Lab 6 Electrical to Mechanical Energy
Objective: To investigate how electric motors convert electrical energy into mechanical energy.
Part I. Electric Field
1. Attach an electric motor to a support rod. Arrange the motor so that it hangs over the edge of
the lab tables, so that the string can extend from it all the way to the floor. Attach a mass hanger
to the string on the motor.
2. Connect the motor cables to the power supply. Connect an ammeter in series with the motor, so
that you can read the current. (Use the mA scale.) Connect a voltmeter in parallel across the
terminals of the motor, so that you can read voltage and current simultaneously.
3. Make some trial runs with a few grams on the mass hanger, adjusting the power supply until the
speed of rise is slow (no more than walking speed) and the hanger rises at a steady rate. (You
want to be able to accurately measure the time to rise, and at the same time have the kinetic
energy of the hanger remain constant, so the only work being done is going to raise the potential
energy of the mass.)
4. Pick two points, just above the floor and just below the motor, and practice measuring the time
of rise between these two points. (The speed of the mass should be constant between these two
points.) Measure the distance between the two points.
5. With just the mass hanger on the string, measure the time to rise through the measured distance.
Record this time, along with the current and potential as the mass is rising in a table.
6. Calculate the change in Mechanical Energy of the mass as it rises. Calculate the Electrical
Energy used by the motor as the mass rises. (Recall that Energy is equal to Power times time.)
Calculate the efficiency of the motor as the ratio of the Mechanical Energy output to the
Electrical Energy input. Record these in the table.
Mass
(kg)
Time
(s)
Current
(Amp)
Voltage
(v)
Mechanical Electrical
Energy (J) Energy (J)
Efficiency
7. Repeat Step 5 adding an extra 5 gram mass for a total of 10 different masses (up to 50 g).
8. In Graphical Analysis, make a graph of Mechanical Energy gained as a function of Electrical
Energy used. Find the slope of the graph.
9. Make a graph of the Efficiency of the motor as a function of the mass being raised.
Questions:
1. What does the graph of Mechanical Energy gained as a function of Electrical Energy used tell
you about the operation of the motor?
2. What does the graph of the Efficiency of the motor as a function of the mass being raised tell
you about the operation of the motor?