlockwise perpendicular to the field, it would be turning anticlockwise/clockwise in the same plane, so there would be no change in area cut even though the the area cut is a maximum. a little bit confusing and should really have a diagram considering it's not the easiest topic at all (my worst topic infact)

Hey guys, stuck on q.16, 17 and 22 on this paper:
http://store.aqa.org.uk/qual/gce/pdf/AQA-PHYA4-1-W-QP-JAN10.PDF

For 16, the right answer is C but I thought electric potential is 0 whenever the electric field strength is 0 surely?
For 17, how do you do this question?
For 22, what is the exact reason for the answer being C? Is it because since the speed of the electron and the magnetic field are not perpendicular, it has no effect on the electron hence no force is exerted on it, therefore it just continues to move as if there was no magnetic field?

Thanks!

16- Its asking which one is incorrect, so you're right C

17- well ke is 0 cause at that point its velocity is 0
- ep is maximum at p because its getting closer
-energy is always constant cannot be created or destroyed..
- alpha particle cant have zero energy otherwise it would literally be non existant must be C

22- Yeh it cant cut any field lines if it travels parrell, key word is that there in the same plane. answer is C

No but I thought C was correct

For 17, how do you know Ep is a max at P? I thought Ep is max when the distance from the object is greatest?

No but I thought C was correct

For 17, how do you know Ep is a max at P? I thought Ep is max when the distance from the object is greatest?

There is a transfer of potential to kinetic energy happening here. At the point P it is momentarily stationary, and so all of it's kinetic energy has been transferred to potential energy.

Can anyone explain how to do questions where you have 2 point charges (-16x10^-6C and +4x10^-6C), a distance between them (120mm) and you're required to find the point between them where the potential/resultant force is 0? I've got this type of question wrong a few times now and can't seem to get my head round it.

Can anyone help me with these questions:

What would the period of rotation of the Earth need to be if objects at the equator were to appear weightless? radius of earth=6.4 x 10^6

Also, for a step up transformer, why is secondary current less than the primary current?

Thanks

I do them using ratios. From the example above you know that the 16C charge is 4 times larger than the (-)4C charge, so their electrostatic forces will act in the ratio 1:4. So the point of zero resultant force is: (120/5) x 1 = 24mm from the -4C sphere or (120/5) x 4 = 96mm from the 16C sphere.

Can anyone help me with these questions:

What would the period of rotation of the Earth need to be if objects at the equator were to appear weightless? radius of earth=6.4 x 10^6

Also, for a step up transformer, why is secondary current less than the primary current?

Thanks