Electric Whirl Pool

Magneto hydrodynamic propulsion in your coffee cup!

Materials:

• Any AA or C battery.
• Two pieces of wire
• Plastic cup of water (preferably clear plastic for better viewing)
• 1 disc magnet. We recommend using neodymium magnets like those that came with the 2009 PhysicsQuest kit. These are often called rare earth magnets.
•  Salt
• Pepper
• Tape

Instructions:

1. First, we will dissolve some salt into the cup of water. Mix enough salt until the water is saturated.

2. Place the magnet underneath the center of the cup of water. You can also submerge the magnet at the bottom of the cup but this may cause the magnet to corrode in time.

3. Sprinkle some pepper lightly on the surface of the water.

4. Tape the wires to the ends of the battery so that stick straight out perpendicular to the length of the battery.

5. Hold the battery above the cup and stick the wires into the saltwater. Make sure that one wire is in the center of the cup above the magnet and the other wire is touching the inside of the cup.

6. Enjoy!

Questions:

• What happens when you switch the polarity of the battery?
• What happens if you flip the direction of the magnet?
Discussion:

When electric current passes through a magnetic field, it creates a force that we call the Lorentz force. This force is maximized when the current is perpendicular to the magnetic field. This phenomenon was formulated by James Clerk Maxwell in 1861 and further refined by Hendrik Lorentz in 1891.

The electric current travels from the wire into the salt water. During its brief swim in the salt water, the electric current passes through the magnetic field that emanates from the magnet. The current is perpendicular to the magnetic field and hence creates a Lorentz force on the water. This causes the saltwater to swirl around. It is truly amazing to see how the two “invisible” concepts of electric current and magnetic field can combine to move something tangible like Salt water. Even though salt water is more conductive than normal tap water, it is worth experimenting with less or no salt. The pepper allows you to see the motion of the water. You could also try food coloring… or cake mix!

Suggested Resources:

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