The formula for finding the resistivity of a wire is Where: R is resistance P is the resistivity A is the cross-sectional area L is the length The variables involved are the cross-sectional area (A), the length (L), the Resistance (r) and resistivity (P). The resistivity of every wire is constant So the changing variables are the resistance (r=v/I) where the resistance is The dependent variable. Keep the area constant. To calculate how the change in length affects the resistance, we plot a graph Of length against resistance. The resistance should be proportional to the Length, R? L (prediction) according to Ohms law i. e. there should be an
Increase in the resistance as the length increases- the longer the length, the Greater the resistance. DIAGRAM METHOD: Set up the circuit as shown in the diagram above, start the experiment by taping The constantan wire of 100cm the has been measured to the metre rule. Make Sure the wire is carefully tightened at both ends of the metre rule then connects The digital ammeter, and the voltmeter at last (the circuit should be able to Function without it. ) As shown on diagram above. Switch on the variable power Supply box. Record the voltage and current readings that are displayed on the digital screens Of the ammeter and voltmeter respectively.
Repeat the procedure using eight Different lengths ranging from 10cm to 80 cm. Then, repeat the same experiment Two more times to get an average. CALCULATING THE RESISTANCE The formula for calculating the resistance is: Resistance = voltage/current (V/I) CALCULATING THE AREA Using a micrometer screw gauge, measure the diameter of the wire at four different Points and at right angles of each of these points and get an average. Then calculate the area With the formula A=? r Where r is radius=1/2 diameter A is area ?
Is 22/7 APPARATUS A digital ammeter of range 0-20 A, a digital voltmeter, which ranges from gative terminal blocks, available power supply box, A meter rule of 1 – 1000mm graduation, a micrometer screw gauge and a Piece of constantan wire. SAFETY PRECAUTIONS Do no touch the wire when the power supply box is switched on Do not cut yourself while cutting wire Be careful not to start a fire by allowing the wire to overheat Be generally careful and safety conscious during the experiment After plotting the graph, calculate the gradient with the formula G= y2-y1 = y X2-x1 x After finding the gradient R= ?
PRELIMINARY EXPERIMENT During the preliminary experiment, I observed that the resistance of a piece of wire is affected by the length and the Area the longer the wire the greater the resistance R? L The fatter the wire, the less the resistance R? L/A I observed that as the voltage increased, the current decreases, which means that The current is inversely proportional to the voltage.
Infact as the wire is an Ohmic Conductor, the result from the graph can be plotted an I-V graph. An Ohmic conductor Should yield a straight-line graph passing through the origin. MAJOR SOURCES OF ERRORS Human errors Parallax errors Kinks and twists on the wire Change in diameter due to change in temperature ACTIONS PROPOSED TO MINIMISE ERRORS Keep the area constant as much as possible by trying to keep the temperature Constant at all times. This was done by switching the power supply box each Time you take a set of result to let it cool down for a while before switching it On to record another set of results.
Also keep the area constant by using the same Piece of wire throughout the experiment. Try to avoid parallax error by using a digital ammeter and voltmeter instead of a Full-scale deflection ammeter and voltmeter. Ensure that the wire is carefully tightened to both ends of the meter rule and avoid Wires with loads of twists, which could alter the length and cross-sectional area Of the wire. To increase the decree of accuracy of the recorded result and plotted graph, take Three sets of results and calculate the average, even when calculating the area. Record the diameter at a minimal of four different points to be able to measure.
The diameter of the wires accurately as possible. Use terminal blocks instead of crocodile clips because the crocodile clips kept Slipping out from each other and disturbing the circuit. These terminal blocks Make it easier to connect the circuit properly, but on the other hand, they make it Difficult to measure the length accurately using a meter rule, so use larger length So this inaccuracy is almost negligible. Finally try to avoid normal human errors as much as possible by being generally Careful. The recorded results of the voltage and current flowing through the circuit Length Current (A) 1 Current (A) 2.
Current (A) 3 Average current Volt. (V) 1 Volt. (V) 2 Volt. (V) 3 Average voltage 1ANALYSING After plotting the graph of length against the resistance, calculate the gradient, m,With the formula At the end of my experiment, the resistivity of constantan wire Is 5. 75 x 10 EVALUATION: The anomalies seen in my results (graph) were as a result of approximations.
After I calculated the resistance, I approximated the result in order to be able to insert the numbers on my graph sheet and this affected my graph (gradient) because when I plotted the graph without approximating; I got a perfectly straight-line graph. Conclusion: I feel my experiment (practical) went well and was suitable for the Task given. This is shown by the fact that I achieved consistency and proves that This experiment can be both reliable and accurate. My result is reliable as they are consistence with each other and I have managed to refrain from making large errors.