The resistance R of a conductor can be found by measuring the current (I) through it when a p. d V is applied across it and then using R = V. This is called the ammeter-voltmeter method. I Set up the circuit of the experiment in which the unknown resistance (R) is 30cm of SWG 34 constantan wire. Altering the rheostat changes both the p. d. V and the current (I). Record in a table, with three columns, five values of I The current flowing through a metal conductor is directly proportional to the p. d. across the conductor, provided that its temperature remains constant.
If we divide the voltage by the current, we get a number which gives us a measure of resistance. In fact, this is how resistance is defined: Resistance of a conductor = p. d. across the conductor____ Current through the conductor ohms i?? R = V i?? volts I i?? amperes The result R = V is called Ohm’s law. However it is really the definition of resistance. I What Ohm discovered was that the resistance of a metallic conductor is constant, provided its temperature does not change. Not all conductors give a straight-line V-I graph. These ones which do are called ohmic conductors.
The resistance of piece of wire depends on, ; Its length ; Its thickness ; The material with which it is made from Resistance and length The diagram below shows a method for investigating how resistance changes as the length of the wire changes. A 1 metre length of resistance wire is stretched out along the bench. By sliding the contact along, we can vary the length of wire actually in the circuit. The p. d. across the resistance wire stays constant throughout. It is equal if the p. d. of the battery. We measure the current with different length of wire in the circuit and calculate the resistance each time (R = V ).
A graph of resistance against length is a straight line I through the origin. The resistance is proportional to the length. Resistance and thickness To investigate the effect of thickness on resistance, we use several equal lengths of same resistance wire. Two pieces placed side by side will have twice the cross-sectional area of a single wire, three have three times the cross-sectional area, and so on. Using the same battery all the time (p. d. fixed),we find that the current increases as the thickness of the wire increases. It is easier for electrons to flow through the thick wire.
The resistance gets less as the thickness increases. In fact, they are inversely proportional and if the cross-sectional area is doubled, the resistance is halved, and so on. Information from, Book: Understanding Physics by Robin Miller (Pages 148;249) From the above theory I infer that the key factors to vary are the length and the thickness of the wire and the temperature should kept constant. External temperature the wire is almost constant. I assumed that the room temperature does not fluctuate much during the time interval during which I conduct the experiment.
The internal conductor varies as the current passes the wire gets heated. The resistance vary with the temperature. I assumed the internal temperature of the wire is the same for the different length of the wire. I intend to vary the length from 5cm to 25cm and the width from 0. 22mm up to 0. 28mm. I intend to use constantan wire in this experiment. Constantan is an alloy of a mixture of 55% Copper and 45% Nickel. I have selected constantan because the resistance of most metal increases with temperature. Constantan is a copper based alloy and its resistance changes very little unless it is heated strongly.
If two conductors at different potentials are joined by a conducting wire, electrons will flow from one to the other until both until are at the same potential. Free electrons in both conductors then have the same average potential energy. Electrons always flow from lower to higher potential, i. e-towards the more positive potential. When electrons move in this way, there is current flow. The resistance could be seen in another way. The resistance is the hindrance for the flow of electrons. As the length of the wire increases the hindrance for the flow of electrons increases.