Kirchhoff's Laws
Purpose
Introduction
There are two sets of fundamental laws upon which all of our electric circuit
analysis is based. These laws are:
1) Ohm's Law - These provide the relationship between the
voltage across the particular element and the current through the element. These
laws are most frequently described by a mathematical equation (ie. Ohm's Law E =
IR) or a graphical relationship (ie. diode characteristic curve).
2) Kirchhoff's Laws - These laws
relate the currents and voltages of all circuit elements and are based on the way
in which the elements are connected.The two laws which were formulated by
Kirchhoff are stated as follows:
- Kirchhoff's Current Law (KCL): For any lumped parameter network, for any node,
for all time, the algebraic sum of all the currents entering (or leaving) the
node is zero. This is a statement of conservation of
charge where no node can store or radiate a charge.
-Kirchhoff's Voltage Law (KVL): For any lumped parameter network, for any loop,
for all time, the algebraic sum of the voltages around the loop is zero. This is
a statement of the conservation of energy.
Pre-Lab
1. Review the material in your textbook pertaining to Kirchhoff's
laws, series and parallel circuits, voltage and
current dividers.
2. Calculate the resistance that the network in Figure
1 presents at the terminals ab.
3. Calculate the current that would flow in each resistor if a source
of 12 V is connected to terminals ab.
4. Calculate the voltage that would appear across each resistor with a source of 12
V connected to terminals ab.
5. Create a table with lines for each resistor in Figure 1 and columns for
calculated and measured Voltage across each, current through each and the measured resistance of each
resistor. (In all there should be 6 columns.) Fill in the calculated results
from Sections 3 and 4.
6. Repeat 2 through 5 for the circuit in Figure
2.
Equipment
Proto Board
Short pieces of wire
Resistors: color
code
| R1 | R2 | R3 | R4 | R5 | R6 | R7 |
| 1.50kΩ | 3.32kΩ | 475Ω | 2.21kΩ | 4.75kΩ | 2.21kΩ | 1.00kΩ |
Procedure
1. Construct the circuit shown in Figure
1 . Show me
how
2. Measure the resistance Rab.
3. Connect a 12 V source to terminals ab.
4. Measure and record the DC voltage
across each resistor Show me how
5. Measure and record the DC current in each resistor. Show
me how
6. Measure and record the values of each resistor. Show
me how
7. Repeat 1 to 5 above with the circuit in Figure
2 .
Record your readings in a table with the same format as used in the Pre-Lab.
Results
1) Present your written calculations and measured results from the procedure sections.
2)Were the measured resistances within indicated tolerances? (State the percent off each is.)
3)Present a table showing how different (%) the calculated and measured voltages and currents were.
4) The
HP E2377A has an input impedance (input resistance) of 10 Meg. ohms. Taking this
into account perform a calculation to determine, how much would it affect the voltage measured across R7.
Remember, a formal report is required as indicated in the "Introduction".