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Lecture 13: Advanced Examples • Selected examples taken from Problem Set 4 • Magnetic Potential (as an example of working with non-conservative fields) • Plus Maxwell’s Equations (as a concise example of the power of vector calculus) Vector Areas (Problem 4.1) B z z C OA y A B=(1,1,1) B x O y C=(0,2,0) C O x z A=(1,0,0) C Vector Area of Triangle OAB B y O A Vector Area of Triangle OBC x Maximum Projected Area Area Projected in direction of this unit vector Solid Angle (Problem 4.2) • Angle (in radians) is dS/r where S is a circle of radius r centred on the origin z • Solid Angle (in steradians) is dA/r2 where dA is a sphere of radius r centred on the origin y x • Applying divergence theorem to red volume since on sides of closed volume, field is everywhere parallel to the sides so surface integrals are zero • •Solution to this “paradox”: divergence is non-zero at the origin: applying definition of divergence to small sphere around origin and only a fraction of these field lines emerge into the cone; therefore divergence theorem works, if origin (source of field lines) is properly treated Divergence (Problem 4.3) • Heat up body so it expands by linear factor • Note fractional increase in volume is • Divergence is a measure of local expansion/contraction Centre of mass of body: doesn’t move as body expands Curl (Problem 4.6) • Consider first, Stokes Theorem applied to a simple circular loop in the z=0 plane z y Surface S has cylindrical sides + a circular cap (taken to be at +ve infinity) along sides of cylinder since is parallel to sides on cap x • Note that double integral result is the same however the loop is wrapped around the cylinder, so result is always Magnetic Potential • In the curl-free region can define a magnetic potential such that where is the cylindrical polar angle. as required by Ampere’s Theorem Since In 2D polars recalling that • This scalar potential must now be a multi-valued function of position • (i) If loop doesn’t enclose wire • (ii) if loop goes round wire once • (iii) if loop goes round wire n times wire carrying current out of the paper Maxwell’s Equations “No magnetic monopoles” “Gauss’s Theorem” “Faraday’ s Law” since using Stoke’s Theorem emf induced is rate of change of magnetic flux through the circuit But “Ampere’s Law” because Recalling “continuity equation” of Lecture 8 if this were true capacitors couldn’t charge up! Maxwell’s Displacement Current surface • Maxwell added Displacement Current (4) cutting space between capacitor plates I • Now take • which is a continuity equation allowing charge to flow surface cutting wire ... and, applying Stoke’s Theorem to get , and hence , we know that we now get the same answer for for a surface passing through the plates of the capacitor (with zero ) as for a surface, with the same rim, cutting through the current-carrying wire (with non-zero )

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