Topology and Elementary Electric Circuit Theory, I

Introduction

As a Sophomore at MIT I took the course 6.10, Elementary Electric Circuit Theory (required at that time for all would-be Physics majors) from David Huffman. That course was in fact my first introduction to topological concepts even though the word "topology," to my recollection, was never mentioned. It does occur in a text written for this course a few years before: Introductory Circuit Theory by Ernst Guillemin (Wiley, NewYork, 1953). In this column and my next I plan to go over some of this material from the point of view of a topologist, to try and share some of the excitement I felt at seeing these abstract concepts emerge from a completely practical context. This column will look at the hidden role of the Euler characteristic in circuit theory; the next will examine duality.

A restricted class of circuits

In mathematical terms, an elementary electric circuit is a connected, directed graph. The graph is made of conducting wire, and each segment of the graph bears a device which can be a voltage source, a current source, a resistor ($R$), an inductance ($L$) or a capacitor ($C$).
    Most of the phenomena related to topology also occur in circuits with no inductances or capacitors; here we will work just with these.
    If the oriented segment $\langle ab \rangle $ bears a voltage source $V$, then the voltage difference $\Delta V_{ab}$ between the ends of the segment is equal to to $V$; if it bears a current source $I$ then it will carry a current $I$ from node $\langle a\rangle$ to node $\langle b\rangle$. Otherwise $\Delta V_{ab}$ and the current $I_{ab}$ from node $\langle a\rangle$ to node $\langle b\rangle$ are related by Ohm's Law:

$\Delta V_{ab} = R\,I_{ab}$.

Note that $\displaystyle{\Delta V_{ba}= -\Delta V_{ab}}$ and $I_{ba} = -I_{ab}$.

The voltage differences and currents in a circuit must satisfy Kirchhoff's Laws

Remark: if a circuit has $N$ nodes $b_1,\dots, b_N$, the sum of the left-hand-sides of all $N$ equations coming from KCL must automatically be zero, since each current appears twice, as $I_{ab}$ and $I_{ba}$, with opposite signs. It follows that the sum of the first $N-1$ equations is equivalent to the last one.

Solving a circuit

Solving a circuit means calculating the currents in all the segments.

Elementary loops (meshes). A mesh in a circuit is a loop that contains no smaller loops.    One way to organize the solution of the network is in terms of mesh currents, the clockwise (say) currents around the boundary of each mesh. The current in a segment will be the algebraic sum of the currents from the two meshes it belongs to (one if the segment is on the outside). This gives a collection of unknowns $i_1, \dots, i_M$ where $M$ is the number of meshes.
   Applying KVL to each of the meshes gives a set of $M$ equations which can be solved for $i_1, \dots, i_M$, thus yielding the currents in all the segments and the solution of the circuit.

circuit1 An example, adapted from Wikipedia, should show how the procedure works. This circuit has voltage sources of 3 and 4 Volts and resistors of 10, 20 and 30 Ohms linked as in the diagram. Applying KVL to the top mesh, we add the voltage increments as we go around the boundary of the mesh, starting at the node on the left: $-10 i_a - 20 (i_a-i_b) + 3 =0$; similarly the bottom mesh yields $-3 -20 (i_b-i_a) -30 i_b -4 = 0$. Hence the system of linear equations: $$-30i_a+20i_b = -3$$ $$20i_a-50i_b=7,$$ which give $i_a=1/110\, \mbox{A},\,\, i_b=-15/110\, \mbox{A}$ and so
$i_1=i_a=1/110\, \mbox{A},\,\, i_2 =i_b-i_a=-16/110\, \mbox{A},\,\, i_3=-i_b=15/110\, \mbox{A}$. The circuit is solved.

(The Euler Characteristic). This straightforward procedure contains a mathematical subtlety. The two mesh equations yielded three segment currents because implicit in the calculation was another equation: $i_1+i_2+i_3=0$, from Kirchhoff's Current Law. Guillemin explains (p. 54) that the term mesh should not refer to "the contour" but rather to "the space surrounded by that contour." In topological terms, the meshes must be considered as 2-dimensional elements of the circuit. For a planar circuit, the numbers $M$ of meshes, $S$ of segments and $N$ of nodes are related by the Euler characteristic: $M-S+N=1$, so the number $M$ of mesh equations plus the number of independent node equations ($N-1$, as remarked above) is equal to the number $S$ of unknown segment currents, allowing for a unique solution. This point is nicely explained, from a slightly different point of view, in a video featuring Peter Lax. K\
(3,3)on torus

A circuit with the topology of the "utility graph" K(3,3) cannot be embedded in the plane but can be embedded in the torus. There will be two mesh equations corresponding to ADBE and DBFC, and 5 independent node equations. Since there are 9 segments, this leaves two undetermined segment currents. These correspond to two independent non-bounding loops, say ADCEA and AFBDA. In fact currents in those loops could be produced (Faraday effect) by varying magnetic fluxes linking the torus inside or outside.

(Homological analysis). A fancier explanation can be given in terms of homology. Geometrically speaking, a network $X$ is a 2-dimensional cellular complex. The nodes are the 0-cells, the segments the oriented 1-cells, and the meshes are the oriented 2-cells. The $k$-dimensional chain complex $C_k(X)$ (here $k=0, 1, 2$) is the vector space of formal linear combinations of the $k$-cells. The boundary homomorphisms $\partial\!: C_1(X) \rightarrow C_0(X)$, which takes the oriented 1-cell $\vec{AB}$ to the 0-chain $B - A$, and $\partial\!: C_2(X) \rightarrow C_1(X)$, which takes the mesh with positively oriented boundary $ABC\dots KA$ to the 1-chain $\vec{AB} + \vec{BC} +\cdots +\vec{KA}$, satisfy $\partial\partial=0$ since each endpoint is counted twice with opposite signs. A $k$-chain $c$ with $\partial c = 0$ is called a $k$-cycle, and one satisfying $c=\partial b$ for some $(k+1)$-chain $b$ is called a $k$-boundary. Since $\partial\partial=0$ the 1-boundaries of $X$ are a subspace of the 1-cycles; the quotient vector space is the first homology vector space of $X$.
   A basic result in homology theory is that for a a subset of the plane like the underlying space of a connected planar circuit, the first homology must be zero: every 1-cycle must be a 1-boundary.    In a planar circuit, let $c_1,\dots c_S$ be the segments, considered as oriented 1-cells, and look at the 1-chain $I = i_1c_1 + i_2c_2 + \dots i_Sc_S$ where the coefficient of each segment is its current, with the sign adjusted to get the flow in the right direction. Then Kirchhoff's Current Law becomes the statement that $\partial I = 0$ (the coefficient of each node is the algebraic sum of the incident currents, hence 0). Since the topology of the circuit forces every 1-cycle to be a boundary, there must be a 2-chain $J$ with $\partial J = I$. Writing $J=a_1e_1 + a_2e_2 +\cdots a_Me_M$ where now $e_1, \dots e_M$ are the oriented 2-cells corresponding to the meshes of $X$, the coefficient $a_p$ is the mesh current associated to the $p$th mesh. This is the homological justification of the mesh-current method used in our example. (In general, the 2-chain $J$ could be varied by an arbitrary 2-cycle $H$, since $\partial (J+H)=\partial J + \partial H = \partial J = I$ but ...)

Duality

A 2-dimensional cellular complex $X$ with underlying space the sphere has a dual complex $X^*$ defined as follows. Suppose $x$ has 0-cells $b_1,\dots,b_N$, 1-cells $c_1, \dots c_S$ and 2-cells $e_1, \dots e_M.$  $X^*$ can be defined abstractly, but it is useful to draw it in the same space as $X$.   
duality

Dual complexes: here $X$ is in black, $X^*$ in green. $X$ is drawn as a planar graph with applications to circuits in mind, but the total space is a sphere, with an additional hexagonal cell $e_x$.

  sphere/duality