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Transcript
Theme 6 – Electricity at home Aim : Experiment to investigate the dependence of resistance on length of conducting wire. OBJECTIVE: The object is to study the dependence of resistance on the diameter, material and length of wire connected between the pairs of the terminals. Focus Questions What are the factors that affect the resistance of a material ? What do you need to measure in order to find the resistance? Do different materials have the same or different resistance? Do you expect the resistance of a substance to increase or decrease with I. Increase in length II. Increase in cross section area (thickness) Is the change of resistance of electrical devices used in applications? Can you give some examples? Safety: The power supply output should be turned down between readings to minimize heating of the resistors. Heat causes the resistance to increase and this could decrease the accuracy of your results. Coil A is made of nichrome wire of diameter 0.35 mm. Coil B is made of nichrome wire of diameter 0.18 mm. Coil C is made of Copper wire of diameter 0.35 mm. They all have same length. Ninu Cremona Lyceum – Gozo College – Physics Department Page 1 Date: ____________ Theme 6 – Electricity at home Aim : Experiment to investigate the dependence of resistance on length of conducting wire. Apparatus : D.C. power supply, wire, variable resistor, ruler, switch, ammeter , voltmeter, various wires, different coils of wires. Sources of error and precautions 1. 2. Method 1. Part A - Connect the wire to the power supply. Adjust the power supply voltage so that a current of 0.5 Amps or less flows through the circuit. 2. Record the voltages and currents for lengths, of the wire of 20, 40, 60, 80, and 100 cm. Simply press the end of one of the voltmeter leads to the wire to make contact for a voltage reading. Press at different points along the wire to change the length. 3. Calculate the resistances of the various lengths of wire (using V=IR). Note whether the incremental change in resistance is nearly constant. 4. Part B – Now connect the coil A . Record the voltages and currents for the coil. Calculate the resistance for this coil of wire using Ohm's law. 5. Repeat this procedure for coils B and C. Be sure to write down the specifications of the wires connected between each pair of terminals. Results : Ninu Cremona Lyceum – Gozo College – Physics Department Page 2 Table 1 Length of wire in cm 0 Voltage - v Current - I 0 0 Resistance = V/I 0 20 40 60 80 100 Table 2 Coil of wire Material, Diameter and length of wire A B C Nichrome wire of diameter 0.35 mm. Nichrome wire of diameter 0.18 mm. Copper wire of diameter 0.35 mm. Voltage - v Current - I Resistance = V/I 1. Fill in table 1 and 2 by calculating the resistance. Determine the length of coil a from the resistance measured. Coils B and C are the same length as coil A. 2. Plot a graph of resistance against length for data from Table 1. As best as you can, fit a straight line through the points. Ninu Cremona Lyceum – Gozo College – Physics Department Page 3 Conclusion/Evaluation 1. Describe or express in some way the relation between the resistance of a conductor and its length as illustrated by your graph. Is this relation clearly visible from the graph? 2. How would you expect that the resistance of a conductor changes with different materials, different diameter and length of wire used. 3. Does your results support this relation: in particular, do the resistance values of your coils support the dependance of R on each of the three parameters material,diameter and length.(HINT: compare coils that differ by only one parameter) 4. As always, identify the sources of experimental uncertainty in this experiment, and consider how these uncertainties affect your results. Ninu Cremona Lyceum – Gozo College – Physics Department Page 4