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I've got a problem with question 5 on this paper:

http://www.freeexampapers.com/past_papers.php?l=Past_Papers%2FAEA%2FPhysics%2F2006/

Download AEA-PHYS-PP-MayJune-2006-AEA-Paper-1342.pdf.

Starting from b) i), we got that:

ε=Δ∅/Δt → W=εe

Where W is denotes work, ε is e.m.f

Also, for ii) work can be written as:

W=2∏rF

in iv) we use F=Δp/Δt=2erΔB/Δt

as during one cycle mementum changes its direction to the opposite twice. From all that we got:

Δ∅=4∏r^2ΔB

Where 4 comes from? It should be deltaB times area of loop. I tried to check it if 2s does not cancel each other but they don't, where's the mistake?

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# Homework Help: Betatron, derivation of expressions

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