The material of the wire consists of layers of atoms whose positive centres (ions) are surrounded by a sea of electrons. These electrons are very mobile (free to move). However, when the electrons start moving, carrying current, they will keep on bumping into the ions. These ions, vibrating in fixed positions, cause obstruction to the free flow of electrons. This obstruction is termed resistance. Resistance opposes current and therefore the higher the resistance, the smaller the current.
Therefore the factors that affect the resistance are the same factors that control the flow of current through a wire. In the above diagram the circle represent the ions and the red line represents the flow of electrons. The factors affecting the flow of current, which you could investigate, are: Area of cross-section of wire When the area of cross-section of the wire is increased the current through it will increase. This is because the electrons can flow more freely through a large cross-section space available, with the same length of wire, and so avoid colliding Length of wire.
When the length of wire is increased the current is expected to decrease. This is because the electrons will experience more obstruction to their free flow due as a result of colliding into more ions along a longer length. Material of wire Some materials provide more resistance to the flow of current than others. This depends on the arrangement of atoms in the material and the sea of electrons surrounding it. Pure conducting metals, such as copper, have very little resistance and therefore is a very good conductor while some others such as nichrome, manganese and constantan have higher resistance.
Those with higher resistance are used as fuse wires, etc. Temperature When the temperature is increased more and more the ions vibrate faster and faster. This increases the obstruction to the free flow of electrons and therefore current will decrease. Out of the many factors, which affect the flow of current through a wire, the one that I am investigating is the effect of length of wire. The reason I have chosen to do this is that it is by far the easiest and probably the most convenient.
In order to make this a fair test when I change the length of wire. I will keep all the other factors, i. e.the material of wire, area of cross-section and the temperature exactly the same. In all measurements of current, I will keep the voltage across the wire unchanged (constant).
Also I have to consider of using a constant temperature, i. e. room temperature (25i?? C), and also I have to keep the cross-section of the wire constant. Prediction I predict that when I double the length of wire the current will halve and if I make the length of wire three times, the current will approximately be a third of its initial value. That is my quantitative prediction is that the current will be inversely proportional to length of wire.
Therefore, if I plot (V=Voltage, I=Current) vs. length of wire, a straight line will be obtained, since V is constant. The scientific reason for my prediction is that the resistance is caused by the obstruction to the path of freely moving electrons by the ions. When I double the length the number of ions will also double. The numbers of ions along the material of constant thickness will be proportional to the length and therefore the collision experienced by the electrons will also be proportional to the length. Hence the resistance is expected to increase while lengthening the wire.
Method The apparatus that I will use are: Powerpack- This gives power to the whole appliance. Ammeter – This records the current of the circuit. Voltmeter – This records the voltage of the circuit. Variable resistor – This changes the resistance of the circuit Constantan wire (0. 9mm) – This is what I am testing. This is the experiment Connectivity wire – This connects all the apparatus together. Two Crocodile clips – This hold the wire up Micrometer Screwgauge – This measures the area of cross-section of the wire. 1 metre ruler – You use this to measure the constantan wire length.