answer n explaination plz~~ help me out guys~! (1 Viewer)

[32cc]

this is the Q from STANSW 2002:

a student is rapidly turning the handle of a small generator. another student attaches an electrical lead directly between the two output terminals of the generator. the student turning the handle notes that there is now a noticeable drag acting on the handle. which of the following would best explain this observation?

(A) the second student has created a short circuit in the generator

(B) it is the result of the law of coservation of evergy

(C) when the wire is connected, a back-emf is produced

(D) it is the result of Faraday's law



wot is the answer ar?

 

[Grizzly]

Faraday's law is when an induced EMF experiences a change in magnetic flux, and is relative, or proportional to the rate of change. Therefore answer is D.
Notice (c), a back EMF is produced, according to Lenz's law, but the student notes a "drag", not because of a back EMF, but because it requires more "drive" to rotate through the magnetic flux.

I think im right :|

 

[+:: $i[Q]u3 ::+]

sorry grizzly..
it's B.. (and i have the success book wit the stansw answers.. so yeh)

in the beginning, ur just turning it, and ur not producing any electrical energy. when u connect it up, and ur generating emf, then you are producing electrical energy. COE - that energy has to come from somewhere. And since the only energy coming in is the mechanical energy from u turning the handle, you're gonna have to turn the handle a bit harder to generate that electricity.

 

[Grizzly]

OH ic
but im right about c) not being right
yeah..thats it ;P

 

[32cc]

oh thanx guys~

i hav dasuccess book 2... but it didnt hav explaination... and dats wot i need...

thanx +:: $i[Q]u3 ::+ ~! ^^

 

[ND]

It's an application of Lenz's law, where's that option?

Before the circuit is closed, there is an induced emf, but no induced current. When the circuit is closed, a current flows - this current flows in the direction which sets up a magnetic field opposing the magnetic field causing it. It is the interaction of the induced magnetic field with the generator's magnetic field that causes the drag.

 

[32cc]

ah... damn....

so ND... do u agree with (B) being the answer???

ah... im confused... @_@

 

[Constip8edSkunk]

the question asks for the option that BEST explains the phenomenon, and the law of conservation of energy (in that u cant produce energy out of no where) is the best option as it directly relates to why the resistance is felt. While both C and D have applications in explaining the phenomenom, it does not directly explain why it happen, therefor b is the best answer.

 

[Grizzly]

..guess thats why mutliple choice is the easiest place to lose marks...converesely, easy to pick up marks