Vol. Electrical Systems
Chapter Basic Direct Current (DC) Theory

Kirchhoff’s Current and Voltage Laws

Conservation of energy allows us to account for voltage and current entering and leaving every circuit and junction point. Kirchoff’s Laws quantify these principles, allowing us to mathematically analyze circuits.

Two extremely important principles in electric circuits were codified by Gustav Robert Kirchhoff in the year 1847, known as Kirchhoff’s Laws. His two laws refer to voltages and currents in electric circuits, respectively.

Kirchhoff’s Voltage Law (KVL)

Kirchhoff’s Voltage Law states that the algebraic sum of all voltages in a closed loop is equal to zero. Another way to state this law is to say that for every rise in potential there must be an equal fall, if we begin at any point in a circuit and travel in a loop back to that same starting point.

Kirchoff’s Loop Rule

An analogy for visualizing Kirchhoff’s Voltage Law is hiking up a mountain. Suppose we start at the base of a mountain and hike to an altitude of 5000 feet to set up camp for an overnight stay. Then, the next two days we set off from our previous camps and proceed to hike farther up the mountain. Deciding we’ve climbed high enough for three days of hiking, we set up camp again and stay the night. The next day we hike down 7300 feet to a fifth and final location and camp once again. On the last day we hike back to our original starting point at the base of the mountain. We can summarize our hiking adventure as a series of rises and falls like this:

 Day Path Altitude gain/loss Day 1 A to B +5000 feet Day 2 B to C $$-$$1200 feet Day 3 C to D +6000 feet Day 4 D to E $$-$$7300 feet Day 5 E to A $$-$$2500 feet (Total) ABCDEA 0 feet

Of course, no one would tell their friends they spent five days hiking a total altitude of 0 feet, so people generally speak in terms of the highest point reached: in this case 9800 feet (which, as a note of general trivia is roughly the elevation of McDonald Peak near Glacier National Park in Montana, US).

However, if we track each day’s gain or loss in algebraic terms (maintaining the mathematical sign, either positive or negative), we see that the end sum is zero (and indeed must always be zero) if we finish at our starting point.

If we view this scenario from the perspective of potential energy as we lift a constant mass from point to point, we would conclude that we were doing work on that mass (i.e. investing energy in it by lifting it higher) on days 1 and 3, but letting the mass do work on us (i.e. releasing energy by lowering it) on days 4 and 5 (and also on day 2, since the result of that day was a negative gain). After the five-day hike, the net potential energy imparted to the mass is zero, because it ends up at the exact same altitude it started at.

KVL Example

Let’s apply this principle to a real circuit, where total current and all voltage drops have already been calculated for us:

If we trace a path ABCDEA, we see that the algebraic voltage sum in this loop is zero:

 Path Voltage gain/loss A to B $$-$$ 4 volts B to C $$-$$ 6 volts C to D + 5 volts D to E $$-$$ 2 volts E to A + 7 volts ABCDEA 0 volts

We can even trace a path that does not follow the circuit conductors or include all components, such as EDCBE, and we will see that the algebraic sum of all voltages is still zero:

 Path Voltage gain/loss E to D + 2 volts D to C $$-$$ 5 volts C to B + 6 volts B to E $$-$$ 3 volts EDCBE 0 volts

Kirchhoff’s Voltage Law is often a difficult subject for students, precisely because voltage itself is a difficult concept to grasp. Remember that there is no such thing as voltage at a single point; rather, voltage exists only as a differential quantity. To intelligently speak of voltage, we must refer to either a loss or gain of potential between two points.

Our analogy of altitude on a mountain is particularly apt. We cannot intelligently speak of some point on the mountain as having a specific altitude unless we assume a point of reference to measure from. If we say the mountain summit is 14,000 feet high, we usually mean 14,000 feet higher than sea level, with the level of the sea being our common reference point. However, our hiking adventure where we climbed 9800 feet in three days did not imply that we climbed to an absolute altitude of 9800 feet above sea level.

Since I never specified the sea-level altitude at the base of the mountain, it is impossible to calculate our absolute altitude at the end of day 3. All you can tell from the data given is that we climbed 9800 feet above the mountain base, wherever that happens to be with reference to sea level (they refer to the height above the base as ‘prominence’).

So it is with electrical voltage as well: most circuits have a point labeled as ground where all other voltages are referenced. In DC-powered circuits, this ground point is often the negative pole of the DC power source. Voltage is fundamentally a quantity relative between two points: a measure of how much potential has increased or decreased moving from one point to another.

Kirchhoff’s Current Law (KCL)

Kirchhoff’s Current Law is a much easier concept to grasp. This law states that the algebraic sum of all currents at a junction point (called a node) is equal to zero. Another way to state this law is to say that for every electron entering a node, one must exit somewhere.

Kirchoff’s Junction Rule

An analogy for visualizing Kirchhoff’s Current Law is water flowing into and out of a “tee” fitting:

So long as there are no leaks in this piping system, every drop of water entering the tee must be balanced by a drop exiting the tee. For there to be a continuous mis-match between flow rates would imply a violation of the Law of Mass Conservation.

KCL Example

Let’s apply this principle to a real circuit, where all currents have been calculated for us:

At nodes where just two wires connect (such as points A, B, and C), the amount of current going in to the node exactly equals the amount of current going out (4 mA, in each case). At nodes where three wires join (such as points D and E), we see one large current and two smaller currents (one 4 mA current versus two 2 mA currents), with the directions such that the sum of the two smaller currents form the larger current.

Just as the balance of water flow rates into and out of a piping “tee” is a consequence of the Law of Mass Conservation, the balance of electric currents flowing into and out of a circuit junction is a consequence of the Law of Charge Conservation, another fundamental conservation law in physics.

REVIEW:

Lessons in Industrial Automation

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