Introduction

Copper ions in solution moving between two electrodes in a magnetic field produce a swirling motion that can easily be displayed on an overhead projector.

Equations

Cu  Cu2+ + 2e-

Cu2+ + 2e-Cu

Building the motor

Prior to Lecture

  1. Place two bar magnets (available from Radio Shack) in a petri dish with the same poles facing up.
  2. Prepare a second petri dish by adding a shallow layer of 1M copper sulfate solution.
  3. Place a short piece (about 3 or 4 cm in length) of 1/2" diameter copper pipe in the center of the copper sulfate solution.   Bend some heavy uninsulated copper wire (about 3 or 4 mm diameter, available at McGuckin's Hardware) around the inside edge of the petri dish.
  4. Put the petri dish with the copper sulfate solution and electrodes on top of the dish holding the magnets.
  5. Provide the lecturer with a 6 to 12 volt, 1 to 2 amp battery or power supply, two wires with alligator clips on the each end, and some pepper to sprinkle on the surface of the copper sulfate solution.

To Conduct Demonstration

  1. Place some pepper on the surface of the copper sulfate solution to make it visible. 
  2. Place the apparatus on an overhead projector and focus on the floating pepper.  There should be enough solution visible between the magnets to see any movement of the pepper.
  3. Connect the positive and negative wires from a DC power source to the copper electrodes.  The solution should start to slowly rotate.  Reverse the positive and negative wires and the solution will begin to rotate in the opposite direction.  Finally, without reversing wires, turn over the magnets below the solution, thereby reversing the magnetic pole facing upwards toward the liquid.  Again, the direction of rotation should reverse.

Note:  The movement of charged particles in a magnetic field produces a force.  In an electric motor, electrons moving through the armature wires in a magnetic field produce a torque that turns it.  In this demo, the copper ions are produced at the positive electrode and move through solution to be reduced at the negative electrode.  These ions are moving through the field of the magnets situated below the copper sulfate solution; thus a torque is applied to the liquid.

Safety and Disposal

Avoid contact of copper sulfate with skin.  Collect solutions of copper compounds for proper disposal through EH&S collection procedures.