In electric current is the flow of electrons around a circuit through a conducting wire of metal. The wire is made up of positive ions, atoms that have lost electrons. Figure 2- Cross- section of a wire When they have enough energy, the electrons in the outer shell of the atoms become free, leaving the atoms as positive ions. They get this energy from the power supply. The sea of free electrons flows past the ions as they gain more energy. However ions are obstacles and they often collide with the electrons, as there is not enough space. Each collision results in a loss of energy.
The longer the wire the more atoms it contains. When a long piece of wire is attached to a power supply, electrons encounter more ions and collide more often, so there is more resistance. Each collision uses up more energy and so the electrons slow down resulting in a smaller current. The electric current is the rate at which charge (electrons) flow. The power supply works by forcing the sea of free electrons to flow in a particular direction. Figure 3- Diagram of how a battery works The thick membrane stops the negative and positive terminals attracting each other.
In the right terminal of the battery the free electrons are repelled because it has a negative charge. The left terminal has a positive charge so it attracts the negative electrons. This gives the electrons in the current a direction to flow in. PREDICTION: I predict that the longer the conducting wire length, the smaller the current shall be (lower value in Amps). So if the wire length is doubled then the current shall be halved because the resistance increases (V= I x R) I = k X 1 L I ?? 1 L As the wire gets longer (and the temperature remains constant), the resistance shall increase proportionally.
The formula for resistivity is: R = p l A R – Resistance P – Rho- Resistivity of material L – Length of Wire A – Area of cross-section I – Current Using this formula: When l=1 and A=1, R= p So when the length is doubled, the resisitivity is doubled as well: When l= 2 and A=1, R=2 p. This suggests that the length is proportional to the resistivity, and the resistance in a circuit is dependant upon the voltage and current Figure 4- Graph showing that the increase in Current is proportional to the increase in Voltage. This is because the higher the electrical pressure the more electrons go around the circuit.
Therefore the line showing resistance is straight and follows Ohms law Ohm’s Law: V = I x R A simple and fair test is needed to investigate the effects of the conducting wire length on a current. PRELIMINARY EXPERIMENT: Preliminary work was carried out to help clarify the method of the experiment and make changes. It also helped to decide the range of observations and measurements to be made. RESULTS Table 1- Data collected in Preliminary experiment Length/m Preliminary I/A Expected.
Figure 5- Graph showing Preliminary results against what was expected As shown in the graph, my preliminary results were not very accurate. The following changes had to be made to my method of procedure: Set needle of voltmeter accurately Measure the length of wire accurately Clean the contacts and put it in perfect position. Be careful to connect the positive (+) side of the ammeter to the positive side (+) of the power supply, which is colour coded otherwise the ammeter will not work Tape the wire down to keep it in place.
Wind wire around crocodile clip tightly, from near end of 0. 1m otherwise each reading will have an extra 0. 1m(shown below) KEY RULER CONDUCTING WIRE LENGTH CROCODILE CLIP A suitable range of measurements was selected as 1m, 1/2m, 1/3m, 1/4m, 1/5m, 1/6m, 1/7m, 1/8m, 1/9m and 1/10m because the reciprocal of these values is 1,2,3,4,5,6,7,8,9 and 10. It provides a useful scale for plotting the graph and the data will not be clustered in one area. So that the results are as accurate and reliable as possible the readings should be carried out 3 times and these result would be plotted to draw a line of best-fit graph.
The average will also be calculated and shown on the graph APPARATUS NEEDED: 1m20 Constantine Wire SWG (Standard Wire Grange) 38 – [Also known as eureka. Resistance does not change as temperature increases. Resistivity at room temperature is 49 x 10 (Muncaster, 1986 p487)] Low Voltage Variable DC Supply Voltmeter Ammeter 1 metre rule Tape Wire cutter Wires Crocodile clips METHOD 1. Connect and set up equipment as in the Circuit diagram (Figure 1) Using the wire cutter and ruler, measure 1m of Constantan Wire SWG 38 to be the variable wire. 2. Set the voltage to 1V using the variable DC Power Supply.
Keep at 1V throughout. 3a) Measure the size of the electric current by noting down the ammeter reading. 3b) Be sure to connect the correct terminal of the ammeter to the matching terminal of the power supply 4. Vary the size of the wire to 1/2 of 1m and take the ammeter reading. 5. Vary the wire length to 1/3, 1/4, 1/5, 1/6, 1/7, 1/8, 1/9 ; 1/10 of the original value 6. Repeat this 3 times and take the average as the result to make the experiment as fair as possible. 7. To make it a fair and accurate test, the wire length must be the only variable. All other factors must be kept constant.
The cross-sectional area and material of wire must stay the same so Constantan SWG 38 will be used throughout. The results must be all taken on the same day so that the atmospheric temperature is the same. The needle of the voltmeter should be set perfectly at 1V. SAFETY: It is very important and the following precautions must be carried out to prevent any accidents: Working area should be dry and clean Move bags and coats away Keep bare hot wire away from paper; could ignite When wire length is at its shortest be careful when touch wire as it might be hot. SOURCES OF INFORMATION:
1) Mr Allnutt 2) Science Club 3) “A-Level Physics Second Edition” Text Book by Roger Muncaster 4) CGP Physics Revision Guide for GCSE Double Science. ANALYSING EVIDENCE AND DRAWING CONCLUSIONS From Table 2 it can be seen that as predicted the longer the length of the conducting wire, the smaller the current is as it has a lower value in amps. As the wire length gets shorter the current appears to increase proportionally as predicted.
When the length of the wire is 1m the average (mean) current is 0. 033A When the length of the wire is 0. 1m the average (mean) current is 0.350A The 0. 1m wire is 1/10 of the length of the 1m wire and so its current should also be 10 times the current of the 1m wire. 10 x 0. 033 = 0. 330A My experimental value is 0. 350A My experimental value of the current at 0. 1m appears to be just a little more than 10 times the current of the 1m wire.
When the length of the wire is 1m the average (mean) current is 0. 033A When the length of the wire is 0. 5m the average (mean) current is 0. 069A The 0. 5m wire is 1/2 of the length of the 1m wire and so its current should also be 2 times the current of the 1m wire.
My experimental value is 0. 069A My experimental value of the current at 0. 1m appears to be just a little more than 2 times the current of the 1m wire. Both calculations show that as predicted the wire length and current are indirectly proportional. It also shows the accuracy of my results as the experimental and theoretical current values are very close together. The small gap between they allows for little experimental error I = k X 1 L I ?? 1 L The graph in Figure 6 also shows the current as inversely proportional to the length of the wire.