Energy, Machines, and Motion
Part One: Energy
Lesson 9: Power of a Motor
Students learn how to calculate the power of a small electric motor. Students then determine the relationship between the number of batteries connected in series and the power of the motor.
Focus Question: How do scientists describe the term power?
3. Lesson Set-up and Management
EALR 2: Inquiry
Core Content: Questioning and Investigating
6-8 INQC Collecting, analyzing, and displaying data are essential aspects of all investigations.
EALR 4: Physical Science
Core Content: Balanced and Unbalanced Forces
6-8 PS1D The same unbalanced force will change the motion of an object with more mass more slowly than an object with less mass.
- Students learn power is the rate at which work is done.
- Students learn the common units of measure for power.
- Students investigate how a motor’s power is related to the number of batteries connected in series to the motor.
- Students investigate how the time it takes a motor to lift a load is related to the number of batteries connected in series to motor.
3. Lesson Set-Up and Management
Materials:
- Students will be required to accurately measure the time it takes to lift a load of washers. Consider having students practice using the stopwatches. Having students time dropped objects is good practice for the timings they will need to gather in the lesson.
- Make sure batteries are fully charged for lesson.
- Have students secure string to the nail with a small piece of tape. Be careful to not increase the size of the shaft by using too much tape.
- Make sure motors are in good working order. Occasionally if a motor is not working lightly turn the shaft when it is hooked up to a battery.
- Have students make sure the batteries are hooked up correctly in
series if their motors are not working.
Student Management:
- Having students recognize the work will be the same in all of their tests. Emphasize for students that in this lesson the work will be the same for all three tests, the same load will be lifted the same distance, the difference in time is then the amount of power when additional batteries are lifted.
- Students will connect one battery and lift the three washers. Do all three trials and average. Review data to see if timings are accurate between groups.
- After students have finished gathering data for one battery have them predict the amount of time needed to lift the three washers with two batteries. Have them write their predictions in their notebook.
- Students will lift three batteries to the table top. In a classroom you may have different heights from group to group, this might cause a bit of confusion between groups if not addressed.
- When students calculate their work, make sure that students convert centimeters to metric units.
- If the motors can not lift three motors with one battery there is no work. You could have students make this generalization and move on to two and three batteries. One other possibility is to calculate the power of two washers for one, two and three batteries in series.
- In the final reflection questions students are asked if the amount of work has changed. Many students still think that different batteries will produce different amounts of work. Additional examples may be necessary to develop this concept.
Writing Support:
- Students will need to calculate the amount of power for each
battery. Consider making a chart to assist those students having
difficulty:
Number of Batteries
Distance Lifted (m)
Mass of Washers (N)
Work to Lift Washers (N-m)
Time Need to Lift Washers (s)
Power Generated to Lift Washers (W)
1
2
3
- Have students sketch the pegboard system picture into their
notebook. Have students reflect on the energy transformations that
occur as the batteries lift the washers. Include where the
transformations take place. A good source of information regarding
transformations is included in the Teacher’s Edition under Measuring
Power.
Reading Support:
- Read Work, Energy, and Power together as a class. The information regarding how to calculate power is necessary for this lesson.
- The reading selection How Many Horses describes how James Watt developed the unit of horsepower.
- The Power of Nature conveys the amount of energy in natural events and how humans harness some of that power.
Math Support:
- Students learn how to calculate power through the reading Work, Energy, and Power then practice with the questions in the Getting Started portion of Lesson 9.
- Have charts posted in the room for students to reference how to calculate work and power.
- Students need to convert centimeters to meters (10 cm = 0.1 m).
- Students need to graph both the average time to lift a load and the power of the motor. Students may still need support in creating these graphs. There are excellent examples in the Teacher’s Edition.