A-Level Physics OCR B H557

the action of a transformer: magnetic flux from a coil; induced e.m.f = rate of change of flux linkage

the action of a dynamo: change of flux linked produced by relative motion of flux and conductor

electromagnetic forces; qualitatively as arising from tendency of flux lines to contract or interaction of induced poles; quantitative calculation limited to force on a straight current-carrying wire in a uniform field

simple linked electric and magnetic circuits: flux produced by current turns, need for large conductance and permeance and the effect of increasing the dimensions of an electromagnetic machine; qualitative effect of iron and air gap.

permeance is understood as a magnetic equivalent to electrical conductance and with analogous dependence on the dimensions and nature of the magnetic medium

Make appropriate use of:

(i) the terms: B-field, magnetic field, flux, flux linkage, induced e.m.f, eddy currents

by sketching and interpreting:

(ii) graphs of variations of currents, flux and induced e.m.f

(iii) diagrams of lines of flux in magnetic circuits; continuity of lines of flux.

$ϕ = BA$, $ε = -\dfrac{d(ϕN)}{dt}$

In the context of Faraday’s and Lenz’s laws

$F = ILB$

Calculations restricted to current perpendicular to uniform magnetic field

$\dfrac{V_1}{V_2} = \dfrac{N_1}{N_2}$ for an ideal transformer

i.e. no energy losses for an ideal transformer Learners will also be expected to recall the equation relating V and N

$\dfrac{I_2}{I_1} = \dfrac{N_1}{N_2}$ for an ideal transformer

Learners will also be expected to recall the equation relating I and N

observing induced e.m.fs produced under varying conditions such as dropping a magnet through a coil attached to a data logger or oscilloscope

links to 6.1.1a(ii), 6.1.1c(i)

determining the uniform magnetic flux density between the poles of a magnet using a rigid current carrier and digital balance

links to 6.1.1a(iii), c(ii)

investigate transformers

links to 6.1.1a(i), c(iii)(iv)