Resistance of a Wire
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GCSE Physics Coursework - Resistance of a Wire Theory What is resistance? Electricity is conducted through a conductor, in this case wire, by means of free electrons. The number of free electrons depends on the material and more free electrons means a better conductor, i.e. it has less resistance. For example, gold has more free electrons than iron and, as a result, it is a better conductor. The free electrons are given energy and as a result move and collide with neighbouring free electrons. This happens across the length of the wire and thus electricity is conducted. Resistance is the result of energy loss as heat. It involves collisions between the free electrons and the fixed particles of the metal, other free electrons and impurities. These collisions convert some of the energy that the free electrons are carrying into heat. How is it measured? The resistance of a length of wire is calculated by measuring the current present in the circuit (in series) and the voltage across the wire (in parallel). These measurements are then applied to this formula: V = I ´ R where V = Voltage, I = Current and R = Resistance This can be rearranged to: R = V I Ohm's Law It is also relevant to know of Ohm's Law, which states that the current through a metallic conductor (e.g. wire) at a constant temperature is proportional to the potential difference (voltage).
By allowing the wire to cool between experiments a fair test could be assured. Safety In order to perform a safe experiment, a low voltage of 3V was chosen so that overheating was minimilised. Furthermore, lengths lower than 10cm were not tried, which also helped to avoid overheating. Results Wire 1, Set 1: Length (cm) Voltage (V) Current (A) Resistance (W) (to 2 d.p.) 10 0.66 1.22 0.54 20 0.84 0.89 0.94 30 0.97 0.70 1.39 40 1.06 0.57 1.86 50 1.16 0.50 2.32 60 1.22 0.44 2.77 70 1.25 0.38 3.29 80 1.27 0.35 3.63 90 1.31 0.29 4.52 100 1.33 0.29 4.59 Wire 1, Set 2: Length (cm) Voltage (V) Current (A) Resistance (W) (to 2 d.p.) 10 0.51 1.02 0.50 20 0.79 0.79 0.97 30 0.91 0.65 1.40 40 1.02 0.55 1.85 50 1.08 0.48 2.25 60 1.15 0.42 2.74 70 1.19 0.37 3.22 80 1.22 0.33 3.70 90 1.26 0.30 4.20 100 1.27 0.28 4.54 Having completed two sets of results for one wire, it was noticed that these was a large black mark towards one end of the wire, where it appeared that it had been melted to some degree at some point. It was therefore decided to conduct experiments on an additional piece of wire that was checked for integrity prior to investigation: Wire 2, Set 1: Length (cm) Voltage (V) Current (A) Resistance (W) (to 2 d.p.)
At 50 and 80cm it is possible that the length was shorter, causing a lower resistance, and at 90cm it is possible that it was longer, causing a higher resistance. The solution to this is to measure the lengths more carefully and ensure that the wire is pulled tight against the metre rule. o For a particular result, one or more of the connections could have been faulty, causing extra resistance at the connections. A solution to this would be to, before each experiment, connect the connections together without the wire in place and measure the resistance then. If it is higher than it should be then the connections could be cleaned. o Whilst extremely unlikely, it is conceivable that the power supply was providing a different voltage for some of the results. This is unlikely to be a problem in this investigation but it might have been an issue had we used batteries instead. NB: If one were to assume that Ohm's Law applies, then another possible explanation could be that at some points (more likely in the lower lengths), the wire was not allowed to cool completely so that the temperature was higher for that measurement. Whilst unlikely (due to the two sets of results), this would cause a higher resistance as explained previously. However, it is now known, after researching the metal alloy "constantan," that the resistivity (the electrical resistance of a conductor of particular area and length) of this alloy is not affected by temperature. Therefore, in these experiments Ohm's Law does not apply.
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