Phys-112 Lab 10: Electrical to Mechanical Energy
Objective: To investigate how electric motors convert electrical energy into mechanical energy.
Procedure
1. Attach an electric motor to a support rod. Arrange the motor so that it hangs high 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. You can read voltage and current
simultaneously on the digital display built into the power supply.
3. Make some trial runs with a few grams on the mass hanger, adjusting the voltage of the
power supply until the speed of rise is slow (less 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 (A)
Voltage (V)
Mech. Energy (J)
Elec. Energy (J)
Efficiency
7. Repeat the procedure, adding an extra 5 grams of mass for each trial, to obtain a total of
10 trials (up to 50 g).
8. In PASCO Capstone, make and print a graph of Mechanical Energy gained as a function of
Electrical Energy used. Find the slope of the graph.
9. Make and print 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?