# Magnetic field and capacitor questions

## Homework Statement

PROBLEM 1
For question 5bi) 5why is the lower pole north and the upper pole south?

could someone also please explain 5aii)

PROBLEM 2
How do I do questions
5 ai, b

thanks

## The Attempt at a Solution

PROBLEM 1: I need to use flemmings left hand rule. I am confused because I found north to be the upper pole

PROBLEM 2:
5ai, b) I am not sure if a current is felt in a uniform field. Flux is still being cut but apparently no current is induced in position 2 and the current is greater in position 3 than 1 BUT I DON'T KNOW WHY?

thanks

Last edited by a moderator:

Why is the lower pole N and the upper S?
I think you have done exactly the right thing but you must realise that the LH rule gives you the force on the WIRE.... so which way is the force on the MAGNET ?
The reading on the scales has INCREASED

Problem 2
First of all do you realise that only the sections QR and PS will experience any effect due to motion through the magnetic field?
Look at QR coming into the magnetic field. From Problem 1 can you see that electrons in this wire will be forced down.... the wire forms part of a complete loop so electrons will flow round this loop from Q to R to S to P to Q. NOW this is the way ELECTRONS are flowing (so there is a current!!!).... which way is the CURRENT?
When the complete loop is in the field moving can you see that QR and SP will 'cancel out'.... that is not a good explanation.... can you word it better.
And when PS is leaving the field area?.... You can do it
KEY POINTS
1) Fleming's LEFT HAND rule for force on a moving charge
2) CURRENT is the direction + charges would move. In metal wires the charges that move are electrons (- charges). In solutions it is possible to get both -ve electrons and +ve ions moving so you need to get a good feel for the variety of moving charges.

for the problem 1:
the force on the magnet is DOWN
the current is right
thus the magnetic field is out of the plane of the page? how is this bottom to top?

problem 2:
why do the electrons in QR experience a downward force. the magnetic field is into the plane of the page thus the field is at right angles. Why cant it be up?

You have all the right ideas at hand!!!The left hand rule gives you the direction of the FORCE on the Current carrying conductor in a magnetic field.
You see that the force on the magnet is DOWN.... the reading on the balance increases. This must be because the current in the WIRE causes a downwards force on the magnet.... so the Wire must be experiencing an UP force.
The wire pushing UP means the magnet is being pushed DOWN.
Problem2
IMAGINE that the section QR is full of + charges, it is moving to the right and the field is INTO the page.
Now do the LEFT HAND rule.... Field into paper... point your first finger into paper.... + charges moving right ....point your second finger to the right.... your thumb should be pointing UP.
This means that + charges (REPEAT>>>> + Charges) would experience an UP force.
BUT.... the charges free to move in a wire are electrons - charges. So the force on electrons in QR is DOWN.
Electrons (-charges) will be forced around clockwise.... this means the CURRENT will be anti-clockwise (read that again) so will flow UP from R to Q

This can be very confusing physics because all of the rules are based on the fact that CURRENT is a flow of +charges but what flows in wires is a stream of -charges. !!!!

If you live in England you drive on the left..... can you cope with driving on the right!!!
(I can't!!!)

but the thumb represents motion NOT second finger?

oh I get it

thanks