# Phya4... again..

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#1
Hey, stuck on question 23 of the multiple choice question of june 2011's phya4 paper.

http://filestore.aqa.org.uk/subjects...W-QP-JUN11.PDF

It says that a bar magnet is pushed through a coil generating a current, firstly, i get that when the coils magnetic flux is cut (does this just mean pushed through?) by a magnet, an emf is induced, but i really don't understand why... something to do with lenzs law? :s

Secondly, the question says that the ammeter briefly shows a non zero reading because the MFL in the coil increases then becomes constant, why doesn't the MFL increase then decrease back to 0 when the magnet has left the coil?

Wouldn't an Emf be induced in one direction the an equal magnitude emf be induced in the opposite direction as the magnet leaves the coil? Does is matter which end of the magnet enters the coil first?

if Anyone can answer ANY of these question i'd be seriously grateful! xxxxxxxxxxxxxxxx
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5 years ago
#2
(Original post by physicso)
Hey, stuck on question 23 of the multiple choice question of june 2011's phya4 paper.

http://filestore.aqa.org.uk/subjects...W-QP-JUN11.PDF

It says that a bar magnet is pushed through a coil generating a current, firstly, i get that when the coils magnetic flux is cut (does this just mean pushed through?) by a magnet, an emf is induced, but i really don't understand why... something to do with lenzs law? :s
Cause and effect:

The coil of wire is a conductor, meaning it is made up of atoms that have electrons in their outer shells which can leave the parent atoms.

Electrons are charged particles and are affected by magnetic fields. Pushing the magnet through the coil will force the electrons in the coil to be deflected in accordance with Flemings rules.

Any movement of electrons is defined as a current. So by pushing a magnet through the coil, lines of flux are cut by the coil and a current in the coil will flow.

From ohms law V = I/R so the movement of electrons against the resistance of the coil and any load connected to it will create a voltage drop across the coil.

(Original post by physicso)
Secondly, the question says that the ammeter briefly shows a non zero reading because the MFL in the coil increases then becomes constant, why doesn't the MFL increase then decrease back to 0 when the magnet has left the coil?

Wouldn't an Emf be induced in one direction the an equal magnitude emf be induced in the opposite direction as the magnet leaves the coil? Does is matter which end of the magnet enters the coil first?

if Anyone can answer ANY of these question i'd be seriously grateful! xxxxxxxxxxxxxxxx
You need to read the questions very carefully:

It says "Pushed into the coil..................until it comes to rest inside the coil"

You are correct that if the magnet goes through the coil and leaves the other side, then the emf induced will first be in one direction as it enters the coil and then as it leaves the coil the induced emf would be in the opposite direction.

However, the magnet stop moving inside the coil.

When stationary, no lines of flux are cut and therefore no emf is induced.
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#3
(Original post by uberteknik)
Cause and effect:

The coil of wire is a conductor, meaning it is made up of atoms that have electrons in their outer shells which can leave the parent atoms.

Electrons are charged particles and are affected by magnetic fields. Pushing the magnet through the coil will force the electrons in the coil to be deflected in accordance with Flemings rules.

Any movement of electrons is defined as a current. So by pushing a magnet through the coil, lines of flux are cut by the coil and a current in the coil will flow.

From ohms law V = I/R so the movement of electrons against the resistance of the coil and any load connected to it will create a voltage drop across the coil.

You need to read the questions very carefully:

It says "Pushed into the coil..................until it comes to rest inside the coil"

You are correct that if the magnet goes through the coil and leaves the other side, then the emf induced will first be in one direction as it enters the coil and then as it leaves the coil the induced emf would be in the opposite direction.

However, the magnet stop moving inside the coil.

When stationary, no lines of flux are cut and therefore no emf is induced.

Ahhh cool, yeah cheers, for some reason PHYA4 is the only paper i ever have to rush... but anyway.

Could you give me a really quick (simple as possible) explanation as to why the emf is induced according the lenz law? i know there will be a force on the magnet that slows it down through the magnet and it's opposite to the direction of the induced emf but other than that i'm stuck... does it depend on which pole enters the coil first? :s
0
5 years ago
#4
(Original post by physicso)
Ahhh cool, yeah cheers, for some reason PHYA4 is the only paper i ever have to rush... but anyway.

Could you give me a really quick (simple as possible) explanation as to why the emf is induced according the lenz law? i know there will be a force on the magnet that slows it down through the magnet and it's opposite to the direction of the induced emf but other than that i'm stuck... does it depend on which pole enters the coil first? :s
A conductor in a changing magnetic field will have a current induced in it provided the flux lines are cut.

But you also know that a current in a conductor will produce it's own magnetic field.

So the fact that the external magnetic field produced a current in the conductor, that very same current will produce it's own magnetic field which will oppose the motion or field that created it in the first place. i.e. a so called back emf is also generated. Lenz's law in other words.

Yes it depends on the pole polarity. Use Flemings right hand rule for generators which will tell you the direction of the induced current, and Flemings left hand rule for motors which tells you the direction of the opposing force.
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