# Resistance Relationships: Ohm�s Law and Resistance by 8ubrKB

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Resistance Relationships II PreLab
Instructions: This PreLab activity MUST be completed before arriving for your scheduled lab activity.
No time will be given during lab to complete this exercise. PreLabs must be turned in upon arrival in lab;
late submissions will not be accepted. Each PreLab is part of the laboratory and will be scored
accordingly. Please circle the letter of the best response for each question.

1. Which of the following sets of resistors are wired in series? Circle all correct responses.

a.                                                 b.

2. Which of the following circuits is properly wired for measuring the indicated variable (A =
ammeter, V = voltmeter)? Circle all correct responses.

a.                                                      b.

3. An amp meter measures current. Would it have a high or low internal resistance and why?
a. Low – the meter must be connected in series with the circuit.
b. Low – the meter must be connected in parallel with the circuit.
c. High – the meter must be connected in series with the circuit.
d. High – the meter must be connected in parallel with the circuit.

4. A volt meter measures voltage. Would it have a high or low internal resistance and why?
a. Low – the meter must be connected in series with the circuit.
b. Low – the meter must be connected in parallel with the circuit.
c. High – the meter must be connected in series with the circuit.
d. High – the meter must be connected in parallel with the circuit.
Resistance Relationships II Lab

Objectives: At the conclusion of this lab activity, the student should be able to clearly and accurately:
 state the rules for determining equivalent resistance in series and parallel circuits.
 state, using a concept of conservation, an explanation for why the above two rules work the way they do.

Caution: The voltage and current sensors by Vernier that you will be using can withstand a maximum of 10 volts
and 0.6 amps respectively. Be certain not to exceed these limits or the units will be damaged.

Task I. Determine equivalent resistances in series and parallel resistors.

a. Using only resistors and connecting wires, set up a two series circuits using two and then three resistors. Using
experiments of your own design, determine the relationship between individual resistances and the total resistance
of a series circuit. Measure the total resistance using voltage and current sensors, and your knowledge of Ohm’s
law. You need not generate a graph for this section, but you should write your data in the table on the next page.

Resistor #       Individual Resistances ()             Combinations              Total Resistance ()
R1                                                      R1+R2

R2                                                      R2+R3

R3                                                      R3+R1

Q1. Based on the evidence above, what is the relationship between individual resistances, (e.g., R1, R2, R3) and the
total resistance (Rt) of a series circuit?

b. Using a fixed resistor, a resistor box (variable resistor), and wires as necessary, set up a parallel resistance
circuit. Using Ohm’s Law (V=IR) and an experiment of your own design, determine the relationship between
individual resistances and the total resistance of a parallel circuit. Note carefully the resistance parameter (a fixed
variable) of your system before starting. Collect data, generate a graph, perform regression analysis, and find the
relationship between the independent and dependent variables. Print the graph and data; label this Graph 1. (Hint:
Consider graphing 1/Rn on each axis to linearize the data.)

Q2. What is the physical meaning of the y-intercept in your regression equation? Does it somehow relate to a
system parameter? Which? How are these related?

Q3. What is the relationship between individual resistances, (e.g., R1, R2, R3) and the total resistance (Rt) of a
parallel circuit?
Task II. Explain series and parallel equivalent resistance laws.

Q4. Now that you have experimentally determined the form of the series and parallel resistance laws, you need
explain why they work that way. Given your knowledge of Ohm’s law, why is it that the series law for resistors
holds? Show your work below. (Hint: Substitute Vn/In for Rn for each R-value in your empirical series equation and
cancel identical terms. You will see that something is conserved - remains constant - in series circuits. What is it?)

a. You have probably noticed how your prediction indicates that the sum of the voltage drops over each of the
resistors in a circuit should be equal to the voltage impressed on that circuit. Collect and provide experimental
evidence to show that this is the case. Use the space below to draw a schematic diagram. Include experimental
values.

Q5. Given your knowledge of Ohm’s law, why is it that the parallel law for resistors holds? You will see that
something is conserved in parallel circuits as in series circuits. What is it? Show your work below. (Hint: Use an
approach similar to but different from that for series circuits.)
b. You have probably noticed how your prediction indicates that the total current over all the branches of a parallel
circuit sum to a constant value. Collect and provide experimental evidence to show that this is the case. Use the
space below to draw a schematic diagram. Include experimental values.

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