Introduction: An electric current (D. C. – direct current) is a flow of charge from the positive terminal of a cell to the negative terminal. The charge is carried by conduction electrons, which are not fixed to a particular ion – they are called free electrons. When p. d. (potential difference) is applied across the ends of a wire (by a battery etc. ) these electrons will move towards the positive terminal. Some of the electrons in metals are not bound to particular atoms, they are shared between them. These ‘delocalised’ electrons are free to move around the metal and are what actually carries the current.
As the delocalised electrons are charged and can move, they can carry a current. The most important property which controls how well a certain metal conducts electricity is the number of delocalised electrons there are per atom. The more of these electrons there are in the metal the better it conducts electricity. Basically, a copper wire consists of millions and millions of copper atoms. Each copper atom has one or two electrons which are not tight to the atom but are loosely held – and seeing as electrons have negative charges, once an atom loses an electron it becomes positively charged and is now called an ion.
These ions vibrate as atoms always do, with these “free” electrons moving randomly from one ion to the next – this is true of all metals. A battery is applied to the circuit like so in this simple diagram… … the free electrons are repelled by the negative terminal and attracted by the positive terminal. The free electrons still move randomly but now they move with a steady velocity towards the positive terminal and away from the negative one. This is now called a flow of charge. Having explained the nature of the flow of charge, resistance is simply how hard it is for the electric charge to flow through something.
The higher the resistance, the more energy is used up just getting the current through the wire/conductor. The resistance of a conductor (in this case the wire) is the ratio of the potential difference across it, to the current flowing through it. A resistor does not stop current from flowing; it just slows down the rate at which it flows. Charge (Q), flow (V) and resistance (R) are all linked, which is demonstrated by the fact that when current is decreased, resistance increases. We know the following formulae: I=Q/T and R=V/I This shows me that if the charge changes then the current will change.
Therefore the resistance will change because to calculate resistance I use the current (i. e. if the current decreases than the resistance will increase). There are many factors which affect the amount of resistance: Length: If I increase the length of the wire, the resistance increases: the longer the wire the more resistance. This is because it takes longer for the electrons to flow through the wire… as explained earlier the more atoms there are in the electrons’ way, the harder it is to get through which increases the resistance of the wire.
Temperature: The higher the temperature, the higher the resistance. As with all metals, when their temperature increases they begin to vibrate more side to side. As a result, they get in the way of the electrons which makes it harder for them to get through which consequently increases the resistance of the wire. Cross sectional area (thickness): The cross sectional area of a length of wire is inversely proportional to its resistance – when the cross sectional area is doubled, the resistance is halved. This is because there are more electrons to carry the current.
It works like a hose – the wider the hose is, the easier and faster water passes through it. If one was looking to use a piece of wire with the optimum (least) amount of resistance the wire would be thick, as short as possible and as cold as possible. For this investigation, as mentioned in the aim, I have chosen to investigate what effect the length of the wire has on the resistance thereof. I chose this as my variable because it is by far the easiest to measure – all the other factors are relatively difficult to work with, harder to control and may produce inaccurate results.