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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: Allygator on October 03, 2011, 10:41:08 pm

Title: Right Hand Slap Rule
Post by: Allygator on October 03, 2011, 10:41:08 pm

Hi,
I've found that on practically every practice exam I've done I get any question involving induced current direction wrong and I cannot for the life of me figure out what I'm stuffing up. I think I'm getting confused as to when Lenz's law has to be applied.
Ie. Does the induced current in a generator oppose the change in magnetic field?

Anyway, would someone be able to run me through how to work out the question attached so I can see where I am going wrong?
Thanks heaps!!  :D

Title: Re: Right Hand Slap Rule
Post by: Lasercookie on October 03, 2011, 10:49:53 pm

So the thing rotates clockwise.

That means a force must be pushing down on QR.
You know the field goes from North to South.

So take your right hand, and face your fingers horizontal and rightwards.
Your palm should be facing downwards.
Your thumb is the direction of current, and it should be pointing "north".
In terms of what you've been given, it'll be R to Q.

This is where Lenz's Law comes in. I just simplify things and think "the current is induced and therefore must be opposite". So you had R to Q, but the direction of the induced current is reversed.

So the answer is that the induced current will flow from Q to R.

Title: Re: Right Hand Slap Rule
Post by: Allygator on October 03, 2011, 10:54:28 pm

thanks so much!! :D
makes so much sense

Title: Re: Right Hand Slap Rule
Post by: tha boss on October 04, 2011, 02:16:15 pm

isnt it parallel

Title: Re: Right Hand Slap Rule
Post by: Lasercookie on October 04, 2011, 02:21:32 pm

Quote from: tha boss on October 04, 2011, 02:16:15 pm

isnt it parallel

No, we are talking about the length QR and the force.

For flux, it would be parallel, since the angle we look at there is between the field and the normal to the coil, but with force we look at the field and the wire.

So QR is clearly perpendicular.