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# CAlling all physicists to the rescue... watch

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1. Ok, this is the third time i attempt to post this thread, because...bla bla bla...i need to stop my self from waffeling on.

OK...Can someone please explaim Flemings Left hand and Right hand rule i know about movement field current lol...but other than that im really confused, when do you use which.

Also if anyone would be kind to explain anything about eletromagnetism, that would be greatly appreciated, je suis tres confused! Especially about why wires move up and down if they are flowing through magnets.

Thank you very much for your help.

2. (Original post by ThornsnRoses)
Ok, this is the third time i attempt to post this thread, because...bla bla bla...i need to stop my self from waffeling on.

OK...Can someone please explaim Flemings Left hand and Right hand rule i know about movement field current lol...but other than that im really confused, when do you use which.

Also if anyone would be kind to explain anything about eletromagnetism, that would be greatly appreciated, je suis tres confused! Especially about why wires move up and down if they are flowing through magnets.

Thank you very much for your help.

Fleming's Left Hand Rule is for Force experienced by a moving charge (i.e.current) in a magnetic field.

Fleming's Right Hand rule...you use for finding the direction of an induced current when magnetic flux linkage changes (opposite of above). This might be when a magnet moves (hence the filed changes) and there's a wire in the constantluy changing field.

G
3. I vaguely remember that from GCSE physics - dont think i understood it tho - sorry
4. (Original post by gzftan)
Fleming's Left Hand Rule is for Force experienced by a moving charge (i.e.current) in a magnetic field.

Fleming's Right Hand rule...you use for finding the direction of an induced current when magnetic flux linkage changes (opposite of above). This might be when a magnet moves (hence the filed changes) and there's a wire in the constantluy changing field.

G
So...FLHR is just to find out the force using the current...so i point my thrid finger the way the current is moving and i can work out the force. Can i also use it to work out the field and other things that way?

FRHR...i dont get what an induced current is...how do you know that the magnetic flux thingy has changed (can you please explain it)....

thanx...
5. (Original post by ThornsnRoses)
Ok, this is the third time i attempt to post this thread, because...bla bla bla...i need to stop my self from waffeling on.

OK...Can someone please explaim Flemings Left hand and Right hand rule i know about movement field current lol...but other than that im really confused, when do you use which.

Also if anyone would be kind to explain anything about eletromagnetism, that would be greatly appreciated, je suis tres confused! Especially about why wires move up and down if they are flowing through magnets.

Thank you very much for your help.

the LEFT hand one is to remember the direction of movement and the right hand one is to remember the direction of the induced current
6. (Original post by ThornsnRoses)
So...FLHR is just to find out the force using the current...so i point my thrid finger the way the current is moving and i can work out the force. Can i also use it to work out the field and other things that way?

FRHR...i dont get what an induced current is...how do you know that the magnetic flux thingy has changed (can you please explain it)....

thanx...
Ok..if you put a wire carrying current into a magnetic field....the movement of charge means is cuts the lines of flux of magnetic field. This means that the particles carrying the current (ie the wire) will experience a force.

The way to remember it is...
first finger....field direction
second finger.....current direction (conventional + => - )
thumb........movement of wire/particle

FHRH...when there is no current in a wire.....but the wire moves relative to the magnetic field (ie lines of flux are cut)....there is said to be a change in magnetic flux linkage in the wire. This means that an EMF is induced across the wire....the EMF causes the electrons to move..and thus a current flows.

G
7. (Original post by gzftan)
Ok..if you put a wire carrying current into a magnetic field....the movement of charge means is cuts the lines of flux of magnetic field. This means that the particles carrying the current (ie the wire) will experience a force.

The way to remember it is...
first finger....field direction
second finger.....current direction (conventional + => - )
thumb........movement of wire/particle

FHRH...when there is no current in a wire.....but the wire moves relative to the magnetic field (ie lines of flux are cut)....there is said to be a change in magnetic flux linkage in the wire. This means that an EMF is induced across the wire....the EMF causes the electrons to move..and thus a current flows.

G
WOW im starting to get it...i hope this actually doesnt come up. So correct me...but the left hand rule is if there is current already flowing and it affects the magnetic field, so there is a force experienced so the wire either goes up or down...and you use the LHR to work it out.

The RHR is if there is no current, but the magnetic 'induces' an electro magnetic force...so there is a current and electricity is flowing through the wire
8. (Original post by ThornsnRoses)
WOW im starting to get it...i hope this actually doesnt come up. So correct me...but the left hand rule is if there is current already flowing and it affects the magnetic field, so there is a force experienced so the wire either goes up or down...and you use the LHR to work it out.

The RHR is if there is no current, but the magnetic 'induces' an electro magnetic force...so there is a current and electricity is flowing through the wire
Smacked it!!!! Btw- which exam is this for???

G
9. (Original post by gzftan)
Smacked it!!!! Btw- which exam is this for???

G
I love you...physics GCSE tomorrow...
So now for the hardest bit...i need to apply this knowledge in questions. Are commutators to make the wire spin in the direction of the force?
10. So in an exam do they tell you things, like there is already a current flowing through the wire or that a current has been induced?
11. (Original post by ThornsnRoses)
I love you...physics GCSE tomorrow...
So now for the hardest bit...i need to apply this knowledge in questions. Are commutators to make the wire spin in the direction of the force?
Lol...in that case..you don't need to know about the flux linkage i don't think!! Lol..that's my A2 physics...whoops

Ok...if you have a DC current flowing throught the wire of the solenoid in the 'motor'...using FLHR..u'll find that it will rotate in one direction....

However....past the vertical....because the current flow has been reversed on each side of the solenoid...ie...past the vertical...the point rotating upwards will want to stay at the vertical...as it still experinces an upwards force on the other side of the solenoid axis.

Therefore..you uise a split ring commutator....what this does is reverse the current in the wirse every half turn it goes through...which means that the motor will continue to rotate in the same direction..as opposed to stopping at the vertical position.

Hope this helps

G
12. (Original post by ThornsnRoses)
So in an exam do they tell you things, like there is already a current flowing through the wire or that a current has been induced?
Sorry..i can't remember much from GCSE format of papers...twas a long time ago. But i'd assume there'd be question like giving you current flow in a wire...and field direction..and then ask you to figure out what happens to the wire.

G
13. (Original post by gzftan)
Lol...in that case..you don't need to know about the flux linkage i don't think!! Lol..that's my A2 physics...whoops

Ok...if you have a DC current flowing throught the wire of the solenoid in the 'motor'...using FLHR..u'll find that it will rotate in one direction....

However....past the vertical....because the current flow has been reversed on each side of the solenoid...ie...past the vertical...the point rotating upwards will want to stay at the vertical...as it still experinces an upwards force on the other side of the solenoid axis.

Therefore..you uise a split ring commutator....what this does is reverse the current in the wirse every half turn it goes through...which means that the motor will continue to rotate in the same direction..as opposed to stopping at the vertical position.

Hope this helps

G
OK...so if there was no commutator then the current will alternate...amd the spinning thing would go up the vertical and then down the same way instead of spinning around?

Also am i right so assume that if they ask about generators...i used the FRHR, because the magnets induce the current, and it its a motor it would be the LHR
14. (Original post by ThornsnRoses)
OK...so if there was no commutator then the current will alternate...amd the spinning thing would go up the vertical and then down the same way instead of spinning around?

Also am i right so assume that if they ask about generators...i used the FRHR, because the magnets induce the current, and it its a motor it would be the LHR
No...only for DC current you'd use a split ring commutator....cos the current doesn't change direction..but needs to change every half turn to keep it spinning. Without the commutator..the current will remain constant..and the motor will stop at the vertical position. I don't recall ever having to use motor with AC current.

Motor = FLRH
Dynamo = FRHR

G
15. (Original post by gzftan)
Motor = FLRH
Dynamo = FRHR

G
Btw- dynamos and generators are essentially the same..only difference is that generators rotate coil in a magnetic field to induce a current....dynamos rotate a magnet within a field..both induce current

G
16. G,

do you by any chance go to tiffin boys?
17. (Original post by ThornsnRoses)
G,

do you by any chance go to tiffin boys?
Yep...although..not for much longer!!!! you're at TGS right?

G
18. (Original post by gzftan)
Yep...although..not for much longer!!!! you're at TGS right?

G
how did i guess! YES...but not for much longer either...im going to be CLSG now...hehe

You must be so upset about leaving though...im kinda devastated...its like the end of an era for me...oh well.
19. (Original post by ThornsnRoses)
how did i guess! YES...but not for much longer either...im going to be CLSG now...hehe

You must be so upset about leaving though...im kinda devastated...its like the end of an era for me...oh well.
CLSG???

Yeh...leaving when i've spent 7 years there is...odd..to say the least...oh well..onwards to pastures new and all that stuff....i'm so looking fwd to uni though!!!!

G
20. (Original post by gzftan)
CLSG???

Yeh...leaving when i've spent 7 years there is...odd..to say the least...oh well..onwards to pastures new and all that stuff....i'm so looking fwd to uni though!!!!

G
A City of London School Girl...yeah 5 years means a life time...but its gone by so quick, and you really dont appreciate it till its gone. The prospect of never being a tiffin girl again is quite sad...now im a city girl lol, which probably suits me much better than town girl.

What you doing and where are you going for uni.

G...i just got 100% on the electromagnetic BBC bitesize test, i know it means nothing, but im ecstatic....lol...

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