# EMF v time graph vs change in flux v time

1. Jun 11, 2012

### alv

I'm actually a teacher and I can't explain this problem from the 2007 HSC. I've attached a copy of the question and the two graphs. The question shows a graph of change in magnetic flux vs time and asks you to choose which graph of induced emf best fits it. I don't understand why the induced emf is at max even while the change in flux increases (or decreases).

I've searched for help on this but can't find it. I'd appreciate any explanation you could give me.

Thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

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2. Jun 11, 2012

### ehild

emf= -ΔΦ/Δt, more accurately, the negative of the time derivative of the flux. The magnitude of the emf is maximum when the flux-time graph is steepest. ΔΦ is the change of the flux in time interval Δt.
As the graph consists of linear segments, the emf is constant within one segment. The vertical lines do not mean anything.
ehild

3. Jun 11, 2012

### azizlwl

Actually there's no maximum or minimum value. Just polarity change due to changes in gradient.Normally verticall line is dotted.

4. Jun 15, 2012

### alv

Thanks heaps for this.