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Exercises with Magnetic Monopoles - Kurt Nalty

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Field Angular Momentum due to Charge/Monopole<br />

Interaction<br />

This section follows homework problem 8, chapter 3 of Julian Schwinger’s<br />

Classical Electrodynamics. Place an electric charge at (0, 0, R/2). Place a<br />

magnetic charge at (0, 0, −R/2).<br />

The electric field is<br />

⎡ ⎛<br />

⎤<br />

⃗E(x, y, z) = q e<br />

4πɛ<br />

⎞<br />

⎣−∇<br />

⃗ ⎝<br />

1<br />

√<br />

⎠⎦<br />

x 2 + y 2 + (z − R/2) 2<br />

= q e x⃗a x + y⃗a y + (z − R/2)⃗a z<br />

4πɛ<br />

(<br />

x2 + y 2 + (z − R/2) 2) 3/2<br />

The magnetic field is<br />

⃗B(x, y, z) = µq m<br />

4π<br />

= µq m<br />

4π<br />

The power flux Poynting vector is<br />

⃗S = 1 µ ( ⃗ E × ⃗ B)<br />

⎡ ⎛<br />

⎞⎤<br />

⎣−∇<br />

⃗ ⎝<br />

1<br />

√<br />

⎠⎦<br />

x 2 + y 2 + (z + R/2) 2<br />

x⃗a x + y⃗a y + (z + R/2)⃗a z<br />

(<br />

x2 + y 2 + (z + R/2) 2) 3/2<br />

= 1 q e x⃗a x + y⃗a y + (z − R/2)⃗a z<br />

µ 4πɛ<br />

(<br />

x2 + y 2 + (z − R/2) 2) × µq m x⃗a x + y⃗a y + (z + R/2)⃗a z<br />

3/2<br />

4π<br />

(<br />

x2 + y 2 + (z + R/2) 2) 3/2<br />

= q eq m 1<br />

yR⃗a x − xR⃗a y<br />

16π 2 ɛ<br />

(<br />

x2 + y 2 + (z − R/2) 2) 3/2 (<br />

x2 + y 2 + (z + R/2) 2) 3/2<br />

The momentum density is<br />

ɛµ ⃗ S = µq eq m<br />

16π 2<br />

yR⃗a x − xR⃗a y<br />

(<br />

x2 + y 2 + (z − R/2) 2) 3/2 (<br />

x2 + y 2 + (z + R/2) 2) 3/2<br />

3

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