Metals have a high density of conduction electrons. The aluminium atom for example has three valence electrons in a partially filled outer shell. In metallic aluminium the three valence electrons per atom become conduction electrons. The number of conduction electrons is constant, depending on neither temperature nor impurities. Metals conduct electricity at all temperatures, but for most metals the conductivity is best at low temperatures. Resistance in electricity is the property of an electric circuit or part of a circuit to transform electric energy into heat energy by opposing electric current.
Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors; this can be seen in the diagram below. Resistance is often considered as localized in such devices as lamps, heaters, and resistors, in which it predominates, although it is characteristic of every part of a circuit, including connecting wires and electric transmission lines. Diagram 1: The collisions of electrons with ions in a circuit Electron Ion direction in which the electron is moving
Here the electron is going to collide with the ion and so the speed at which it is traveling will decrease and since the electron is one of the current-carrying charged particles the current in the circuit will decrease. The spread of electric energy in the form of heat, even though small, affects the amount of electromotive force, or driving voltage, required in order to produce a given current through the circuit. In fact, the electromotive force V (measured in volts) across a circuit divided by the current I (amperes) flowing through that circuit defines quantitatively the amount of electrical resistance R.
In other words, R = V/I and this is referred to as Ohm’s law. Thus, if a 12-volt battery steadily drives a 2-ampere current through a length of wire, the wire has a resistance of 6 volts per ampere, or 6 ohms. The ohm is the common unit of electrical resistance, equivalent to one volt per ampere and is represented by the capital Greek letter omega. Below is a list of the factors that affect the resistance of a wire. From this list, I will pick two factors to investigate. Factors 1. Temperature: If the wire is heated up the ions in the wire will start to vibrate because of their increase in energy.
This causes more collisions between the electrons and the ions, as they will be moving into the path of the electrons more frequently. When the electrons collide with the ions they lose some of their kinetic energy in the form of heat energy and this causes them to move more slowly thus making the current intensity in the wire decrease. This increase in collisions therefore means that there will be an increase in resistance. 2. Material: The type of material will affect the amount of free electrons, which are able to flow through the wire.
The number of electrons flowing 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. Furthermore, if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase. 3.
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 are likely to occur. 4. 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 ions there should be less collisions. From the above I have decided to investigate how a wires width (cross sectional area), length and material affect its overall resistance.
Hypotheses and Predictions As I said previously, as the length of a wire is increased then the distance the electrons will have to travel in an electric current will increase, so more collisions are likely to occur between them and the ions present. Therefore it can be deduced that the longer the wire is the greater its resistance. Furthermore, if the length of a wire is doubled then the distance the electrons must travel in an electric current will double and so the number of collisions between the electrons and the ions that are likely to occur will also double.
So, it can be deduced that the resistance of a wire should be directly proportional to its length. If I were to plot the resistances of different lengths of wire made out of the same element then I think my graph would look as follows: Resistances Lengths of wire used I also predict that the resistance of a wire will be inversely proportional to its cross sectional area. The reason being that if the wires thickness is increased the space for the electrons to travel through will increase and due to this increased space there should be less collisions between the electrons and the ions.
This diminish in the number of collisions would reduce the resistance in the wire. So, if a wires width is doubled then the space that the electrons have to travel through will double and so the number of collisions occurring should halve. If I were to plot the resistances of different wires made out of the same element but with different cross sectional areas on a graph then I think it would look as follows: Resistances Cross sectional areas of wires used The greater the number of atoms in a material, the more obstruction there will be for an electron to flow from one end to the other in an electric current.
For this reason it is possible to say that the greater the density of a conductor, the greater its resistance. For this reason as well as the molecular structure of the conductor and the number of free electrons it has, I predict that not all materials will conduct electricity to the same extent. When one factor is investigated during the experiment, by carrying out the plans written below, all of the other factors mentioned previously must be kept constant to ensure fair testing however the temperature of the wire cannot be controlled since it rises automatically when a current passes through it.
The reason for this is that when a current passes through a wire collisions occur between the electrons and the ions and as I mentioned previously, when collisions occur the electrons lose kinetic energy in the form of heat energy making the overall temperature of the wire increase. However it is possible to minimise heat rise by not using very large currents, this is because it will minimise the number of collisions occurring between the electrons and the ions in any given time and so minimise the amount of heat energy released by the electrons thus reducing the rise in temperature experienced by the wire.
This is as constant as it is possible to keep the temperature. Furthermore, as well as the factors listed above there are other variables that must be kept constant to ensure fair testing and these can be seen in the list below: o The same apparatus must be used throughout the experiment. o The same method must be used to collect every set of results. I will now say what apparatus is to be used in the investigation, how the investigation will be carried out and I will also draw a diagram for a visual aid. APPARATUS: o A 6V DC generator.
o A nichrome wire (Ni 80% Cu 20%) 1 meter long with a cross sectional area of 0. 40 mmi?? o A nichrome wire 1 meter long with a cross sectional area of 0. 08 mmi?? o A nichrome wire 1 meter long with a cross sectional area of 0. 11 mmi?? o A nichrome wire 1 meter long with a cross sectional area of 0. 13 mmi?? o A nichrome wire 1 meter long with a cross sectional area of 0. 25 mmi?? o A constantan (Cu 55% Ni 44% Ma 1%) wire 1 meter long with a cross sectional area of 0. 08 mmi?? o A copper wire 1 meter long with a cross sectional area of 0. 08 mmi??
o A manganin (Ma 12% Ni 2% Cu 86%) wire 1 meter long with a cross sectional area of 0. 08 mmi?? o A rheostat o A voltmeter o An ammeter o 2 crocodile clips o 6 connecting wires o A 1 meter ruler DIAGRAM: Generator ……. Ammeter 1 2 Wire Rheostat Voltmeter PLAN (for investigating how the length of a wire affects its resistance): o The circuit will be set up like the above diagram shows. o The crocodile clips will be attached to the wires at points 1 and 2; these will be used to connect one of the loose wires with the cross sectional area of 0.08 mm2 to the rest of the circuit. o.
The wire will then be placed in the position shown by the above diagram and the two crocodile clips will then be placed at the far ends of the wire. o The rheostat will then be placed in a random position ensuring that the current produced isn’t too high to minimise heat rise and the voltage and current present across the wire will then be read off of the meters and noted. o The rheostat will then be placed in five new positions with the current and voltage across the wire being noted each time using the meters.