What's in and out of Phase in Transformers?
This doesn't crop up often, but I've found pupils who are often either confused or irritated when mark schemes ask questions about the flux or EMF in a transformer set-up and the mark scheme has any resulting minus signs in brackets, showing that they are non-essential in terms of scoring marks. Having spent time learning about how important the minus sign is in
(for example by talking about Lenz's Law, back-EMFs or eddy current braking) then it can seem silly that mark schemes treat it as optional.
In fact, it's quite sensible that they do this, but it involves some Physics which is rarely on syllabuses nowadays. Below is a sheet I wrote to explain it all, partly to clarify my own thinking but mainly to satisfy some pupils who had got rather irritated by it. It requires some knowledge of self-inductance and the way V and I lead or lag each other in an inductor.