# Ohm’s Law and series and parallel circuits

Ohm’s Law and series and parallel circuits

Ohm’s Law

The fundamental relationship among the three important electrical quantities current, voltage, and resistance was discovered by Georg Simon Ohm. The relationship and the unit of electrical resistance were both named for him to commemorate this contribution to physics. One statement of Ohm’s law is that the current through a resistor is proportional to the voltage across the resistor. In this experiment you will see if Ohm’s law is applicable to several different circuits using an ammeter and a voltmeter.

Current and voltage can be difficult to understand, because they cannot be observed directly. To clarify these terms, some people make the comparison between electrical circuits and water flowing in pipes. Here is a chart of the three electrical units we will study in this experiment.

 Electrical Quantity Description Unit Water Analogy Voltage or Potential Difference A measure of the Energy difference per unit charge between two points in a circuit. Volt (V) Water Pressure, or height difference Current A measure of the flow of charge in a circuit. Ampere (A) Amount of water flowing Resistance A measure of how difficult it is for current to flow in a circuit. Ohm (Ω) A measure of how difficult it is for water to flow through a pipe.

Figure 1

objectives

· Determine the mathematical relationship between current, potential difference, and resistance in a simple circuit.

· Compare the potential vs. current behavior of a resistor with that of resistors in series and parallel.

MATERIALS

PRELIMINARY SEtup and QUESTIONS (to be done in small groups)

1. Go to the following website and click run:

2. Try building a few circuits and playing with the controls.

3. Try to create a circuit that lights the lightbulb.

a) Draw your circuit here (don’t worry about making a proper circuit diagram, just sketch what you see on the screen). You can upload a scan of a sketch if you like.

b) What are the important characteristics of this circuit that make it work? List as many as you can.

4. Now that you can make a light bulb light up, design a simple circuit to test the conductivity of the various objects included in the simulation. Test the following objects by placing them in the circuit (in series with the light bulb) and see if the light bulb lights up or not. Classify them as insulator or conductor based on whether or not a current can pass through them.

Wire, Dollar bill, paper clip, coin, eraser, Hand, Dog and pencil.

Any surprises?

5. Make sure you know how to

a) Insert and remove circuit elements (wires, resistors, batteries, etc…)

b) Attach a voltmeter to your circuit to measure the potential difference between two points in your circuit.

c) Insert an ammeter into the circuit to measure the current through the circuit.

d) adjust the resistance of a resistor and the voltage of the battery.

PROCEDURE

PART 1:

For this experiment, you will follow a simple procedure for determine the resistance using the slope of the V vs. I plot. You will do this for four (4) cases and compare what you get to the theoretical formula for adding resistors in series and parallel.

The cases are

(1) a 10 Ω resistor,

(2) a 50 Ω resistor

(3) the two resistors above in series.

(4) the two resistors above in parallel.

First, set up the circuit as shown in Fig. 1 and follow this procedure:

1. Record the value of the resistor in the data table 2.

2. Starting at 0V, increase the voltage in steps up to the maximum allowed, recording the voltage across the resistor, and the current through the circuit, for each voltage.

3. Make sure to collect at least 8 points for each setting. Create a data table 1 of V and I and include this table in your lab report.

4. Plot the voltage vs. current and fit to a linear function. Include the plot with the fit in your lab report. Fill in the data table 2 with linear fit values.

Data Table – 1.1 (you will have 5 of these by end of experiments, you can label them 1.1 to 1.5)

 V(V) I(A)

Similar tables can be created for all 5 cases measured in this lab.

Now, change the resistor value to 50 Ω and repeat steps 1-4. Make sure to create a new data table for this resistance with the V and I values.

For the next two experiments, you will replace the single resistor with a network of two resistors, one of 10 Ω and one of 50 Ω. You will do this with the two resistors in series, and then repeat with the two resistors in parallel. This will result in two more data sets — create tables and plots with fits as before for each of these. Include fit results in the data table below.

DATA TABLE

 Resistance (Ω) Slope of regression line (V/A) Percent difference Resistor 1 10 Resistor 2 50 Resistors in series (value calculated from analysis question 2) Resistors in parallel (value calculated from analysis question 2)

ANALYSIS

1. Calculate the theoretical resistance for the two resistors in series and in parallel using the value you measured from the slope of the graphs of data sets 1 and 2. Put these values in the table column 2.

2. Determine the percent difference between the measured slope from data for the series and parallel circuits (column 3 in data table) to the value for the resistances you calculated in question 2 (in column 2 in data table).