Experiment on Electrical Resistance

Experiment on Electrical Resistance
The electrical resistance of a material is its opposition to the flow of electric current (slowing the flow of electrons down). Resistance occurs when the electrons travelling along the wire collide with the atoms of the wire. These collisions slow down the flow of electrons causing resistance. Resistance is a measure of how hard it is to move the electrons through the wire. A current is the rate of the flow of charge (electrons) and the resistance controls the amount of current flowing. If we want to calculate the current flowing through the circuit, we need to know how much resistance it has. A resistor that has a large resistance only allows a small current through it and a small resistance allows a large current through.
Resistors are usually long coils of wire, or small pieces of material that do not conduct electricity very well, therefore the conductivity of the metals affect resistance. As the potential difference (voltage) between the ends of conductor is increased the current passing through it increases. If the temperature of the conductor doesn?t change, the current that flows is proportional to the voltage applied. This is called Ohms Law. Ohms Law= Potential Difference x Current or Potential Difference = resistance x current or The unit of resistance is measured in Ohms (W). Measuring Resistance The voltage across the resistor is measured using the voltmeter. The current flowing through the resistor is measured using the ammeter. The resistance can then be calculated using the formula: Resistance = Voltage Current The width, length, material and temperature are factors, which affect the resistance of a wire. Temperature: If the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance. Material: The type of material will affect the amount of free electrons, which are able to flow through the wire. The number of electrons depends on the amount of electrons in the outer energy shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase. Wire length: If the length of the wire is increased then the resistance will also increase as the electrons will have a longer distance to travel and so more collisions will occur. Due to this the length increase should be proportional to the resistance increase. Wire width: If the wires width is increased the resistance will decrease. This is because of the increase in the space for the electrons to travel through. Due to this increased space between the atoms there should be less collisions. I am going to investigate how the length of wire affects its resistance. I have done a preliminary experiment to help me make my plan: Preliminary Work Diagram: [image] Method: The apparatus was set up in a circuit as shown in the diagram above. I used a wire with constantan wire swg 32. The lab pack was set to coarse=2.0V and Fine=0.2V. The length of the wire was increased by 5cm each time. The results were recorded and put into a table. Results: Length (cm) Voltmeter (v) Ammeter (I) Resistance (W) 5 0.45 0.66 0.68 10 0.6 0.48 1.25 15 0.7 0.42 1.67 20 0.7 0.38 1.84 25 0.8 0.32 2.50 30 0.8 0.28 2.86 35 0.9 0.26 3.46 40 0.9 0.24 3.75 45 0.9 0.23 3.91 50 1.0 0.22 4.54 55 1.0 0.18 5.56 60 1.0 0.18 5.56 65 1.0 0.18 5.56 70 1.0 0.16 6.25 75 1.1 0.16 6.88 80 1.1 0.14 7.86 85 1.1 0.14 7.86 Conclusion: From these results I?ve learnt that as the wire gets longer, the resistance gets bigger. The preliminary experiment gives me information on what size wire I should use, and what the settings on my lab pack are going to be. I also now know, what type of results I should be getting in my real experiment. Prediction I predict that as the length of the resistance wire increases the current flowing through the constantan wire decreases and the resistance increases. This is because when increasing the length of the wire there is a greater mass of the substance. Therefore there are a greater number of positive ions. Electrons colliding with the positive ions, causing the electrons to slow as their kinetic energy has been transferred to the positive ions, cause resistance. A short length of wire (5cm) has a large current (0.66A) and a small resistance (0.68W) but a long length wire (80cm) has a small current and a large resistance. I can see this from my preliminary experiment. When the given length is doubled, this means that there is double the amount of mobile electrons, and so double the chance of a collision. As an electrical current is passed through the conductor, an electron has to travel double the distance, and will therefore have two times the amount of objects in its path. So, as I increase the length of wire I will increase the resistance, so as I double the length of the wire I will double the resistance, treble the length, treble the resistance. From my scientific background I stated that the resistance is directly proportional to the length. Planning In my experiment I am going to use the following apparatus: ? Constantan Wire swg 32 ? Ammeter ? Voltmeter ? Lab pack ? Crocodile clips ? Wires I am going to set my apparatus in a series circuit, and join the voltmeter parallel to the resistance wire. I am going to use 19 different lengths (5cm-95cm): 5cm 25cm 45m 65cm 85cm 10cm 30cm 50cm 70cm 90cm 15cm 35cm 55cm 75cm 95cm 20cm 40cm 60cm 80cm I will measure these lengths on a metre rule. I am going to put the lab pack to these settings: coarse=4.0V and fine=0.4V. This is different to what I used in my preliminary experiment. I decided to increase the voltage on the lab pack slightly because the current readings I was getting were quite low, so I wanted to see if by changing the lab pack settings the current would get bigger. I will record my results in a table like the one I used in my preliminary experiment, and I will then plot a graph of resistance (Y axis) against Length (x axis). Safety ? I am going to wear safety goggles and an overall throughout the practical work. ? If I use a high voltage setting on the lab pack, there will be a high current flowing through the wire, and therefore, it will heat up, melt and break, therefore I am going to keep the voltages on the lab pack quite low (4.0V and 0.4V=4.4V) Fair Test ? To make my results are accurate and reliable as possible, I am going to take 3 readings at each length and find the resistance in each 3 readings. Then I am going to find the average (mean) resistance at each length. ? For a fair test I will need to make sure that the other variables that I am not testing remain constant. I shall try and keep temperature constant by only turning on the power pack for short bursts so as to avoid the wires temperature going up significantly. The other variables will be very easy to keep constant because I will not be changing the type of wire that in turn means the thickness will not change throughout the experiment and I will not change the voltage I input at all during the experiment. ? I will read off the voltmeter and ammeter readings exactly. v Obtaining Evidence I carried out my plan as I stated, but I had to use constantan swg 30 instead of constantan swg 32, which I had stated. This was because there were no more constantan swg 32 wires left. Length (cm) Voltage (V) Reading 1 Current (A) Reading 1 Resistance (W) (Reading 1) Voltage (V) Reading 2 Current (A) Reading 2 Resistance (W) (Reading 2) Voltage (V) Reading 3 Current (A) Reading 3 Resistance (W) (Reading 3) 5 0.2 0.84 0.24 0.3 0.78 0.38 0.3 0.74 0.41 10 0.4 0.70 0.57 0.5 0.64 0.78 0.4 0.62 0.65 15 0.5 0.56 0.89 0.6 0.50 1.20 0.5 0.54 0.93 20 0.7 0.50 1.40 0.6 0.44 1.36 0.6 0.46 1.30 25 0.7 0.46 1.52 0.7 0.40 1.75 0.7 0.44 1.59 30 0.8 0.40 2.00 0.8 0.34 2.35 0.8 0.40 2.00 35 0.8 0.36 2.22 0.8 0.32 2.50 0.8 0.36 2.22 40 0.9 0.36 2.50 0.8 0.32 2.50 0.8 0.32 2.50 45 0.9 0.32 2.81 0.8 0.28 2.86 0.9 0.30 3.00 50 1.0 0.30 3.33 0.9 0.26 3.46 0.9 0.28 3.21 55 1.0 0.28 3.57 0.9 0.24 3.75 0.9 0.26 3.46 60 1.0 0.26 3.85 0.9 0.24 3.75 0.9 0.24 3.75 65 1.0 0.24 4.17 0.9 0.22 4.09 1.0 0.24 4.17 70 1.0 0.22 4.55 0.9 0.20 4.50 1.0 0.22 4.55 75 1.0 0.22 4.55 0.9 0.20 4.50 1.0 0.20 5.00 80 1.0 0.20 5.00 1.0 0.18 5.56 1.0 0.20 5.00 85 1.0 0.18 5.56 1.0 0.18 5.56 1.0 0.18 5.56 90 1.0 0.18 5.56 1.0 0.18 5.56 1.0 0.18 5.56 95 1.0 0.18 5.56 1.0 0.18 5.56 1.0 0.16 6.25 I repeated the experiment 3 times to get an average reading (like I said in my plan). I worked out the average resistance by adding all 3 readings resistance at each length together and dividing by 3. Length (cm) enen Average (mean) Resistance (W) eeeee 5 0.34 10 0.67 15 1.01 20 1.36 25 1.62 30 2.12 35 2.31 40 2.50 45 2.89 50 3.34 55 3.59 60 3.78 65 4.14 70 4.53 75 4.68 80 5.19 85 5.56 90 5.56 95 5.79 My readings were accurate, because they represent the proportion relationship between resistance and length. This is because current flowing through the wire is proportional to the voltage applied. v Analysis I kept my readings at the same accuracy through out my experiment. For resistance I recorded the readings at 2 d.p. , for current I recorded the readings at 2 d.p. , and for voltage I recorded the readings at 1 d.p. To make it easier to draw my graph I am going to round up the readings for resistance to 1 d.p. Length (cm) enen Average (mean) Resistance (W) 5 0.3 10 0.7 15 1.0 20 1.4 25 1.6 30 2.1 35 2.3 40 2.5 45 2.9 50 3.3 55 3.6 60 3.8 65 4.1 70 4.5 75 4.7 80 5.2 85 5.6 90 5.6 95 5.8 To illustrate my findings I have drawn a graph with a line of best fit, which is in a straight line going through most of my points. The straight line shows that ohm?s law was present in my experiment; therefore resistance is directly proportional to length. The length of the wire affects the resistance of the wire because the number of fixed ions in the wire increases or decreases as the length of the wire increases or decreases in proportion. As stated in my plan, electrical resistance in a wire is caused by collisions between the free electrons and the fixed ionic structure of the metal. The more collisions there are, the more difficult it is for the electrons to pass through and therefore the greater the resistance. By increasing the length, the number of fixed ions is also increased and consequently more collisions occur. This has the effect of increasing the resistance. My prediction was correct. The resistance of the wire is directly proportional to its length. From the graph, we can also find out the gradient, which gives the value of the resistance per unit length of the wire. This is useful because it allows me to predict the resistance of any length of this wire and so make resistors that can be used in circuits by cutting the appropriate amount of wire v Evaluation Overall, my experiment went according to my plan. From my graph it is clear the results follow a good straight line, which supports my prediction. There were however, 8 anomalous points on my graph, which I have circled. These fell slightly off of the best straight line. There could be many causes for these anomalies: firstly, by reading the potential difference to 1 d.p. , slight variations in the P.d. could have been missed; secondly, the ammeters and voltmeters could have been damaged and reading falsely on both the meters used (however, this seems unlikely as the results were as I expected), thirdly, Measuring the lengths of the wire is also a inaccuracy as the rulers used are not exact, and it is difficult to get an accurate reading of length by eye, as the wire might not be completely straight, it may be of different thicknesses throughout the length, so the length may have been longer than I had measured, lastly whenever current flows through a wire it will heat up.As I stated in my introduction, this heating effect will also tend to increase the resistance. I feel that this last point is the most likely cause of the errors I have. To improve the reliability of my experiment, I would ensure that these possibilities were overcome. I would use a voltmeter that was able to read to 2 d.p., which would be more accurate. An even better method for measuring the resistance would be to use a multi-meter to read the resistance directly. By taking just one reading for the resistance instead of two (current and p.d.), I will be able to reduce any errors I have in reading the scales. I would also ensure that the temperature of the wire stayed as constant as possible. I could do this a number of ways. One method would be to cool the wire to a constant temperature by immersing it in ice for all of the readings. A better method however, would be to ensure that the current flowed in the wire for as short a time as possible (as I stated in my plan) but this time I?d try and turn the lab pack off even quicker, and so the heating effect was as small as possible. By making these improvements, the reliability of my experiment would be improved and so I could be even more confident of my conclusions. I am confident in my data. I took 3 readings for each of my 19 lengths and so gained an average resistance. It is clear that all of my results follow the same pattern and so I am confident in my conclusion that resistance is proportional to length. To extend my investigation, I would like to examine one of the other factors from my introduction. As I feel that the heating effect of the current gave me anomalous results, I would test to what extent temperature affects resistance. To do this, I would take identical lengths and thicknesses of wire and place them at different temperatures using a water bath. In this way I could control the temperature and by using the same circuit as before, determine the affect that temperature has on the resistance.

Experiment on Electrical Resistance 6.8 of 10 on the basis of 912 Review.