/0's physics phread

[TrueTears]

By moving the wire, you are only changing the DIRECTION of the electrons not the speed, v stays the same.

[/0]

By moving the wire, you are only changing the DIRECTION of the electrons not the speed, v stays the same.

Hmmm? Wouldn't v change?

Ok let's say the wire is already in motion, moving at a constant speed v perpendicular to the field. Is there an emf then?

[TrueTears]

Is the wire moving in a straight line or rotating?

Say the wire is moving up, with infinite long uniform magnetic field around it (say left of it is north and right of it is south), yes there would be a force on the wire, this means the electrons are moving up, using the left hand slap rule we get the forcing facing you (out of the page), as in the wire is being pushing towards you. There is no emf because the electrons are not separated on either end of the wire.

If it was rotating then yes there would be a induced emf due to change of flux.

[/0]

Just moving in a straight line

[TrueTears]

Edited last post, check.

[/0]

Is the wire moving in a straight line or rotating?

Say the wire is moving up, with infinite long uniform magnetic field around it (say left of it is north and right of it is south), yes there would be a force on the wire, this means the electrons are moving up, using the left hand slap rule we get the forcing facing you (out of the page), as in the wire is being pushing towards you. There is no emf because the electrons are not separated on either end of the wire.

If it was rotating then yes there would be a induced emf due to change of flux.

Wait, sorry what I mean is if the magnet is as it is in question 11 Page 274, but imagine the dots go on till infinity.

Now if the wire moves in the direction of A, is there an emf?

[TrueTears]

In response to your post before you edited/deleted it.

If the wire was coming out of the page that would be impossible wouldn't it?

You initial condition was that the wire was surrounded by a magnetic field, it can only move up (considering the magnetic field was like a hallow tube, and the wire fits in the hallow tube)

If it comes out of the page it would just hit the side of the hallow tube?

If only the "sides" of the wire was surrounded by magnetic fields and the wire comes out of the page, then yes there would be an emf, check with Lenz Law, it also proves there to be a induced emf

[/0]

Sorry I was edited my previous post to show what I meant.

Also in response to your previous post, if the poles of the magnet were at a very long distance away, I don't see why the wire wouldn't be able to move in any direction it liked.

[TrueTears]

If the poles of the magnets were very far away, the magnetic field at the wire would be so weak that it'd approach 0?

[/0]

lol thx tt but don't worry about that it's a stupid question

Another though, how does decreasing the area of the rotating coil decrease the speed of the motor?

[TrueTears]

Reducing the area reduces 2 things, the distance to the axle of rotation and the length of the coil.

the rotational torque is dependent on the length to the axle of rotation and force applied.

smaller length of coil implies smaller force which implies smaller torque which implies slower

smaller distance to axle of rotation implies smaller torque which implies slower.

[/0]

pr0 thx man

[/0]

A current is flowing in the coil between the magnets the coil is rotating like a motor side AB of the coil is rotating towards us.

Which of the following best describes the current in the coil?
A. It is flowing from A to B along this side.
B. It must be alternating.
C. It is flowing from B to A along this side.
D. It could be flowing in either direction, one cannot tell.



(: (:

[TrueTears]

I thought answer would be just C.

I just used the right hand slap rule.

the force on BA is coming out of the page, magnetic field is from right to left. Your fingers point up, hence current flows from B to A.

Why must it be alternating? I thought the diagram could represent DC motor or AC/DC generator [since it doesn't show slip rings or split ring commutator]. But DC motor uses DC current and DC generator is pulsating DC current, so it may not be alternating? (even though the commutator reverses the current in a DC motor, the +ve and -ve terminals don't switch.)

[Mao]

I thought answer would be just C.

I just used the right hand slap rule.

the force on BA is coming out of the page, magnetic field is from right to left. Your fingers point up, hence current flows from B to A.[/tex]

my apologies, I thought it was a generator

Why must it be alternating? I thought the diagram could represent DC motor or AC/DC generator [since it doesn't show slip rings or split ring commutator]. But DC motor uses DC current and DC generator is pulsating DC current, so it may not be alternating? (even though the commutator reverses the current in a DC motor, the +ve and -ve terminals don't switch.)

The current in the coil must always be alternating (every half turn) to maintain the torque in the same direction. This is the purpose of a split-ring commutator, to alternate the current inside the coil.