Matching law

In electrical engineering, electrical terms are associated into pairs called duals. A dual of a relationship is formed by interchanging voltage and current in an expression. The dual expression thus produced is of the same form, and the reason that the dual is always a valid statement can be traced to the duality of electricity and magnetism.

Here is a partial list of electrical dualities:

• voltage — current
• parallel — serial (circuits)
• resistance — conductance
• impedance — admittance
• capacitance — inductance
• reactance — susceptance
• short circuit — open circuit
• Kirchhoff's current law — Kirchhoff's voltage law.
• Thévenin's theorem — Norton's theorem

History

The use of duality in circuit theory is due to Alexander Russell who published his ideas in 1904.

Examples

Constitutive relations

• Resistor and conductor (Ohm's law)

$v=iR\iff i=vG\,$

• Capacitor and inductor – differential form

$i_{C}=C{\frac {d}{dt}}v_{C}\iff v_{L}=L{\frac {d}{dt}}i_{L}$

• Capacitor and inductor – integral form

$v_{C}(t)=V_{0}+{1 \over C}\int _{0}^{t}i_{C}(\tau )\,d\tau \iff i_{L}(t)=I_{0}+{1 \over L}\int _{0}^{t}v_{L}(\tau )\,d\tau$

Voltage division — current division

$v_{R_{1}}=v{\frac {R_{1}}{R_{1}+R_{2}}}\iff i_{G_{1}}=i{\frac {G_{1}}{G_{1}+G_{2}}}$

Impedance and admittance

• Resistor and conductor

$Z_{R}=R\iff Y_{G}=G$

$Z_{G}={1 \over G}\iff Y_{R}={1 \over R}$

• Capacitor and inductor

$Z_{C}={1 \over Cs}\iff Y_{L}={1 \over Ls}$

$Z_{L}=Ls\iff Y_{c}=Cs$

See also

• Duality (electricity and magnetism)
• Duality (mechanical engineering)
• Dual impedance

References

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• Turner, Rufus P, Transistors Theory and Practice, Gernsback Library, Inc, New York, 1954, Chapter 6.