Physics-Lab 9 Ohm’s Law, Resistance and Resistivity

Lab 9Ohm’s Law, Resistance and Resistivity1. Objective: (i)(ii)(iii)The purpose of this experiment are: To measure the resistance of a given length of wire. To study the variation of resistance with length of the conductor. To measure the reistivity of the material of a conductor. 2. Equipment: (i)(ii)(iii)Five nickel-silver wires of length 40 cm, 80 cm, 120 cm, 160 cm, 200 cm anddiameter 0. 00026 mOne ammeter, one voltmeter, one rheostat and a 6. 0-V batterysix connecting leads and four crocodile clips. 3. Theory: Ohm’s law states that the current (i) passing through a conductor is proportionalto the potential difference (V) across its ends. This can be expressed symbolicallyas: V = RI…(i)Where R is a constant for the conductor and is called its resistance. Resistance ismeasured in volts per ampere and is called ohm (). The resistance of aconductor depends on the type of material, its length (l), area of cross section (A)and the temperature at which it is maintained. The resistance of a conductor at agiven temperature is given by: RlAwhere is a constant for the material of the conductor and is called the resistivityof the conductor. It is measured in m. The area of cross section of the wire isgiven by: A = r2Thus the above equation for resistance becomes: Rlr2…(ii)4. Procedure: VbatteryconductorArheostatSet up the circuit as shown in the figure above. The voltmeter is connectedparallel to the conductor, and the ammeter, rheostat and the battery are connectedin series with it. The voltmeter and the ammeter are pole sensitive and so aconnection originating from the positive end of the battery must be connected tothe red terminal. If the voltmeter or ammeter reads backward, reverse theconnections on it. Adjust the rheostat so that the ammeter and voltmeter show the smallest readablevalue. Record the current and the voltage values. Take 5 more reading of currentand voltage, each time adjusting the rheostat for a higher value. Record your I andV values in a table. Repeat the procedure for the remaining four wires andmake separate tables for I and V values for each conductor. 5. Interpretation of the Data: Identify the variables in equation (i) and draw a suitable graph for each of theconductors. Use the slope of the graph to obtain the resistance of each conductor. Record the resistance of each conductor and its length in a table. Write equation (ii) in the form y = mx + bIdentify x and y and draw a suitable graph. Use the slope of your graph to obtain avalue for the resistivity of nichrome-silver. If the actual value is 3. 3 x 10-7 m,calculate the percentage error in your experiment. 6. Questions: 1. A wire of diameter 1 mm has a resistance of 0. 5 . What is the resistanceof another wire of the same material and the same length but of diameter0. 5 mm?2. What is the potential difference across a 100 m length of copper wire thathas a diameter of 0. 5 mm when it carries a current of 15 A? The resistivityof copper is 1. 7 x 10-8 m. 3. How long is a copper wire that has a resistance of 2 and a diameter of0. 01 mm?4. A wire of length 1. 0 m has a resistance of 5. 0 . It is uniformly stretchedto a length of 2. 0 m. What is its new resistance?