Hi! 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?