physics question (1 Viewer)

[14494]

can someone please explain to me why the right answr is (a) in Q.7 2005 Physics exam. I used the right hand palm rule, with my palm(force) towards the right on the left wire, and my fingers (magnetic field) downwards so i get current flowing as it would in answer (d).

thanks

 

[brettymaccc]

As the coil begins to rotate, a current is induced in the coil as it has a relatively changing magnetic field. Thus, the direction of the current induced will be such that it will oppose the change that created it (Lenz's Law) and consequently flow like it is in A as opposed to D.

 

[OmmU]

Right technique, you just forgot lenz's law (because it is a generator not a motor).

The induced current will be set up to oppose the original change in flux. Therefore you switch the direction of the current.

Does that help?

 

[14494]

so do we always use the right hand rule and switch current direction in generators (due to lenzs law), but leave it the same in motors?

and yes it does help, thanks

 

[khorne]

so do we always use the right hand rule and switch current direction in generators (due to lenzs law), but leave it the same in motors?

and yes it does help, thanks

The reason it's switched is not because of the current. That's the result. The force on the generator coil must be in the opposite direction of motion, because if it was in the same, the conservation of energy would be violated, like a snitch in a prison shower. So if the coils left side is travelling upward, the force would be downward, which basically results in a switch in current direction (if you look at your hand rule).

 

[14494]

The reason it's switched is not because of the current. That's the result. The force on the generator coil must be in the opposite direction of motion, because if it was in the same, the conservation of energy would be violated, like a snitch in a prison shower. So if the coils left side is travelling upward, the force would be downward, which basically results in a switch in current direction (if you look at your hand rule).

but how come whatever direction of force i get (direction of palm) in motors is also the direction of motion, this is confusing me, can u please explain more??

 

[khorne]

but how come whatever direction of force i get (direction of palm) in motors is also the direction of motion, this is confusing me, can u please explain more??

In motors, electrical energy is being converted to motion, rotational, energy of the arm. The energy here is being conserved, so one form is going into another. Thus, you just use your normal hand rule.

In a generator, motion is being transformed into electrical energy. If the force created by the movement of charge (which is current) was in the same direction as motion, the arm would accelerate, induce more EMF (emf = change in flux per time), and basically result in infinite energy being created, as it would accelerate itself. Thus, to conserve energy, the force has to be opposite to motion.

 

[14494]

In motors, electrical energy is being converted to motion, rotational, energy of the arm. The energy here is being conserved, so one form is going into another. Thus, you just use your normal hand rule.

In a generator, motion is being transformed into electrical energy. If the force created by the movement of charge (which is current) was in the same direction as motion, the arm would accelerate, induce more EMF (emf = change in flux per time), and basically result in infinite energy being created, as it would accelerate itself. Thus, to conserve energy, the force has to be opposite to motion.

thankyou so much, i get it now, i really appreciate it

 

[jamesfirst]

Right technique, you just forgot lenz's law (because it is a generator not a motor).

Just so you know, Lenz's law applies in motors as well

 

[khorne]

Just so you know, Lenz's law applies in motors as well

Back EMF bby