An investigation into the theory of resistance Resistance of a wire An electric current is the flow of electrons through a material. The Current through a wire is proportional to the potential difference across it. Plotting a graph of P. D against current would give a straight line graph through the origin. P. D (V) [image002. jpg] I (A) The gradient of the line (V/ I) is a constant and is called resistance. Resistance is measured in ohms (Ohm). The graph above is an ideal case where there is no temperature change of the wire during the time that measurements are being taken.

This will not be the case as passing an electric current through a wire causes it to heat up, as in an electric kettle or electric fire. By setting the following circuit it is possible to determine the resistance of a wire: [image004. jpg] ( [image006. jpg] = wire sample) Planning I shall apply an electrical current through a piece of wire one hundred centimeters long. I will be taking readings of current and P. d. every 10 centimeters. With the information I gather, I will then be able to calculate the resistance using the formula: Voltage (V) = resistance (Ohm) Current (I) Aim.

I aim to carry out an experiment which will enable me to show a relationship between length of a wire and resistance of a wire. In this investigation I will see how resistance of a wire varies with the extending length. The things I could change in this experiment are called the variables, these are: – Material of the wire – Width of the wire – Starting temperature of the wire – Current in the circuit Prediction I predict that as I take my readings moving further along the wire, the greater the resistance will become.

This is because the longer the wire, the more times the free electrons will collide with other electrons, such as the particles making up the metal. From this more electrical energy should be transferred into thermal energy. This in short, means less electricity can pass through the wire giving it a greater resistance. If my prediction appears to be correct then the results of the resistance against length graph will be directly proportional and show positive correlation. Here is a simple diagram showing a metal wire that shall be used: [image008. jpg] It is the interaction between these positive atoms that causes the wire to have resistance. The inside of the metal has a regular array of positive ions (+ve);

this is when an ion is a metal atom, which has lost its free electrons. The free electrons can swim about in the space between the ions like gas molecules. When voltage is applied across the ends of a wire the negative ions, (-ve) electrons, are attached towards the positive end of the wire and current flows. Longer wires will cause an increase in resistance because the electrons have to travel past more atoms, because of this, collisions between the electrons and the atoms are more likely in longer wires then in shorter wires, also because of more collisions occurring, the wire may heat up, this in turn may cause an alteration of the results, creating anomalies. Ohms law will also have an effect on my results.

Ohms law states that the following will happen: Resistance (Ohm) [image010. jpg] Length Of Wire (cm) Also, because the results will be altered through heating, I have decided to take certain precautions. In an attempt to discard this alteration I will disconnect the wire for each new reading, therefore preventing heating up of the wire which should allow the experiment to obey ohms law, which states that resistance will be directly proportional to length, in a temperature controlled environment.

From this though, ultimately, increasing the length of wire should increase the total amount of positive ions inside the wire, resulting in a higher resistance. Apparatus The equipment I will use is: – 100 centimeter piece of wire – Ammeter – Voltmeter – Crocodile clips – Connecting wire – Ruler Method 1. Collect the required equipment for the investigation. 2. Connect the equipment as shown in the circuit diagram. 3. Place crocodile clips along the wire every 10 centimeters. 4. Take preliminary readings from the voltmeter and ammeter. 5. Record the results shown on voltmeter and ammeter in a table.

6. Calculate the resistance and evaluate. 7. Take note of preliminary results and modify investigation where necessary. 8. Do the experiment 3 more times and repeat steps 5. and 6. in more depth. Pre-Test I have done a preliminary experiment to allow me to ensure that things will run smoothly, the results are as follows: Length (cm) P. d. (V) Current (A) Resistance (Ohm) .

From this table I have realised that my prediction was actually correct, the further the length the higher the resistance. My table reinforces that prediction except for two readings (IN BOLD), these are the anomalies. These readings at 70cm, which is too low, and at 90cm, which appears to be far too high, could be due to many reasons. These reasons could be endless, one for example could simply be a poor connection.

From completing my pre-test trial I have decided I will make no alterations whatsoever, I believe my preliminary investigation was quite successful and straight forward. Moving on from this somewhat simply completed pre-test I will carry out my actual results within the main experiment, in this main bulk of my investigation I shall redo the experiment and record my results as shown above 3 times, calculate the average, draw up a series graphs and evaluate (as explained in the method). Results Table The following table is my recorded three experiments and the calculated average’s and the average resistance: Length of wire (cm) Test 1 Test 2 Test 3 Average Resistance (Ohm) P.

Conclusion From my investigation, I have established that the length of a wire directly affects the total resistance measured in proportion to it. I have seen that as the length of the wire increases so does the resistance, this also perfectly backs up my original prediction. In my prediction I foreseen that if you doubled the length of the wire the resistance would also double, this is because you are doubling the number of collisions that would occur between the free electrons and the atoms in the metal.