Circles of Magnetism IV
Two parallel, current-carrying wires exert forces on each other.
When an electric current flows through a wire, a magnetic field is created around the wire. If you place two current-carrying wires near each other, the magnetic field around each wire exerts a force on the current flowing in the other wire. These forces can push two current-carrying wires apart, or pull them together.

(15 minutes or less)

Make a stand from wood or Tinkertoys™ (see the photo on page 14 and the diagrams below), or build a stand of your own design from available materials.

Cut a strip of aluminum foil measuring about 2 feet (60 cm) long and 1/2 inch (1.3 cm) wide. Tape one end of the foil strip to your support. Run the strip down and back up to the support, making a loop, then tape the other end in place. Be sure the ends of the strip do not touch.

Attach one clip lead to each battery terminal, but do not attach the other ends of the lead wires to the strip yet.

(15 minutes or more)

Touch the two clip leads to the ends of the foil strip. The descending and ascending portions of the loop will repel each other. The closer you can hang the descending and ascending portions of the loop to each other - without allowing them to touch - the larger the repulsion.

Now hang the foil strip from the support with the two ends overlapping, so they make a good electrical contact. Connect one of the clip leads to these overlapping ends. Separate the two sides of the loop and briefly touch the other clip to the bottom of the loop. Notice that the sides of the loop are attracted to each other when the current flows. (This step requires a little coordination and a delicate touch to clearly demonstrate that it is the current flow in the strips that makes them move together and not forces that you create when you touch the clip to the bottom of the loop.)

A current-carrying wire generates a magnetic field that circles the wire (See the "Circles of Magnetism I" Snack.)

When a current flows in a magnetic field, the field exerts forces on that current. (See the "Motor Effect" Snack.) So each current-carrying wire in this Snack generates a magnetic field at the position of the other wire and thus exerts a force on the current in the other wire. Two parallel wires will either attract or repel each other, depending on the direction of current flow in each wire. If both currents flow in the same direction, the wires will attract; if they flow in opposite directions, they will repel.

The forces produced on the aluminum foil are small. This is because the electrical current flowing through the foil is small, only a couple of amperes. Larger currents produce larger forces. The Exploratorium exhibit, for example, uses wires carrying 400 amperes, which produces forces that are more than 10,000 times stronger than the forces you produce with this Snack.

The ampere, the fundamental unit of electrical current, is defined by the force exerted by one wire on another. The definition of the ampere is as follows: A current of 1 ampere flowing in each of two infinitely long parallel wires separated by 1 meter will produce an attractive force of 2 x 10-7 newton on each 1-meter length of wire. For comparison, a force of 1 newton is approximately the weight of a quarterpound of hamburger.