Hot Wires. Topic. Introduction. Time required. Materials. Heating effect of wire in an electrical circuit

Transcription

1 Hot Wires Topic Heating effect of wire in an electrical circuit Introduction In previous experiments in this section, you may have noticed that some electrical components heat up when a current passes through them. In this experiment, you will find out more about the heating effect of an electric current. You will apply different voltages to a length of Nichrome wire (wire with a high electrical resistance) to study how the heat generated by the wire changes as the current through it changes (as a greater voltage is applied, the current through the wire increases). Fuses use this heating effect for safety purposes. A fuse is a small piece of wire in a glass tube with metal connectors at each end. A fuse is designed to be used in a circuit to allow a current of a certain size to pass through it. If the current in the circuit increases above this point, the fuse heats up and breaks. This causes the current in the circuit to stop. The fuse is a very useful safety device as it protects appliances from surges of current. Time required 45 minutes for Part A 10 minutes for Part B Materials For Part A: 40 cm Nichrome wire (approximately 32 or 36 gauge) 30 cm wooden ruler 2 pieces of cardboard (approximately 3 cm square) transparent tape 2 clip leads either: volt cell batteries plus 8 connecting leads and a switch; or: a low voltage power supply with variable DC voltage up to 12 volts For Part B: 2 amp fuse 2 clip leads either: volt cell batteries plus 8 connecting leads and a switch; or: low voltage power supply with variable DC voltage up to 12 volts The appearance of the components may vary among suppliers (see Appendix A for website addresses of possible suppliers). Circuit diagrams are given in this experiment to show the arrangement of the components. Diagram 1 on the next page shows the symbols used in these diagrams.

2 1 positive pole negative pole cell Symbols used in circuit diagrams switch fuse Safety note Do not use an electrical outlet. Do not touch the hot wire. If you are unable to use sleeved clip leads for Part A, make sure you do not touch the jaws of the clips as they will get hot. If using the cells in Part A, do not leave the switch closed for longer than necessary each time you make an observation, as the high current used will drain the cells quickly. Procedure Part A: Electric heating of wire 1. Roll the pieces of cardboard and attach them to the ends of the ruler with transparent tape as shown in diagram 2 below. 2 transparent tape Nichrome wire transparent tape rolled cardboard ruler rolled cardboard Nichrome wire and card attached to ruler 2. Attach the Nichrome wire to the ruler by securing each end to the back of the ruler using transparent tape (the wire is 10 cm longer than the ruler). The cardboard helps to support the wire slightly above the ruler. 3a. Using 1.5 volt cells: Connect six of the cells (total 9 volts) and the switch as shown in diagram 3 below. Make sure the cells are connected positive to negative and that the switch is open. 3A total 9V + 3B switch clip lead clip leads length of wire being tested clip lead Nichrome wire connected in a circuit with cells and switch: wiring diagram (A) and circuit diagram (B)

3 3b. Using power unit: Connect one end of one of the clip leads to one terminal of the power unit. Connect one end of the other clip lead to the other terminal of the power unit. Set the voltage to 9 volts. 4. Connect one of the free clip leads to the wire on the ruler. 5. Connect the other free clip lead to the wire at a point 10 cm away from the first clip as shown in diagram 3 on the previous page. 6. Close the switch (or switch on the power unit). 7. If the wire glows red, proceed to step Open the switch (or switch off the power unit), move the second clip 1 cm closer to the first and close the switch (or switch on the power unit). If the wire glows red, proceed to step 9. If it does not glow red, repeat this step. 9. Open the switch (or switch off the power unit). 10. In data table A below, check the box glows red for the row labeled 9 volts. 11a. Using 1.5 volt cells: Disconnect four cells and leave the other two connected (total 3 volts). 11b. Using power unit: Reduce the voltage to 3 volts. 12. Close the switch (or switch on the power unit) and record your observation of the wire in the 3 volts row of data table A. 13. Open the switch (or switch off the power unit). 14. Repeat steps 12 and 13 for different voltages (each cell supplies 1.5 volts, so four cells give 6 volts and eight cells give 12 volts). DATA TABLE A Voltage Doesn t glow Glows dully Glows red Glows bright red 9 volts 3 volts 6 volts 12 volts Part B: Using a fuse 1. Look carefully at the fuse. Record your observations in data table B on the next page. 2a. Using cells: Set up the equipment as shown in diagram 4 on the next page. 2b. Using power unit: Take two leads. Connect one end of each lead to opposite metal ends of the fuse. Connect the other end of each lead to the terminals of the power unit. 3. While observing the fuse closely, close the switch (or switch on the power unit). Record your observations in data table B on the next page. 4. Open the switch (or switch off the power unit).

4 4A total 9V + 4B switch clip leads clip lead fuse Circuit with fuse: wiring diagram (A) and circuit diagram (B) DATA TABLE B Appearance of fuse At beginning of experiment At the end of experiment Analysis Part A: Electric heating of wire 1. Did the different voltages produce different effects on the wire? Part B: Using a fuse 1. What did the fuse look like before the experiment? 2. What happened when the switch was closed? 3. What did the fuse look like after the experiment? Want to know more? Click here to view our findings.

5 Part A: Electric heating of wire 1. The wire glowed a brighter red at higher voltages. At higher voltages, a larger current passed through the wire. This caused the wire to increase in temperature and emit a brighter color. A light bulb uses the heating effect of electricity. If you look at a light bulb, the filament at the center is the part that glows with a bright white light when the bulb is lit. The filament is a very fine coil of wire. This wire is far thinner than the wire used in the experiment, and it reaches very high temperatures when electricity passes through it. The wavelength of radiation emitted from a hot object depends on its temperature (see Experiment 5.03: Dark And Dull Or Bright And Shiny?). The light emitted by the bulb is composed of much shorter wavelengths than the red light emitted by the heated wire in Part A; the wire making up the filament of the bulb is much hotter than the wire in this experiment. Part B: Using a fuse 1. The fuse consists of a very fine length of wire inside a transparent tube with metal connectors at each end. 2. The wire in the tube quickly became red and then separated into two pieces. The wire became dark again. It becomes dark as it cools down.

6 3. At the end of the experiment, the wire in the fuse has a gap in the center. There may be a small lump of metal on one of the broken ends of wire. This is a result of the metal becoming very hot and melting as the current passed through it before the fuse wire broke and caused the current to stop.