# Lecture 13- Applying Kirchoff's Rules

## Kirchoff's Rules

Kirchoff's junction rule comes from conservation of charge and says that for any junction in a circuit the sum of all currents entering the junction must equal the sum of all currents leaving the junction.

Kirchoff's loop rule comes from conservation of energy and states that the changes in potential around any closed loop of a circuit must be zero.

The way to use these rules is to use them set up a sufficient number of equations based on the junctions and loops in a circuit to solve for the required unknowns.

## More lightbulbs

Which glows more brightly?

A. The 60W bulbs around the edge
B. The 23W bulb in the center
C. All the bulbs have the same brightness

## Wheatstone Bridge

The circuit we have just seen is in fact the Wheatstone Bridge.

With a Wheatstone bridge the value of a unknown resistor $R_{x}$ can be measured to high precision by the use of an adjustable resistor, $R_{3}$.

The idea is that when there is zero potential measured across the voltmeter in the middle of the bridge, the values of $R_{3}$ and $R_{x}$ are the same.

The equations that explain the operation of the bridge can be derived from Kirchoff's rules.

$I_{1}+I_{G}=I_{2}$     $I_{3}=I_{G}+I_{x}$     $-I_{3}R_{3}-I_{G}R_{G}+I_{1}R_{1}=0$     $I_{G}R_{G}+I_{2}R_{2}-I_{x}R_{x}=0$

If we balance the bridge then $I_{G}=0$ so the equations become

$I_{1}=I_{2}$     $I_{3}=I_{x}$     $I_{3}R_{3}=I_{1}R_{1}$     $I_{x}R_{x}=I_{2}R_{2}$

$\frac{I_{x}R_{x}}{I_{3}R_{3}}=\frac{I_{2}R_{2}}{I_{1}R_{1}}$

$R_{x}=\frac{I_{2}R_{2}I_{3}R_{3}}{I_{1}R_{1}I_{x}}\to=\frac{R_{2}R_{3}}{R_{1}}$

## Meters

One reason for using a Wheatstone bridge is that it removes any effects that can arise to non-idealities in meter characteristics.

When we want to measure something we want to have as little effect on the measurement as possible.

When using a voltmeter we want to measure the potential between two points, but without having any current flow through the voltmeter.

By contrast, we place an ammeter in series with the device we want to measure, and want the potential drop across it to be as small as possible.

Therefore the resistance of a voltmeter should be as high as possible, and the resistance of an ammeter should be as low as possible.

## Making Meters

Both meters can be assembled by combining a Galvanometer with resistors.

A Galvanometer is a meter which measures a small current based on electromagnetic induction (we'll come back to this later!).

For an ammeter a shunt resistor is placed in parallel to allow most of the current to flow through the shunt.

For a voltmeter a resistor is placed in series to reduce the total amount of current that flows through the meter.

As the resistor of the galvanometer itself is known the actual current or voltage can be worked out (the meter scale usually takes care of this automatically!).

## Basic RC circuit

We consider a circuit with a resistor and capacitor in series with a battery and a switch.

Consider the case where the capacitor is initially uncharged.

Let us close the switch at $t=0$.

## Basic RC circuit

We consider a circuit with a resistor and capacitor in series with a battery and a switch.

Consider the case where the capacitor is initially uncharged.

Let us close the switch at $t=0$.

What is the potential across the capacitor $V_C(t=0)$?

What is the potential across the resistor $V_R(t=0)$?

What is the current in the circuit $I(t=0)$?

Now we let the capacitor charge up over a period of time.

What is the potential across the capacitor after a long time $V_C(t=\infty)$?

What is the potential across the resistor $V_R(t=\infty)$?

What is the current in the circuit $I(t=\infty)$?

Qualitatively, we are ready to plot $I(t)$ and $Q_C(t)$ and hence $V_C(t)$ and $V_R(t)$

## RC circuit: Mathematical Solution

From Kirchoff's loop rule

$\mathcal{E}-I(t)R-\frac{Q(t)}{C}=0$

Writing this in terms of charge

$\mathcal{E}-\frac{dQ}{dt}R-\frac{1}{C}Q=0$

and then

$\mathcal{E}C-Q=\frac{dQ}{dt}RC$

$\frac{dQ}{\mathcal{E}C-Q}=\frac{dt}{RC}$

At time $t=0$, $Q=0$ and at time $t=t$, $Q=Q$

$\int_{0}^{Q}\frac{dQ}{\mathcal{E}C-Q}=\int_{0}^{t}\frac{dt}{RC}$

$[-\ln(\mathcal{E}C-Q)]_{0}^{Q}=[\frac{t}{RC}]_{0}^{t}$    ⇒    $\ln(\frac{\mathcal{E}C}{\mathcal{E}C-Q})=\frac{t}{RC}$    ⇒    $\ln(\frac{\mathcal{E}C-Q}{\mathcal{E}C})=-\frac{t}{RC}$    ⇒    $\frac{\mathcal{E}C-Q}{\mathcal{E}C}=e^{-\frac{t}{RC}}$

$Q=\mathcal{E}C(1-e^{-\frac{t}{RC}})$

The voltage on the capacitor is given by $V_{C}=\frac{Q}{C}$

$V_{C}=\mathcal{E}(1-e^{-\frac{t}{RC}})$

$I=\frac{dQ}{dt}=\frac{\mathcal{E}}{R}e^{-\frac{t}{RC}}$

## RC circuit Solution Plots

$V_{C}=\mathcal{E}(1-e^{-\frac{t}{RC}})$    $I=\frac{\mathcal{E}}{R}e^{-\frac{t}{RC}}$    $V_{R}=\mathcal{E}e^{-\frac{t}{RC}}$

## Linear algebra for solving circuits

The application of Kirchoff's laws results in a system of linear equations.

As systems become increasingly complicated, the work in solving these equations as we have done earlier becomes tedious and painful.

We should note however that we can write a set of equations in terms of matrices and vectors and use linear algebra techniques to accelerate our problem solving).

For example, the circuit with two sources of emf we looked at earlier was characterized by the 3 equations

$\mathcal{E}_{1}-I_{3}R_{3}-I_{1}R_{1}=0$
$\mathcal{E}_{2}-I_{3}R_{3}-I_{2}R_{2}=0$
$I_{1}+I_{2}=I_{3}$

A small rearrangement gives us

$R_{1}I_{1}+R_{3}I_{3}=\mathcal{E}_{1}$
$R_{2}I_{2}+R_{3}I_{3}=\mathcal{E}_{2}$
$I_{1}+I_{2}-I_{3}=0$

which can be written as

$$\left[ \begin{array}{ccc} R_{1} & 0 & R_{3} \\ 0 & R_{2} & R_{3} \\ 1 & 1 & -1 \end{array} \right] \left[ \begin{array}{c} I_{1}\\ I_{2} \\ I_{3} \end{array} \right]= \left[ \begin{array}{c} \mathcal{E}_{1}\\ \mathcal{E}_{2} \\ 0 \end{array} \right]$$

This is now a Matrix equation to which linear algebra techniques can be applied.

Perhaps the most attractive feature of Matrix equations is that very efficient computational algorithms exist for solving them.

## Gaussian Elimination

In Gaussian elimination the idea is to reduce the matrix to reduced row echelon form via elementary row operations. You can think of these as the things you are allowed to do to a matrix

1. Swap the positions of two rows.
2. Multiply a row by a nonzero scalar.
3. Add to one row a scalar multiple of another.

These operations are performed on the augmented matrix which combines both the left and right hand sides of our system of equations.

$$\left[ \begin{array}{ccc} R_{1} & 0 & R_{3} \\ 0 & R_{2} & R_{3} \\ 1 & 1 & -1 \end{array} \right] \left[ \begin{array}{c} I_{1}\\ I_{2} \\ I_{3} \end{array} \right]= \left[ \begin{array}{c} \mathcal{E}_{1}\\ \mathcal{E}_{2} \\ 0 \end{array} \right] \to \left[ \begin{array}{ccc} R_{1} & 0 & R_{3} \\ 0 & R_{2} & R_{3} \\ 1 & 1 & -1 \end{array} \right| \left. \begin{array}{c} \mathcal{E}_{1}\\ \mathcal{E}_{2} \\ 0 \end{array} \right]$$

The approach to follow during Gaussian elimination is described here.

However, when solving numerical problems your calculator can do this very effectively for you.

For instructions on doing this on the TI-83 see here.

Of course, when you have access to a computer, programs like Maple and Mathematica have a huge range of powerful linear algebra functions!

## Eliminate!

$$\left[ \begin{array}{ccc} R_{1} & 0 & R_{3} \\ 0 & R_{2} & R_{3} \\ 1 & 1 & -1 \end{array} \right| \left. \begin{array}{c} \mathcal{E}_{1}\\ \mathcal{E}_{2} \\ 0 \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & R_{2} & R_{3} \\ R_{1} & 0 & R_{3} \end{array} \right| \left. \begin{array}{c} 0 \\ \mathcal{E}_{2} \\ \mathcal{E}_{1} \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & \frac{R_{3}}{R_{2}} \\ 1 & 0 & \frac{R_{3}}{R_{1}} \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}} \\ \frac{\mathcal{E}_{1}}{R_{1}} \end{array} \right]$$

⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & \frac{R_{3}}{R_{2}} \\ 0 & -1 & \frac{R_{3}}{R_{1}}+1 \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}} \\ \frac{\mathcal{E}_{1}}{R_{1}} \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & \frac{R_{3}}{R_{2}} \\ 0 & 0 & \frac{R_{3}}{R_{1}}+\frac{R_{3}}{R_{2}}+1 \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}} \\ \frac{\mathcal{E}_{1}}{R_{1}} + \frac{\mathcal{E}_{2}}{R_{2}} \end{array} \right]$$

⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & \frac{R_{3}}{R_{2}} \\ 0 & 0 & \frac{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}}{R_{1}R_{2}} \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}} \\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{1}R_{2}} \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & \frac{R_{3}}{R_{2}} \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}} \\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} \end{array} \right]$$

⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}}{R_{2}}- \frac{R_{3}}{R_{2}} \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}}\\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 1 & -1 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} 0 \\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{2}R_{3}-\mathcal{E}_{1}R_{3}}{{R_{3}R_{2}+R_{1}R_{2}+R_{3}R_{1}}}\\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} \end{array} \right]$$

⇒ $$\left[ \begin{array}{ccc} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} - \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{2}R_{3}-\mathcal{E}_{1}R_{3}}{{R_{3}R_{2}+R_{1}R_{2}+R_{3}R_{1}}} \\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{2}R_{3}-\mathcal{E}_{1}R_{3}}{{R_{3}R_{2}+R_{1}R_{2}+R_{3}R_{1}}}\\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} \end{array} \right]$$ ⇒ $$\left[ \begin{array}{ccc} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} \frac{\mathcal{E}_{1}R_{2}+\mathcal{E}_{1}R_{3}-\mathcal{E}_{2}R_{3}}{{R_{3}R_{2}+R_{1}R_{2}+R_{3}R_{1}}} \\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{2}R_{3}-\mathcal{E}_{1}R_{3}}{{R_{3}R_{2}+R_{1}R_{2}+R_{3}R_{1}}}\\ \frac{\mathcal{E}_{2}R_{1}+\mathcal{E}_{1}R_{2}}{R_{3}R_{2}+R_{3}R_{1}+R_{1}R_{2}} \end{array} \right]$$

## With numbers

$20-I_{3}470-I_{2}3=0$
$12-I_{3}470-I_{1}2=0$
$I_{1}+I_{2}=I_{3}$

$3I_{2}+470I_{3}=20$
$2I_{1}+470I_{3}=12$
$I_{1}+I_{2}-I_{3}=0$

$$\left[ \begin{array}{ccc} 0 & 3 & 470 \\ 2 & 0 & 470 \\ 1 & 1 & -1 \end{array} \right| \left. \begin{array}{c} 20\\ 12 \\ 0 \end{array} \right]$$

By the magic of a computer/calculator…

$$\left[ \begin{array}{ccc} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array} \right| \left. \begin{array}{c} \frac{-49}{31}\\ \frac{50}{31}\\ \frac{1}{31} \end{array} \right]$$