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Edexcel PHY 5 Revision - need help with any questions?

Here is the thread for revising Unit 5 (Written) of Edexcel Physics: Fields and Forces.

Post (relevant to PHY5):

- Questions
- Answers
- Tips and techniques

Good luck all. :wink:

Scroll to see replies

Reply 1

Mohit_C

Ok, I need help with one question.
"Explain why, if a satellite slows down in its orbit, it nevertheless gradually spirals in towards the Earth's surface."
I need to prove this mostly with equations. Can someone help?

Reply 2

Eau

F =mv^2/r

If it slows down, then v decreases - then r also have to decrease (at a faster rate) to maintain the centripetal/resultant force provided by the Earth's gravitational force.

This is my guess - not certain - answer.

Reply 3

Mohit_C

It seems somewhat right to me. Can anyone confirm?

Reply 4

TSR Community Staff

If posting in this thread, please make sure you either ask, answer or comment about a purely acadmeic question (eg asking for help on a specific exam question, or asking about why something works - like Mohit_C and Eau have done). Any questions/comments about how you are revising, how you are feeling about this exam etc should be done in the A Level sub-forum. Requests for past papers/mark schemes etc should be made in the past paper thread in the A Level sub-froum.

Reply 5

Eau

Question:

How do I know when magnetic fields repel? When they are acting in different directions right?

But don't they cancel and form a neutral point?

Reply 6

Mohit_C

Well as far as I know, you need to take some form of measurement to ensure that they repel or attract, usually a top pan balance. Take a reading from it and note another reading when the presence of another magnet exists in its surroundings. If there's an increase or decrease in readings, they repel or attract respectively.

Well if the magnets are both of the same kind with same magnetic field strength, they would create a neutral point in the region between them but what you would be measuring is the change in reading from the repelling.

Not too sure about this second one. Can anyone confirm?

Reply 7

sufiankane

Eau

Question:

How do I know when magnetic fields repel? When they are acting in different directions right?

But don't they cancel and form a neutral point?

Even though a neutral point is formed, i dont believe the magent itself lies in the neutral area and hence repels.

Reply 8

sng

The magentic field is a vector, it has both a direction and a magnitude associated with it. The direction of the field is the direction which a north pole would move if placed at this point. At neutral point occurs when two north/south poles are facing each other, and we get a point between them where the magentic fields of the two magnets is equal in magnitude and opposite in direction. If another north pole were placed here, it would experience no NET force as there is no NET magnetic field acting at this point (of course, we have a conductor in a magnetic field and so we would get forces acting on it, but no net force).

Reply 9

Eau

*bump*

Any questions?

Reply 10

VariableResistor

can someone explain neutral points to me. i must sound like a complete moron but this is one part of PHY5 that has always got me. also im still majorly struggling with all forms of homogenous equations with whatever past paper i try

Reply 11

Eau

Neutral point is where two (or more) (grav/electric/magnetic) fields cancel each other, so there is no resultant field - no force is felt (by mass/charge/moving charge).

Cancelling occurs, say, when two fields at that point have the same resultant field strength, but in the opposite direction.

For homogeneity of equations, there are some tricks to work out units, such as the units of energy, you could determine with work done = force * distance, then force = mass * acceleration. Practice makes improvement.

Reply 12

VariableResistor

thanks for that eau.

also, in a couple of papers, i've seen questions on transformers. i know the actual basis of the question is about electromagnetic induction, but do we need to know transformers for PHY5?

Reply 13

sebbie

I think we need to know roughly how it works, here is what you get 5 marks for in one paper:

Ac/Changing current in the primary
Produces a changing B field
B field carried through core (to secondary)
Changing B field over secondary induces emf
Explanation in terms of the turns ratio formula

How do I know what directions forces and magnetic fields act given a current? I can never work it out

Reply 14

Mohit_C

Using the Right Hand rule only works for a wire, where thumb is the direction of the current and the way your fingers point indicate whether the magnetic field is clockwise/anti-clockwise.
Is this what you mean?

Reply 15

fabbo

question from phy5 june 05 question 3d

http://homepage.ntlworld.com/keith.taggart/physics/revisionASA2.htm

i understand the spreading/deflection of electrons but not which is the fastest - the mark scheme just says fastest labelled.
would it be the one that is most deflected as F = Bqv or the one that is least deflected as it has been in the field the shortest time?

Reply 16

Mohit_C

I would think that the fastest electrons are the ones that deflect the least as they essentially last a shorter period of time within the field but according to the equation F=Bqv, it seems like the fastest electrons are the most deflected :s-smilie:

+++EDIT+++ Okay I just realized that you're confusing electric and magnetic fields. F=Bqv is for magnetic fields...which isn't the case with two plates. So basically the fastest electrons are those that deflect the least.