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Lecture 13: Advanced Examples

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). y. x. z. z. x. z. y. y. x.

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Lecture 13: Advanced Examples

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  1. 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)

  2. y x z z x z y y x Vector Areas (Problem 4.1) B C OA B=(1,1,1) A B O C=(0,2,0) C O A=(1,0,0) C Vector Area of Triangle OAB B Vector Area of Triangle OBC O A Maximum Projected Area Area Projected in direction of this unit vector

  3. z y x Solid Angle (Problem 4.2) • Angle (in radians) is dS/r where S is a circle of radius r centred on the origin • Solid Angle (in steradians) is dA/r2where dA is a sphere of radius r centred on the origin • Applying divergence theorem to red volume • 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 since on sides of closed volume, field is everywhere parallel to the sides so surface integrals are zero

  4. Divergence (Problem 4.3) • Heat up body so it expands by linear factor • Note fractional increase in volume is Centre of mass of body: doesn’t move as body expands • Divergence is a measure of local expansion/contraction

  5. 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) x along sides of cylinder since is parallel to sides • Note that double integral result is the same however the loop is wrapped around the cylinder, so result is always on cap

  6. In the curl-free region can define a magnetic potential such 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 Magnetic Potential where is the cylindrical polar angle. as required by Ampere’s Theorem Since In 2D polars recalling that wire carrying current out of the paper

  7. 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 if this were true capacitors couldn’t charge up! Recalling “continuity equation” of Lecture 8

  8. Maxwell’s Displacement Current surface cutting space between capacitor plates • Maxwell added Displacement Current • Now take • which is a continuity equation allowing charge to flow (4) I 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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