Apparatus Competition

2007 AAPT Summer Meeting

Greensburo, NC

Demonstrating Electrical Resistance Factors of Metal Conductors

William E. Semrau

VanBuren Intermediate School District

250 South St.

Lawrence, MI 49064

269-674-8001

techscienceguy@yahoo.com

Assembly Directions:

1. Gather materials from local building supply / home improvement store.
2. Screw metal boxes onto board.
3. Cut 1 meter (approx) off the female end of the extension cord, leaving the male end to plug into the wall when you are finished. You will need this section later.
4. Strip back 30 cm of the outer insulation off the cut end of the cord
5. Knock out 2 holes on both metal boxes & install romex clamps into these 4 holes.
6. Feed hot/black wire from cord through one romex clamp and attach to one of the brass screws on the fuse/switch.
7. Cut off 20 cm (approx) from the female end of the extension cord. Strip back the outer insulation. Strip back 1.5 cm (approx) of insulation from each end of the black wire (according to strip gauge on back of duplex).
8. Attach one end to one of the screws on the fuse/switch. Then put the other end through the 2^nd romex clamp and attach one of the alligator clips.
9. Screw fuse/switch down into box and tighten romex clamps.
10. Feed white and bare wires from the extension cord through one romex clamp on the 2^nd box. Strip back wire insulation and attach to neutral (white) and ground (bare) screws.
11. Cut another 20 cm (approx) from the female end of the extension cord and attach to hot screw on the outlet. Push though 2^nd romex clamp and attach alligator clip.
12. Attach duplex outlet to box and install outlet cover plate.
13. Wrap alligator clips where hot wire attaches with electrical tape as well as the wires between the fuse and outlet for safety.
14. Install fuse. Apparatus is now ready to use.

Parts List

Materials	Supplier	SKU #	Quantity	Cost
Assembly of Demonstration Apparatus
30cm x 50 cm board (approx) "Bargin Boards"	Menards	1029666	1	1.69
Metal gang box	Menards	361-1218	2	1.58	.79 each
Duplex outlet	Menards	363-3348	1	0.39
Duplex outlet cover plate	Menards	361-1221	1	0.59
T-type fuse & switch	Menards	367-6868	1	5.96
15 Amp T-type fuse, reset	Menards	367-7249	1	6.47
14 gauge indoor/outddor extention cord	Menards	370-0433	1	3.48
Alligator clip (2/package)	Menards	364-1987	2	0.89
Romex cable clamps (4/package)	Menards	365-1003	4	1.19
Multi-purpose screws #8 x 3/4" (25/package)	Menards	234-6270	4	1.97
Electrical tape (3rolls/package)	Menards	364-6004	1	1.29
Equipment:
Digital Multi Meter with clamp	Harbor Freight Tools	42396	1	15.99
Wire: Consumables
18 gauge Aluminum 50'	Menards	235-1720	1	1.99
19 gauge Stainless Steel 30'	Menards	235-1725	1	3.36
16 gauge Brass 25'	Menards	235-1715	1	5.49
24 gauge Copper 100'	Menards	235-1711	1	6.99
28 gauge Brass 75'	Menards	235-1716	1	2.49
22 gauge galvanized Steel 100'	Menards	235-1948	1	1.19
MIG Welding filler wire 10'	VBISD Welding Program, or any MIG weld shop		1	0.50
			Total Cost:	63.50
24 gauge Copper Speaker wire (stranded) 100'	Menards	301-4941	1	4.98

Demonstrating Electrical Resistance Factors of Metal Conductors

Directions for Usage:

1. Leave devise in “OFF” position anytime you are changing or installing a wire between the alligator clips.
2. Cut a section of the “female” end of the extension cord. It needs to be long enough to reach between the 2 alligator clips. Strip back insulation and attach to clips.
3. Plug apparatus into standard 120Vac outlet, turn switch to “ON” position.
4. Set clamp meter to 20 Amps AC. Clamp around hot wire between fuse and outlet. (Use a student volunteer to be in charge of observing and announcing the amperage readings)
5. Plug in any tool: drill, grinder, toaster, hair dryer, etc and turn on. ( 2^nd student volunteer)
6. Record Amperage.
7. Plug in a second tool and record amperage. (3^rd student volunteer)
8. Run both tools and record amperage.
9. Students will see that amperage goes up as more tools are running (a reinforcement of earlier concepts; adding loads in parallel increases amperage).
10. Use DMM’s thermocouple and set to Celsius or Fahrenheit.
11. With tools off (amperage = 0), hold the thermocouple a few cm from the end so body heat is not being read as well as a safety factor to keep fingers from current carrying wires. Place on exposed edge of wire and record temperature. (4^th student volunteer)
12. Turn 1^st tool back on and record temperature of wire. Repeat for 2^nd tool only, as well as both tools together.
13. Students should see that temperature goes up with increased amperage.
14. SHUT SWITCH OFF
15. Replace wire with one of the smaller copper wires purchased separately.
16. Turn switch on, run tool, and record amperage and temperature data.
17. Amperage will be the same as before, yet a noticeable temperature increase will occur with each successively thinner copper wire.
18. Ask students to explain why the temperature increased. Most of my students, by now, can explain resistance in terms of friction, and with the smaller wire, to move the same number of electrons (same amperage) they will be more collisions in the smaller wire and therefore a higher temperature. Explain in terms of electrical resistance.
19. Replace copper wires with other wires (aluminum, steel, etc.) Repeat experiment. Record temperate data.
20. Have students make note of what they observe as they compare data.
21. Summarize class demo/lab:
1. Thinner wires get hotter to carry the same amperage
2. Different wires have different temperatures even with the same amperage
3. The more amperage a wire carries, the hotter it gets.
4. Don’t use extension cords that are thinner than the tool you are plugging in
5. Safety, safety, safety!
22. Extension: have students calculate the resistance of the wire