Apparatus Competition

2007 AAPT Summer Meeting

Greensburo, NC

The Electric Slinky

Hector Maciel, Maria Salazar, Anatol Hoemke, Doug Johnson

Cal Poly Pomona

Physics

3801 W. Temple Ave

Pomona, CA 91768

909-869-3171

mariasalazar@csupomona.edu, hmmaciel@csupomona.edu, dcjohnson@csupomona.edu, ahoemke@csupomona.edu

Abstract

The Electric Slinky consists of a coil about 35 turns, which is then connected to a 20V battery or a power supply capable of delivering 15-20 Amps. As the current passes through the coil, it creates an attractive force that pinches the coil together. Each loop essentially has a current flowing in the same direction thus illustrating the force between parallel current carrying conductors.

Construction of Apparatus:

Materials Needed:

3/8" - 1/2" thick clear Plexiglass/Acrylic, cut to 3"x6"

1 Small Slinky (Slinky Jr. ~1.75" O.D.), cut to ~30.5 - 36.5 loops **It is important to cut to the 1/2 loop for proper assembly

2 Pieces 14 AWG bare solid copper wire, cut to ~2.5" length

2 12 AWG metal crimps (Crimp-on ring terminals with the ring cut off)

2 Binding posts, with the bottom lead cut off (so it doesn't stick out the bottom of the baseboard)

Overhead projector

20V DC Power supply and leads capable of delivering 15 - 20 Amps

Tools Needed:

Radial saw (only to cut the base)

Sander (for sanding only the edges of the base)

Drill press with various drill bits

2 Needle-nose pliers

Crimper

Assembly Instructions:

1. Cut the base and sand only the edges and corners (the top and bottom faces need to be clear for the overhead projector).

2. Drill two holes in the base as diagrammed, 1/2" from the ends and .65" from alternating sides. Our binding post had a base post diameter of .305" so we drilled the holes at 19/64", or .297". Depending on the diameter of your binding post, drill the hole slightly smaller so that the post will fit into the hole tightly and not move. Cut the bottom of the post off so that it doesn't stick out the bottom of the baseboard. You can also countersink around the hole on the bottom of the baseboard so that you can use the binding post's nut to secure it to the board. Push the binding posts through the baseboard so that the wire holes in the posts are parallel to the long sides of the baseboard.

3. Using pliers, bend the two ends of the slinky so that the slinky lays flat and the ends are horizontal and also laying flat against the baseboard, but on opposite sides of the slinky's diameter. The ends of the slinky should line up with wire holes in the binding posts.

4. Crimp the copper wires and each end of the slinky with a metal crimp, so that the wire is attached away from the slinky. Bend the wire as diagrammed so that it will pass through the wire hole in the binding post. Feed the copper wire through the binding posts. You might have to do some additional wire bending and positioning to get them to fit properly through the binding posts. Using needle-nose pliers, bend the ends of the copper wires into a loop as shown. This will keep the wires from snagging and will make it easier for you to adjust them.

** The copper leads are there so that you can adjust the stretch of the slinky to get the desired effect. As you move the leads in and out of the binding posts you may need to twist the leads a bit to keep the slinky properly aligned.

Use of Apparatus:

Place the slinky apparatus on the overhead projector. Connect one end of the slinky with an alligator lead to the power supply. (We use three large Edison batteries that have about 20 Volts together and 15 amps). Connect the other end to the lead and then slightly tap that lead to the battery or power supply. Only close the circuit momentarily, for about a second each time. You should be able to repeat the demonstration 5 or 6 times in succession before it gets too hot.

Caution: THE SLINKY WILL HEAT UP VERY QUICKLY!!

Operating Specifications:

Slinky resistance ~= 0.044 Ohms/loop

Current necessary for effect 15 - 20 Amps

Parts List

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
Slinky Jr.	Toy Store, Ebay, Amazon	7154701258	$2.00 to $3.00 EACH
(2) 12 AWG metal crimp on ring terminals	Hardware store		$0.16 each
(2) Pieces 14 AWG bare solid copper wire	Scrap	Scrap	$0.10 each
Binding Posts	Ocean State Electronics	90-799 (black), 90-800 (red)	$1.25 each
Plexi-glass or Acrylic (3/8” thick)@ 3”x6” blocks	S & W Plastics, Ontario ,CA	N/A	$ 5.00
Total Cost			$9.25