Download LAB D07: CURRENTS and VOLTAGE DROPS IN SERIES and

LAB D07: CURRENTS and VOLTAGE DROPS IN SERIES and PARALLEL
CIRCUITS PH354, NCSSM, 1/03/07
Reading: Walker 21 (2) Ohm’s Law and 21 (5) Kirchhoff’s ( Loop ’s) Rules
Equipment
· battery pack
· breadboard
· multimeter
· alligator clip and/or other wires
· 3 resistors of different resistances (each with 15000 W > R > 150 W ); maximum R
value should be no more than 3x minimum R value and the middle R should ideally fall
near the middle of the maximum and minimum R values. THIS RELATIONSHIP IS
IMPORTANT!
Calculations
****The key to writing a report that conveys the work and information involved in
a clear manner is to first read this lab in entirety. By doing so, you can plan the
layout of your report and data tables so that the sections concerning each of the
four separate circuit analyses are self-contained. Poor planning will result in a
report that hops around in a confusing way. Poorly planned reports obviously
don’t lead to maximum credit or, more importantly, to good study material for
you.****
1. Measure and record the actual resistance values of the three individual resistors.
Choose one good battery and measure and record its voltage. Either you or your
partner should keep the same resistors until this entire lab is complete; switching
resistors mid-lab is NOT OK. You also want to use a battery that has about the same
voltage difference in each circuit.
2. You will follow the procedure listed below for each of the 4 circuits you designed in
part C of Lab D6 and then build each circuit and make the measurements
corresponding to the things that you calculate below.
A) Draw the circuit diagram (it should take up half a page and ideally start on the upper
left page, so as to minimize page turning); label the resistors R1, R2, R3 and the battery
voltage, DVb. Appropriately label (mnemonically) all currents and voltage drops in the
diagram.
Then write
· Conservation of energy-- a.k.a. the loop rule; write the conservation equation for each
loop in the circuit.
· Conservation of charge-- a.k.a. the junction rule; in your circuit diagram, label each
junction for which you write a conservation equation (labels A, B, etc. are good) and
refer to the label next to the equation. for example, for junction A:
· Ohm’s law for each resistor
B) Calculate all of the following, (be sure to use the appropriate subscripts!):
- current through each resistor and through the battery
- voltage drop across each resistor
When solving for any of these quantities, do not substitute any numbers into your
formulas until you have expressed the unknown quantity entirely in terms of knowns.
However, due to the complexity of solving in terms of only the originally given variables
R1, R2, R3 and Vb, , you may express your formulas in terms of any quantities you have
already found numerically {e.g., if you have already clearly solved symbolically for I 2,
you can solve for I3 in terms of I2}. Organize your work clearly!
C) Organize your predicted currents, predicted voltage differences and predicted
powers neatly in tabular form. You should create a separate table for each of the
four circuits. As you create each table, leave space for additional columns for
measured currents and voltages, and percent differences. Do NOT cram
everything together…. leave room for grader comments
D1) Build each of the circuits and measure and record the current through and the
voltage drop across each resistor. Record these values in your data table(s). This
should not take long at all if you have properly prepared!
D2) Properly compare (% differences) the predicted values to the directly measured
values. Enter the results in your already prepared table(s).
E) How should the power produced by the battery relate to the power dissipated (as
light or thermal energy) in the individual resistors? Why?
Determine (symbols first, then values!) the power dissipated in each resistor and
that supplied by the battery. Show and discuss how your predicted values do or
do not support your answers to part 2E. Do you need to modify your 2E answer?
Of course, you need a CONCLUSION to this lab in your journal.