Phy 213 General Physics III1

Instructor: Tony Zable

Experiment: Faraday’s Law & the DC Motor

Objective: To investigate electromagnetic induction and use it to build a simple DC motor.

Equipment:

File name: 213Lab10-FaradaysLaw-DCmotor

Phy 213 General Physics III1

Instructor: Tony Zable

  • solenoid
  • connection wires
  • analog galvanometer
  • 4-6 strong block
  • 2 bar magnets
  • 5V DC power supply (or battery)
  • a drinking straw or dowel
  • 1 empty thread spool
  • 2 needles
  • masking tape
  • bare copper wire
  • 4-6 strong block magnets

File name: 213Lab10-FaradaysLaw-DCmotor

Phy 213 General Physics III1

Instructor: Tony Zable

Part 1: Electromagnetic induction

1. Set-up the simple solenoid circuit as shown below:

2. Move a pair of block magnets (pole facing solenoid) back and forth along the axis of the solenoid opening. What do you observe?

3. How does the motion of the magnet affect the current observed by the meter?

Part 2: Faraday’s Law

1. Connect Differential Voltage Probe to Ch1 of LabPro interface.

2.Start LoggerPro software and set the data collection interval to 5s and the rate to maximum setting (~1000 samples/s).

3.Set-up the simple solenoid circuit shown below:

a.Attach Magnetic Field Sensor to bottom of ring stand with white dot pointed upward

b.Position solenoid above the magnetic field sensor (about 9 cm above table top)

c.Attach a spring to a support rod and attach the stacked block magnets to the spring, using a binder clip and tape.

d.Position the spring so that the magnets suspend about halfway down into the solenoid (the only concern is that the magnetic field sensor signal does not saturate and that the magnets do not rub against the sides of the solenoid or bump the field sensor below).

e.Connect voltage probe leads to the solenoid.

4.Contract the spring then release it so that the magnets oscillate up and down. The motion should a small displacement so that the motion is roughly sinusoidal.

5.Collect Potential vs time and Magnetic Field vs time measurements using LoggerPro. Verify that observed magnetic field and potential are roughly sinusoidal. If the signals are not sinusoidal in shape, adjust your apparatus and re-collect.

6.Cut-and-paste the Potential and Magnetic Field Graphs into Word.

7.Create a Continuous Function (Curve Fit) for the Magnetic Field Data. When you are satisfied with your measurements, you will need to fit your B vs t graph to a continuous function (Bfit) so that the time derivative (dBfit/dt) can be calculated. Fit the sinusoidal region of the magnetic field data to a “Sine” function. In the Curve Fit Window, click on Create Calculated Column then Show Curve Fit. Click Okay. Re-label the new data column to “Bfit” and enter the appropriate units.

8.Create a calculated column to calculate dBfit/dt. You will need to enter the appropriate mathematical definition into the Equation field. Create a plot of V vs dBfit/dt. Unfortunately, you will not yet be able to perform a Curve Fit for this graph.

9.Sort the Data Values using Excel. The LoggerPro data will need to be sorted so that a curve fit for the V vs dBfit/dt graph can be performed. Use CTRL-A to highlight all of the values in the Data Window then cut-and-paste the data into an Excel spreadsheet. With the values still highlighted, sort the data by Potential values (probably column 3, but verify this).

10.Cut-and-Paste the Sorted Data into Graphical Analysis.Export the sorted data into Graphical Analysis and Label the columns appropriately. Create a graph of V vs. dBfit/dt. Fit the graph to an appropriate function and record the fit parameters with uncertainties.

11.Cut-and-paste the graph into Word. Print all of the graphs.

Fit Function:
(i.e. y=mx + B)
Coefficient Label / Value (w/ units) / Uncertainty

Questions:

1. Using your V vs. t and B vs. t graphs as reference, how does the changing magnetic field affect the induced voltage in the solenoid?

  1. Can you explain why the V vs. t graph looks so choppy and granular?
  2. What is the significance of the slope of the V vs. dBfit/dt graph?
  3. Explain how increasing or decreasing the number of suspended magnets would affect the induced voltage in the solenoid? Consider both the mass of the magnets and magnetic fields in your response.
  4. Construct an equation based on your data for the relationship between V & B (this is your interpretation of Faraday’s Law).

Part 2: Building a simple DC motor

1. Set-up the simple solenoid circuit shown below:

  1. Construct the armature. Insert a straw or dowel into middle of spool
  2. Obtain 2 pins. Wrap masking tape individually around each pin (enough to secure the pin in the end of the straw). The pin must be in the center of the tape winding or it will be off-balance and won’t rotate properly. Place a tape wrapped pin into each end of the straw with the flat head pointing outward.
  3. Wrap the armature. Cut a piece of thin copper wire approximately 3 to 5 meters in length. Wrap the wire around the spool as many times as possible and attach the ends of the wire to opposite sides (on the same end) of the straw using tape (see above diagram). Try to keep the windings as tight together as possible.
  4. Set-up a spin support for the motor and attach the armature. Set straw onto the support surface. Attach a support strip to the support surface at each end of the straw and secure them with a thumbtack (as shown in the diagram).
  5. Place the ends of the pins into the support strips. You want the spool & straw to freely rotate while the pins support it.
  6. Attach the motor “brushes”. Secure a couple copper strips to the support structure on opposite sides of the straw near the exposed wire using thumbtacks.
  7. Connect the DC power supply. Connect the motor in series with the power supply. Use a battery or the (5 V, 0-3 A) setting of the electric power supply.
  8. Have instructor check your set-up before turning on the power supply. Once checked, turn on the power supply.
  9. Start your motor. Place several block magnets near opposite faces of the armature and get your motor runnin’. You will need to “fiddle” with the brush positions to get decent performance. Also, you will need to play with the position of the magnets to get the best performance for the motor.

Questions:

1.How does the position of the magnets affect the operation of the motor? Be a specific as possible.

2.Vary the quantity of magnets and observe the effects on motor operation. How does the number of magnets affect the motor operation?

3.Go around the room and compare the motors constructed by other class groups. How does your motor compare with the motor constructed by other groups? What could you do to make your motor better?

4.Describe how electrical energy is converted to mechanical energy in your DC motor. Be as specific as possible.

5.Sketch a diagram of the appropriate vectors acting on your DC motor, as described in question #7.

File name: 213Lab10-FaradaysLaw-DCmotor