Hands-On Lab
Making or Breaking A Circuit
Timing: one 90-minute class session
Objective(s):
Students will learn how to use knife-edge switches in simple series or parallel circuits.
Safety Precautions:
Remind students to follow all general lab safety rules, especially those governing use of
electricity.
Materials:
Per group:
• 1.5-V battery
• battery holder
• three resistors, with different values
of resistance
• three knife-edge switches
• light bulb
• multimeter with data probes
• several insulated wire pieces, with
bare ends
Teacher Preparation:
• Gather materials in advance of students performing the lab.
• If possible, collect resistors of different values of resistance.
Procedure
Part 1: Series Circuit
1. Each team is to connect the following series circuit, including an open switch:
Page 2 of 6
Discovery Education Science
© Discovery Communications, LLC
2. Turn multimeter to measure voltage.
3. Using probes, measure voltage across 1.5-V battery.
4. Measure voltage across the open switch in the open circuit.
5. Close switch, and see that light bulb comes on.
6. Measure voltage across each resistor in the closed circuit.
7. Turn multimeter to measure current.
8. Measure current across each resistor in the closed circuit.
9. Calculate power expended in each resistor. This is a product of current and voltage.
10. Students should make a chart of their observations. A chart might look like:
Chart of Resistor Data
Quantity
Resistance, ohms
Voltage, volts
Current, amps
Power, watts
Resistor #1
1
0.09
0.09375
0.0088
Resistor #2
5
0.47
0.09375
0.044
Resistor #3
10
0.94
0.09375
0.088
Part 2: Parallel Circuit
1. Each team is to connect the following parallel circuit, including three open switches:
2. Turn multimeter to measure voltage.
3. With all switches open, measure voltage across each resistor in the open citrcuit.
Page 3 of 6
Discovery Education Science
© Discovery Communications, LLC
4. Close all switches, and see that light bulbs come on.
5. Measure voltage across each resistor in the closed circuit.
6. Switch multimeter to measure current.
7. Measure current across each resistor in the closed circuit.
8. Open switch in one branch, and repeat Steps 5 through 7.
9. Measure voltage across both open switch and resistor in open branch.
10. Close the switch that was opened in Step 6, open the switch in the other branch, and
repeat Steps 5 through 9.
11. Students should make a chart of their observations. A chart might look like:
Chart of Resistor Data
Quantity
Resistance, ohms
Voltage, volts
Current, amps
Power, watts
Resistor #1
1
1.5
0.67
0.45
Resistor #2
5
1.5
0.3
0.45
Page 4 of 6
Discovery Education Science
© Discovery Communications, LLC
Analysis and Conclusions
1. What do knife-edge switches do in any circuit?
If a circuit contains a switch, that switch can be opened to break it.
2. What voltage did you measure across resistors in the simple series circuit with the switch turned
off? What voltage did you measure with the switch turned on?
When the switch is open, the circuit is broken, so no voltage is measured across any of the resistors.
When the switch is closed, the voltage across any resistor is a portion of the battery’s voltage. That
portion is equal to the ratio of the resistance of the measured resistor over total resistance in the
circuit.
3. Why was a light bulb included in either circuit?
A light bulb, when lit, shows that the circuit is closed, there is current in a circuit and voltages can be
measured.
4. What voltage did you measure across resistors in the simple parallel circuit with the outer switch
turned off or on? What about with switches turned on or off in either of the two parallel branches of
the circuit?
When the main switch was open, there was no current in either branch of the circuit, and no voltage
was measured across resistors in that branch.
When the main switch was closed, but the switch in either branch was open, there was no current in
that branch, and no voltage was measured across resistors in that branch.
When the main switch was closed, and the switch in either branch was closed, there was no current
in the branch and voltage measured was a little less than that of the battery.
5. When the main switch was closed in the simple parallel circuit, and one switch in either branch was
closed and the other open, why did you measure voltage across even the branch with an open
switch? Can you explain the value of this measured voltage?
Voltage gains (the battery) and voltage drops around any circuit loop total to zero. The branch with an
open switch still formed a circuit loop with the battery, so voltage measured across the entire branch
was a little less than that of the battery. It was less than that of the battery because there was a small
voltage drop across the light bulb.
Page 5 of 6
Discovery Education Science
© Discovery Communications, LLC
In this lab, students will demonstrate the following Inquiry Skills:
•
•
•
Identify
o Develop
ƒ state the expected cause and effect (if-then statement) in an investigation based
on prior knowledge and experience (hypothesis)
Evaluate Evidence
o Drawing and supporting a conclusion by
ƒ Using data to determine the cause-effect relationship observed in the
investigation.
Patterns and Systems
o Patterns and change:
ƒ Patterns in nature may be simple repeating patterns or complex changing
patterns
ƒ Some events can be predicted with certainty, such as sunrise and sunset, and
some cannot, such as storms.
Page 6 of 6
Discovery Education Science
© Discovery Communications, LLC