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Linear conductors with currents
Section 33
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Assume B = mH everywhere.Then
Ignore this term, which is unrelated to the currents
Or better
Since j = 0 outside of the wires
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Since Maxwell’s equations are linear in the fields, the superposition of field solutions is also a solution
= sum over fields from each current alone.
Free self energy
Energy of ath conductor in its own field
Interaction energy
Energy of ath conductor in the field of the bth conductor
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Since
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Also Field at dVa due to ja
Field at dVb due to ja
Field at dVa due to jb
ath conductor
bth conductor
ath conductor
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For a given current-density distribution,
depends only on the total current
a
Cross section
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Both ja and the field Aa produced by ja are proportional to total current Ja
Self energy
Proportionality constantLaa = “self-inductance”Depends only on geometry
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Similarly
and
So interaction energy is
A different proportionality constant.Lab = “mutual inductance”Also depends only on geometry
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Magnitude of free energy depends on relative sense of currents
Total free energy of system
No restriction a>b now, due to factor ½ applied to interaction term
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>0 (Positive definite)
Puts conditions on the coefficients
Laa > 0 for all a (e.g. consider a single conductor)
Laa Lbb > Lab2 Mutual inductance is always less than or equal
to the product of self inductances of two circuits.
Lab = k Sqrt[Laa Lbb ] Coupling coefficient |k|
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Calculation of for arbitrary 3D currents is hard.
If m = 1 in both conductors and surrounding media, the problem simplifies.
Then m(T) is not a factor in , which then is independent of the thermodynamic state of the materials.
Then = U, i.e. “free energy” -> “energy”.
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Non-magnetic m = 1 case
(30.12), same as in vacuum
Then the self energy is
ath conductor
ath conductor
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Similarly, the interaction energy is
Linear circuits
Depends only on shape, size, relative position, and relative direction of currents
Does not require m = 1 for the linear conductors, since their magnetic energy is tiny.If surrounding medium has m > 1, Lab m Lab.
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The total energy is
The vector potential of the total field from all conductors at dla of the ath conductor.
Linear circuits
= F = Magnetic flux through ath circuit
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Then Flux from all circuits through the ath circuit
For a linear current J in an external B-field
No self-energy for the sources of B.No factor of 2 to correct for double counting of interaction pairs.
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For uniform external B-field and non-permeable medium surrounding conductor
External field without linear circuit Magnetic moment of linear circuit
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How to find forces on linear current-carrying conductors.
• If free energy is known as a function of shape, size, and relative positions• Then forces on conductors are found by differentiation with respect to the
proper coordinates.
Which thermodynamic potential should we use?
• It is easy to hold currents constant during the differentiation.• It is difficult to hold fluxes constant when the wires move.• The free energy with respect to currents is
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Generalized force Fq in the direction of the generalized coordinate q.
Omitting terms that are independent of currents, See (31.7)
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About the sign of the force vector…
• Lab can be positive or negative, depending on the relative direction of the currents
• Both ccw: repel
• One ccw and one cw: attract
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Forces exerted on a conductor by its own magnetic field
For given J and T, spontaneous irreversible changes occur that reduce until it reaches a minimum.
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Forces on a conductor, which tend to minimize, will tend to maximize L
L is always positive
[L] = length (HW)L~ dimension of conductor
A magnetic field from J in a conductor tends to increase its size.
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