B. Variation of Resistance with Length, Area, and Material 1. The length, wire m

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B. Variation of Resistance with Length, Area, and Material 1. The length, wire material and B&S; gauge number for each spool of wire on the resistance board is writen on the back of the resistance boand. Recoed each of these values on your data sheet for each wite spool on the resistance 2. Use table 39.1 to determine the diameter of each wire spool. Recoed these valoes in meters for each wire on the re- 3. Use equation 39.3 to calculate the cross-sectional area A in square meters of each wire spool on the resistance Use equation 39.2 to calculate the theoretical resistance R of each wire spool on the resistance board (remember board. sistance board board. Recond each area on your data sheet that the numerical value of resistivity p for different materials is given in table 39.2). Record each value of theoret- ical resistance R on your data sheet S. Connect the apparatus as shown in figure 392. The first spool of wire on the resistance board will be used as the resistamce R. Adjust the dial on the power source until the ammeter reads 100 mA. Convert this current to amperes and record this value on your data sheet. Be sure to read the ammeter on the lowest possible scale seting in onder to obtain the most accurate readings possible. 6. Take the voltmeter reading and record this value on your data sheet as the voltage deop V. Be sure to read the volt- 7. Using equation 39.1 with the values for and V found in steps 5 and 6, calculate the experimental resistance R of 8. Repeat steps 5, 6, and 7 for each of the other spools of wire on the resistance board. meter on the lowest possible scale setting in order to obtain the most accurate readings possible. the spool of wire. Recoed this valoe on your data sheet 9. Copy the theoretical value of the resistance of each wire spool from step 4 to this part of the data sheet. Calculate the percentage error in your value of experimentall sesistance. Record this result on your data sheet. QUESTIONS I. What effect does connecting the ammeter and voltmeter into the circuit have on the experimental results? 2 Why must the ammeter and voltmeter be set on the lowest possible scale setting in order to obtain the most acc rate results 3. Sappose a current significandly larger than 100 mA were passed through a resistance spool for a long period of 4. Which of the following materials has the smallest resistivity? 5. Why are two wires connected to a resistor in a circuit, instead of a single wire? time. What effect, if any, would this have on the resistance of the spool? (a) aluminum, (b) copper, (c) German (nicke) silver, (d) gold (c) iron, (f) lead 017 Experimest 39 Ohm's Law 30

Explanation / Answer

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1. ammeter is connected in series with the circuit. now since the ammeter has an internal resistance of its own ( though it is very low), the net effective resistance oof the circuit increases because of this

Voltmeter, on the other hand, has a relly high resistance and is connected in parallel to the circuit. this will provide an additional path for the current to flow and hence reduce the effective resistance of the circuit by some amount

hence connecting ammeter and voltmeter to the circuit changes the resistance of the circuit ( effective resistance)

2. the ammeter and the voltmeter must be set to the lowest possibel scale, as the lowest scale means the least count is also the lowest and hence the error in the readings will be lowest

3. for i >> 100 mA

when current passes through resistance for a long time, the resistance begins to heat up because of ohmic heating. this heating of resistnace increases it stemperature and hence the resistivity ( which is a function of the temperature)

hence due to increased resistance the net current flowing through the resistance decreases

4. out of all the given option, copper has the lowest resistivity, ( because germal silver is not silver)

5. two wires are connected to resistor in a circuit, instead of one, because one wire is to bring the current to the resistor and the other one is to take it away from the resistor. hence two wires are needed to complete the circuit

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