Lab Exercise: Ohm’s Law
PS253-10
OBJECTIVES
After completing this lab, you should be able to:
v
v
v
v
Explain the function of a battery in terms of potential difference or voltage.
Understand how voltage is distributed in a series circuit.
Understand how voltage is distributed in a parallel circuit.
Explain Ohm’s Law and describe a method for verifying it.
*****IN ORDER TO GET CREDIT FOR THIS LAB,*****
YOUR INSTRUCTOR MUST APPROVE
1. YOUR VOLTAGE TABLE
2. YOUR ANSWERS TO QUESTIONS
3. YOUR GRAPH OF V vs I
INSTRUCTOR INTRODUCTION: Your instructor will be sure you understand
Ohm's law, how to use the voltage probe and how to use the battery eliminator.
PART 1: Voltage in Series Circuits
In this activity, you will explore the role of the battery in establishing electric current in a
circuit. You will also examine the distribution of voltage across the elements in a series
circuit.
PREPARATION
Electric circuits can be analyzed in terms of energy. Batteries provide energy to the
charges moving in a circuit. These charges lose energy as they move through elements,
such as bulbs, in the circuit. Electric current is the rate of flow of electric charge. You
saw in Lab #4 that the charges themselves are not “used up” as they move through the
circuit. The amount of available energy and the conversion of electrical energy into light
and heat depends upon both the number of batteries and bulbs in a circuit and the manner
in which those batteries and bulbs are connected (in series, in parallel, or in some
combination of series and parallel).
Discuss with your partner and answer the following questions:
Ø Q1: Consider a simple circuit consisting of one battery and one bulb. What
should happen to the brightness of the bulb if a second battery is connected
in series with the first? What if the second battery is connected in parallel
with the first?
Ø Q2: Consider a simple circuit consisting of one battery and one bulb. What
should happen to the brightness of the bulb if a second bulb is connected in
series with the first bulb? Explain in terms of the potential difference across
the bulbs.
EQUIPMENT
Battery eliminator
Circuit Board (w/bulbs)
Wires
Voltage Probes
PROCEDURE
Connect the leads from your battery eliminator to either end of ONE light bulb. Observe
the bulb brightness with your battery eliminator set to 1.5 V. Now, change the magnitude
of the supplied voltage to 3.0 V. Observe the bulb brightness, then turn off your voltage
supply.
Q3: What happens to the brightness of the bulb when the second battery is added?
Figure 1
Series Circuit
Now add a second bulb in series with the first, as shown in Figure 1. Turn the voltage to
3.0 V and observe the bulb brightness. Try it again with the third bulb in series.
à Now go back to the circuit that just has two bulbs in series. Using your voltmeter,
measure the voltage across lamp 1 (V1), then across lamp 2 (V2) three times each for
each of the battery eliminator voltages shown listed in the table. Then check the voltage
across the battery eliminator (VB).
NOTE: Make the measurement at 4.5 V a quick one so you don’t burn out the bulbs.
Record your data in the table below:
V1
V2
VB
Measured
Voltage
setting
1.5
3.0
4.5
Ø State a rule that explains how the potential difference of the battery
eliminator is related to the potential differences across each of the bulbs in
series.
PART 2: Voltage in Parallel Circuits
In this activity, you will examine the distribution of voltage across the elements in a
parallel circuit.
Figure 2
Parallel Circuit
Build the circuit shown in Figure 2.
Ø Measure and record the voltage across each of the two bulbs when
the battery eliminator is set to
a.) 1.5 volts
V1=
V2=
b.) 3.0 volts
V1=
V2=
Ø State a rule that explains how the potential difference of the battery
eliminator is related to the potential differences across each of the bulbs in
parallel.
PART 3: Ohm’s Law
In this activity, you will investigate the relationship between voltage and current.
PREPARATION
All circuit elements resist the flow of current to a certain extent. Light bulbs provide
electrical resistance, and we have used them to learn about circuits up to this point. As
the filament of a light bulb gets hotter, its ability to resist the flow of current changes. In
order to understand the relationship between voltage and current, we must have constant
resistance in the circuit. Therefore, instead of using light bulbs, we will use a common
device known as a resistor. You simply need to know that they serve the same purpose
in our circuits as the light bulbs have, but they do not give off light and their behavior
does not depend upon their temperature.
The relationship that we will investigate is known as "Ohm’s Law." Not all circuit
elements follow Ohm’s Law – there are both "ohmic" and "non-ohmic" devices – but a
great many do, so it’s useful in a wide variety of situations.
EQUIPMENT
Battery Eliminator
Resistance substitution box set to 10 Ω
Black and red leads
Voltage and Current Meter
PROCEDURE
To build the single-resistor circuit, turn off the power to the voltage source and connect
each end of the battery eliminator to the plugs on either side of the zig-zag symbol on
your resistance substitution box. Make sure all but the 10 Ω switch are “out.”
Current (Amps)
Voltage (Volts)
Measure the current into the resistor and the potential difference across the resistor with
six different applied voltages.
GRAPH: Using the supplied graph paper, draw a graph of voltage (on the y-axis) vs.
current (on the x-axis). Draw a best fit line through your data points. Determine the
slope and y-intercept of the line, and write an equation for your best fit line.
PLEASE CLEAN UP YOUR AREA AND STRAIGHTEN YOUR CHAIRS, SO THE
STATION IS READY FOR THE NEXT STUDENTS TO USE IT.