May 13, 2008 Physics - Electricity & Magnetism Title: Hot Dog Circuits Demonstration 1 Abstract: This demonstration is a way to help students understand Ohm's Law and relationships between power and current. A simple electrical circuit apparatus is used to pass an alternating electrical current through a hot dog held at its ends by two nails, which act as electrodes. The resistance of the hot dog enables it to heat up and cook. The apparatus also demonstrates how two hot dogs can be cooked by connecting them in series or in parallel. The demonstration requires the use of 120 volts so beware of the safety issues involved. Key Concepts: Resistance Power Current Voltage Series Circuits Parallel Circuits Ohm s Law Materials & Equipment A simple electrical circuit apparatus (see Figure 1) made with: 5 common steel framing nails, 3.5-inches long (16d) 1 surface mounted switch 1 nine foot household extension cord 1 piece of 12 x 12 x ½ inch thick plywood 1 wire stripper 1 soldering iron and tin-lead solder (optional) Package of hot dogs Safety Precautions: Do not touch the nails when the power switch is turned ON! Always turn OFF the power switch after the hot dogs are cooked and before removing them from the nails. 1. Adopted from Hot Dog Circuits, Laura Henriques, California Science Classroom, April 2008. -1-
Constructing the Electrical Circuit Apparatus: 1. Using a 1/8-inch diameter drill bit, drill 5 holes about 3-inches apart into the 12 x 12 -inch piece of 1/2-inch thick plywood. 2. Hammer the steel framing nails into the 5 holes. 3. Cut off the female end of the household extension cord and separate the two wires for a distance of about 18 inches by simply pulling them apart. 4. Cut off about 12-inches from one of the separated cords and set it aside. 5. Strip the insulation from the short end of the extension cord and connect it to one pole of the surface mounted switch. 6. Cut a 4-inch section from the 12-inch piece of cord from step 4 and strip the insulation from both ends. Connect one end of this wire from the second pole of the surface mounted switch to nail A (see Figure 1). Wrap the bare wire around the base of the nail tightly. 7. Strip the insulation off the long end of the extension cord and connect it to nail E. Wrap the bare wire around the base of the nail tightly. 8. Cut the remaining piece of wire from step 4 into two equal lengths of about 4-inches each. Stip the insulation from the ends of both wires. 9. Connect one wire from nail A to nail B and the second wire from nail E to nail D. Wrap the bare wires around the bases of the nails tightly. 10. For a firm connection, solder the wires to each other and to the base of the nails. Procedure: 1. One Dog Circuit Place a hotdog between B and D to create a circuit with one hotdog as a resistor. Plug in the cord and turn the switch ON. Ask the students to watch the hot dog cook and ask them to measure the time it takes for the center to become brown and the skin to begin bursting open. There will be hissing and bubbling in the center of the hotdog as it cooks. It usually takes about one minute. Turn the switch OFF. The ends of the hotdog will be cool, making it easy to handle since no electricity flowed through it. 2. Two Dogs in a Series Circuit (Figure 2) Using two new hotdogs, place one between B and C and one between C and D to make a series circuit. Increasing the resistance decreases the current. This means it takes longer to cook. Have students predict how long they think it will take for the hotdogs to cook and ask them to time the cooking. Turn the switch ON. Does the new cooking time make sense based on what we know? -2-
3. Two Dogs in a Parallel Circuit (Figure 3) Using two new hotdogs, place one between B and D and another between A and E to make a parallel circuit. Have students predict how long they think this arrangement will take to cook the hot dogs and ask them to time the cooking. Tun the switch ON. Do the results make sense? 4. OPTIONAL - Two Dogs in Series and One in Parallel (Figure 4) Place two new hotdogs between B and C and one between C and D to make a series circuit. Place another new hot dog between A and E to make a parallel circuit. Turn the switch ON. This is a memorable, interactive demonstration which really helps students see the relationship between current and resistance. Accompanying Lesson: Ohm s Law: Voltage (V) = Current (I) X Resistance (R) or V = I R Ohm s Law can also be written: V = I R The unit for V is volts The unit for I is amperes The unit for R is ohms Power = Voltage x Current or P = V I The unit for power is watts If one hot dog has a resistance of 100 ohms, then: Hot Dogs in Parallel: V = 120 volts, R = 100 ohms, I = 1.2 amperes. Power = 120 x 1.2 = 144 watts Hot Dogs in Series: V = 120 volts, 2R = 200 ohms, I = 0.6 amperes Power = 120 x 0.6 = 72 watts The series arrangement uses ½ the power of the parallel circuit, so it takes longer to cook the hot dogs. -3-
Figure 1: Electrical Circuit Apparatus for Hot Dog Cooking -4-
Figure 2: Two hot dogs in a series circuit Figure 3: Two hot dogs in a parallel circuit -5-
Figure 4: Two hot dogs in series and one in parallel -6-