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Pointer: Does an Electric Current Affect a Compass?

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Author: Janice VanCleave

Problem

Does an electric current affect a compass?

Materials

  • 36 inch (1-m) aluminum foil strip
  • small cereal bowl
  • sewing needle
  • bar magnet
  • tap water
  • corrugated cardboard
  • compass
  • size D battery
  • adult helper

Procedure

NOTE: Never touch a compass with a magnet. This can change the polarity of the compass needle, causing all directions to be reversed.

  1. Wrap the aluminum strip around the bowl as many times as possible, leaving about 6 inches (15 cm) free on both ends of the strip. The first layer should go over the top of the bowl.
  2. Magnetize the needle by laying it on the magnet for two minutes.
  3. Fill the bowl 3/4 full with water.
  4. Measure and cut a 3/4-inch × 3/4-inch (1.9-cm × l.9-cm) cardboard piece.
  5. Ask an adult helper to insert the pointed end of the magnetized needle into the middle of one end of the cardboard square and push the needle through the square until it pokes out of the opposite end. Leave equal lengths of the needle extending from each side of the cardboard.
  6. Float the cardboard square gently on the surface of the water, and allow it to come to rest.
  7. Pointer

  8. Use the compass to determine in which direction the ends of the floating needle are pointing.
  9. Turn the bowl so that the aluminum strips are running in the same direction as the needle, north to south.
  10. Stand the flat, negative terminal of the battery on one of the foil strips.
  11. Watch the needle as you touch the free end of the foil strip to the positive terminal of the battery. Remove it from the positive terminal and repeat this step several times.

Results

The floating needle rotates away from its original position when both ends of the metal coil touch the battery, and then returns when one of the ends is removed from the battery.

Why?

The magnetized needle acts like a compass needle, with one end pointing toward the magnetic north pole of the earth. An electric current flows through the coiled strip when it is attached to the battery terminals. Since the aluminum coil is turned in a north-to-south direction, the movement of electrons through the coil produces a magnetic field pointing east and west. The needle is pulled out of position by the attraction of this magnetic field. Disconnecting one end of the coil from the battery stops the current flow. Without the magnetic field around the strip, the needle again lines up with the earth's magnetic field.

Let's Explore

  1. Does the number of windings of the foil strip affect the results? Repeat the experiment twice: first wrap the foil fewer times around the bowl, and then wrap the foil more times around the bowl. Make sure the windings are smooth and lying in the same direction. Compare the speed at which the needle moves each time.
  2. Does the direction of the current affect the results? Repeat the original experiment twice: first reverse the battery connections, and then repeat again reversing the direction the foil is wound around the bowl. Science Fair Hint: Draw diagrams showing the movement of current from the negative end of the battery to the positive end. Indicate the direction that the needle turns.
  3. Would separating the foil affect the results? Repeat the original experiment twice: first cover the aluminum foil with masking tape, and then repeat using insulated 18-gauge or smaller wire instead of the foil strip. Be sure to wind the foil and the wire the same number of times.
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