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Electromagnetic Antigravity

Science Snack
Electromagnetic Antigravity
Change the measured weight of an object—without touching it.
Electromagnetic Antigravity
Change the measured weight of an object—without touching it.

A current-carrying wire changes the weight of mounted magnets resting on an electronic scale—simple but dramatic proof of the Lorentz force, the phenomenon at work in most electric motors.

Tools and Materials
  • Two sturdy pieces of plastic, each approximately 1 1/4 x 3 x 1/4 inches (Delrin or a similar material)
  • Masking tape
  • Pencil or marker (to mark holes)
  • Two rare-earth magnets, each 1 x 1 x 3/16 inches (2.5 x 2.5 x 0.5 cm), with dual countersunk holes (other sizes will work)
  • Four flat-head machine screws and nuts to match the countersunk holes
  • Two #1/4-20 nylon thumb screws, 1 3/4 inches (1.5 cm) long
  • Six #1/4-20 nylon nuts
  • Electric drill and drill bits: 1/4-inch (6.35 mm) bit for the nylon thumb screws, and appropriate bit for the countersink
  • Safety glasses
  • Three feet (1 yard) of 1/8-inch (3-mm) 10-gauge aluminum armature wire
  • Wire cutters
  • Electrical tape or heat-shrink tubing and heat source
  • Battery holder (either commercial or homemade is fine: we used nails pounded into wood)
  • Two alkaline D-cell batteries (do NOT use rechargeable batteries)
  • Two test leads with alligator clips
  • Electronic scale
  • Optional: 10-amp multimeter and an additional alligator clip lead

1. Place the two pieces of plastic one on top of the other and tape firmly together, as shown in the photo below. (This will allow you to drill both pieces at once, so the holes will line up later.)

2. Center one of the magnets on top of the taped plastic pieces, as shown in the photo below, and use a pencil or marker to note the position of the holes on the tape. Set the magnet aside and go on to Step 3.

3. Approximately one-quarter of the way from each end of the plastic, mark holes for the nylon screws on the plastic pieces. Put on safety glasses, and drill all the holes with appropriate bits (see photo below), and then pull off the tape.

4. Using flat-head machine screws and nuts, attach each magnet to a piece of plastic. Be sure the exposed faces of the magnets will attract one another (see photos below).

5. Push the nylon screws all the way through the open holes on one piece of plastic. The heads of the screws should be on the opposite side of the plastic from the magnet The threads should extend outward on the magnet side. Tighten the nuts to secure the screws.

6. Twist a nut down onto one of the screws so it’s approximately 1/2 inch (1.25 cm) from the nut at the base. Repeat on the second screw (see photo below).

7. Set the second piece of plastic onto the assembly so the magnets face one another (see photo below). Add the remaining nuts to hold everything in place. When you’re done, set this assembly aside.

8. With wire cutters, cut the armature wire into three pieces, each about 1 foot (30 cm) long. Cover all but the ends of each wire with either shrink wrap or electrical tape (see photos below).

9. Leave one wire straight, bend another wire into a tight U-shape, and bend the third wire into a zigzag, as shown in the photo below. When you’re done, set the wires aside.

10. Insert the batteries into the battery holder.

To Do and Notice

Put the short end of the magnet assembly on the scale so it stands up vertically. Then tare the scale by setting it to read zero.

Begin by using both batteries in series. Hold the straight wire between the magnet faces, but not touching the magnets or the scale. Attach one end of an alligator clip lead to one side of the straight wire, and the other end to the positive side of the batteries. Use the other alligator clip lead to connect the free end of the straight wire to the negative side of the batteries. Does the reading on the scale change from zero? (Don’t keep the wire connected for longer than necessary to get the reading, as the connection is rapidly discharging the batteries in this and all the rest of the investigations.)

Now reverse the direction of the current in the straight wire by swapping the ends of the alligator clip leads that touch the positive and negative sides of the battery. How do you think that will change the scale’s reading?

Now connect the alligator clips so that they use just one battery. What do you think will happen to the reading on the scale? When the batteries are connected in series, the voltage increases, and the amount of current in the wire will increase.

Disconnect the alligator clips from the straight wire. Place the U-shaped wire between the faces of the magnet, just as you did with the straight wire. Make sure that the bend sticks out past where the magnets are (click to enlarge the photo below). What do you think the scale will read when you reconnect the alligator clips to the U-shaped wire? Will the direction of the current matter? How will it differ between one and two batteries?

Finally, disconnect the alligator clips again, and place the zigzag wire between the magnet faces. Will this wire be more like the straight wire or the U-shaped wire when you reconnect the alligator clips?

What's Going On?

It may seem strange and almost magical that current flowing through the aluminum armature wire can change the weight of the magnet assembly. After all, the two aren’t even touching. To understand how this works, let’s start where the action is: in the wire.

When an electric charge (such as the current in the wire) moves in the presence of a magnetic field, it experiences a sideways force, also known as the Lorentz force.

Here, the Lorentz force tends to push the wire either up or down—depending on how the magnets are oriented and on which direction the current is flowing. In this experiment, the force created by the current is too small for you to notice the force with your hand. If it were larger, you would see and feel the wire pop up or down the moment you connected the batteries.

According to Newton’s Third Law, all forces come in pairs: If there is an upward force on the wire, there must be an equal and opposite downward force elsewhere. This downward force acts on the magnets in the assembly, causing the scale reading to increase. Conversely, if the force on the wire is downward, then the force on the magnets is upward—causing them to “lose weight,” as shown on the scale.

Increasing the current increases the strength of the deflecting Lorentz force, causing a greater change in the scale reading. On the other hand, two wires with the same current in opposite directions will experience opposite forces that cancel each out—hence the minimal weight change measured in the U-shaped wire. In the zigzag wire, two of the segments cancel each other out, leaving one uncancelled, resulting in a net force similar to that of a single wire.

Going Further

Batteries don’t always provide the same amount of current, due to a voltage drop and changes in internal resistance as they get used. To get even more quantitative results, you’ll want to measure the current more precisely with a multimeter. To do this, set the multimeter to the 10 A range and use the additional alligator clip to place it in series with the aluminum armature wire.

Most scales, including the electronic scale used in this Snack, display units of mass, rather than units of force. In fact, a scale can really only measure force (Newtons), but automatically converts this measurement into mass (grams) on the assumption that it’s operating in Earth’s gravitational field. If you want to convert mass back to force, first convert the reading to kilograms, and then multiply by 9.8 N/kg, the strength of gravity on the Earth's surface.