What is Resistance

Technically, resistance is defined as the ability of a substance to prevent or resist the flow of electrical current.

Electricity is conducted through a conductor, in this case nichrome wire, by means of free electrons. The number of free electrons depends on the material and freer electrons results in a better conductor. For example, a piece of metal such as gold would have more free electrons as the atoms do not hold on to their electrons very well in comparison to another conductor such as plastic which has less free electrons therefore, making it a worse conductor of electricity due to the higher level of resistance.

The free electrons are given energy and as a result move and collide with neighboring free electrons. This happens across the length of the wire and thus electricity is conducted.The smaller the cross-sectional area of the nichrome wire is e.g. 20swg, the fewer channels of electrons in the wire for current to flow and as a result of this the resistance will be high but having a lower area of wire leaves it more susceptible to a heating effect. However, when the cross -sectional area increases to a value of 36swg for example the resistance will decrease as there are more channels of electrons in the wire (This can be seen in the following diagrams; figure 1 and figure 2).

Factors Which Affect the Resistance of the Wire?There are various factors which affect resistance of a wire, three of them being the material of the wire e.g. Nichrome or Gold, the cross-sectional area of the wire, the length of the wire and the temperature of the wire.1Below, you can see the effects these factors have upon the relationship between the resistance of a wire and its’ length:- The length of the wire: if a wire is longer, the moving electrons have further to go, so there is a higher chance of an electron colliding with an atom. Thus, increasing the resistance in the wire.- The cross-sectional area of the wire: the moving electrons in a current are spread out over a greater area. There results in a lower chance of an electron colliding with an atom, so more current can flow.

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Increasing the cross-sectional area of a wire decreases its resistance, this can be seen by figures 1 and 2.- The temperature of the wire: if the wires temperature increases, its atoms vibrate more, each one moving around a fixed centre. There is more chance of electrons colliding with the vibrating atoms, so less current can flow. An increase in temperature increases the resistance of a wire.Ohm’s LawResistance is measured in ohms.

One ohm is represented with the symbol: ?The greater the number of ohms, the greater the resistance present in a wire.The image on the right shows the relationship between voltage, current and resistance. It can also be understood through the following equation; voltage = current * resistanceV = I * ?Relationship between current and voltage when a resistor follows Ohm’s LawThe current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. Following those guide lines we can come to believe that if you double the voltage, the current also doubles.

This is called Ohm’s Law. Figure 5 (left) shows what happens to the current and voltage when a resistor follows Ohm’s Law.Relationship between current and voltage when a filament lamp is present in a circuitA filament lamp is a typically bulb found in many circuits. This contains a filament (typically tungsten) which heats up when an electric current flows through it and creates light. This can be seen by figure 7.The filament lamp doesn’t follow Ohm’s law. The resistance of the lamp increases whilst the temperature of it increases and, due to this the current flowing through the filament is not directly proportionate to the voltage across it.

Figure 6 shows the graph which conveys the relationship between the current and voltage for a filament lamp.VariablesDuring this experiment, our independent variable (which will be changed) is the length of nichrome wire.In order to collect our results, we will measure the voltage (dependant variable) passed through a current of 0.5A whilst varying the length at which the electricity is transmitted through the wire. From this we will compare the relationship between the length of the wire and its resistance.In addition to this, several variables must be controlled during the experiment. Essentially, this focuses on ensuring that the flow of energy throughout the current remains the same in each test, ensuring that the same current is emitted for every test. The following table (table 1) explains this.

Control VariableWhy must it be controlled?How will it be controlled?MaterialWe must use the same material due to the fact that various materials have different resistances. If I were to use a material with a high resistance then, one with a low resistance my results would become un-reliable.I will use a 26SWG Nichrome Wire (See justification in pre-tests section.) It will be attached to a 1 metre ruler.TemperatureThe variable power-pack will be turned off after each recording has been made (refer to method), this will allow the wire to cool down so it will not affect my data.Cross-Sectional AreaAs previously mentioned in the background (with figures 1 ; 2) varying the cross sectional area of the wire (thickness) will affect the resistance and make the data collected less reliable.

From the pre-tests and the fact that we did not have enough resources to vary the gauge of wire we will be keeping the Nichrome wire at 26SWGs.Current (0.5A)The current causes the wire to increase in temperature. Keeping the current low at 0.5A lowers the heating affect and ensures the resistance of the wire will not be affected. This will increase the reliability of my results.Through the use of a variable power-pack we can adjust the input of volts and maintain a constant current of 0.5 Amps for each test.

Straightness of WireThe straightness of the wire affects the length of it. If a wire with a crocodile clip on it is connected at 20cm and the wire bends, the length and resistance of the wire changes. Therefore, we must keep the wire as straight as possible, that is why we are using a ruler and some sellotape.The wire chosen (26swg) can be easily straightened along a 1 metre ruler. For that reason, we will place the wire along the ruler and connect it to either end with the use of sellotape. This will lower the chance of the resistance being affected and thus, increasing the reliability of our results.EquipmentEquipment NameJustification for useDiagramVariable Power-Pack (0-12V)The use of a variable power pack allowed me or any member of my group to manually adjust the voltage input through the circuit in order to control and manipulate the number of amps being transmitted in our circuit, in our case it allowed us to keep the number of amps to 0.

5 in order to make our experiment reproducible.Digital AmmeterUsing a digital ammeter allowed for much more precise readings of the amps present in the electric current with only a +/- 0.1 area for mistake in comparison to the traditional ammeter which has an area for error of +/- 0.5 as it requires the use of the naked-eye to record the readings. For that reason the digital ammeter was used in the experiment.Digital Volt MeterDigital voltmeters give a numerical display of voltage to 2 decimal places by use of an analog to digital converter, which lowers the margin of error to +/- 0.1 whereas, an analog voltmeter moves a pointer across a scale in proportion to the voltage of the circuit.

This requires the use of the naked-eye and increases the margin of error to +/- 0.5. For that reason the digital volt meter was used in the experiment.100cm Ruler with wire onThe range of the length of wire was 25-95cm, the use of a 100cm ruler allowed us to easily place the wire straightly upon it and easily record the measurement of length in comparison to laying down four 30cm rulers which would have increased the error of margin in the reading of various lengths such as 35cm or 75cm.

26swg Nichrome WireFrom the pre-tests we found that this wire could be easily straightened along the 1 meter ruler whereas the 36swg piece of wire could not which meant the resistance was not affected as there were next to no bends in the wire. Also, it was not as susceptible to heat as the 20swg wire which when heated would have caused the resistance to increase due to the more energy in the circuit, which would have increased the chances of collisions between free electrons and atoms.2 x Crocodile ClipsCrocodile clips allow us to connect the wires to the circuit (creating a complete circuit) in a safe manner which lowers the risk to the people involved in the experiment.4 x WiresUsing these in our experiment is essential. These allow us to connect each piece of equipment together for example; the voltmeter to thee variable power-pack.

Method1. Clear your workspace, this will make your working area safe for you and your class mates and ensure that there are no accidents.A) Place chairs/stools under desks.B) Clear the desks of paper, books, bags etc. and simply have a pen and results table in order to record your results (following the steps below you will find out how to create a suitable results table).2. Gather all the necessary equipment, as previously mentioned.

The Equipment is necessary in order to undergo the experiment because if all equipment is not used or set up accordingly then the data will not be reproducible.3. Set the equipment up as shown by the circuit diagram below, following this circuit diagram will increase the similarity to our experiment therefore, increasing the reliability of the results collected.4. Now that the equipment is set up, ensure that the variable power-pack is turned off with the adjustable voltage dial is turned to ‘0’, this is to make sure that the wire does not heat up before acquiring data as the temperature will affect the resistance as previously mentioned in the ‘Background’ section of this document.5. Create a results table with the following headings in a similar order.

You should also include the suitable range of lengths at which you will be moving your crocodile clips in the appropriate column (wire length(cm)) For this experiment I chose to use the range of lengths; 25cm-95cm.Wire Length (cm)Current (A)Cross Sectional Area (swg)Voltage (V)Resistance (R = V/I)Test 1Test 2Test 3Average6. Place one wire with a crocodile clip on it at one point on the 1 metre ruler, then place the other wire with a crocodile clip connected at another point 25cm’s apart.7. Turn the variable power-pack on, adjust the voltage dial until the digital ammeter has a constant reading of 0.50A.8. Record the amount of volts as displayed on the digital volt meter on your results table.

for example, ‘1.28’9. Once recorded, turn the variable power-pack off and wait a reasonable amount of time so that the wire has sufficiently cooled down, this is because if the wire increases in temperature the resistance will be effected as previously mentioned in the ‘Background’ section. This could result in our results being less reliable and giving me less confidence in my conclusion.10.

Then, remove one wire and increase the length to 35cm. Repeat steps 7-9. Continue to do this until you reach the length 95cm. Once you have recorded the reading for 95cm repeat the above stages 2 more times until you have repeated the range of 25-95cm with 10cm intervals 3 times.11. Now that you have repeated each length 3 times you must work out the average voltage for each length, do to this you simply add all three results together and divide them by 3 (the number of test carried out).

For example, I have three results – 1.15, 1.17 and 1.12, these add up to make 3.44, then I divide 3.

44 by 3 to get a mean average of 1.15.12. With the average, we can calculate the resistance In the circuit. To do this, we must multiply each average voltage by 2, this is because when using the equation for resistance we will get R = V/0.

5 which is the equivalent to the average voltage multiplied by 2. Continue to do this for each length until you have done all 8 sets of data.Pre-TestsChange Gauge (SWG) of WireLength of Wire (cm)Cross Sectional Area (SWG)Current (A)Voltage (V)Average (V)Test 1Test 2Test 325160.






763.733.73From viewing these results we can interpret the temperatures created when the current of 0.5Amps has been placed through the circuit. For example, when viewing the data for the 16SWG wire we can see that the voltage at 100cm is 7.76V, this high voltage could be due to the low cross sectional area of the wire and because of this the temperature would be much higher in comparison to that of the 26SWG wire which has a voltage of 0.42V at a length of 100cm. Due to the increase in temperature present in the wire the data cannot be counted as reliable as the heat of the wire affects the resistance and therefore, the voltage read by thee voltmeter (for further explanation on resistance and the cross-sectional area of a wire see the ‘Background’ section).

Also, going up a gauge of wire, at 20SWG does not provide reliable results in my opinion. This can be seen with the results at 25cm which has an average voltage of 0.07V and the other at 100cm which has an average voltage of 0.17V. The reading at 100cm was expected to be approximately 0.28V and as you can 0.

17V is over 0.1V under the predicted result. This could have occurred for multiple reasons. For example, the wire may not have been straight and due to this the resistance may have been affected thus, resulting in the lower piece of data collected.Finally, from the data gathered and the selection of resources used in this pre-test I can come to the conclusion that carrying out my final experiment in this manner would not be the best way to collect reproducible data, this is due to multiple reasons; one, there are simply not enough different pieces of wire with varying cross-sectional areas to carry out the experiment to satisfactory standard and two, the increments used are not very good for plotting a graph or conveying accurate results, the increments used in this pre-test were a follows – 4, 6, 10. This is not an even increase and would give me less confidence in my results when it came to plotting them on a graph.


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