Metals conduct electricity because the electrons in the metal can move about inside the structure. These electrons are called free electrons. Electricity is conducted through a conductor by means of free electrons. Atoms consist of protons, electrons and neutrons. The protons and neutrons make the nucleus of an atom while the electrons circle the outer area of the atom. Electrons in metal are able to move freely and are used as current in an electric circuit. This is because they carry a charge and can move all around the circuit with this charge.
While these electrons are travelling around the circuit, atoms are sometimes in the way, causing the two to collide. This takes out some of the energy from the electron and transfers it to the atom. This is how resistance occurs. The number of free electrons depends on the material and the more the free electrons in a substance the better the material as a conductor. All conductors offer resistance to the flow of current. The conductor’s atoms determine this resistance. For example copper atoms offer negligible resistance to an electric current because a significant proportion of its electrons are free to move from electron to electron.
Thus copper is commonly used as a conductor. Current, is the flow of electrons around a circuit. Those materials, which have a lot of “free” electrons, will make it a lot easier for current to flow through, and so there is low resistance. That’s why not all metals are equally as good at conducting electricity. Therefore materials with a low resistance are called good conductors and those with a high resistance are called good insulators. In many materials there is a simple relationship between the voltage across two points and the resulting current.
Such materials are called ohmic materials and obey what is known as ohms law, which sates that: “The current through a metallic conductor at a constant temperature is proportional to the potential difference (voltage)” i. e. V I V = IR R= V/ I V= Potential difference in volts (V) I = Current in amps (A) R= Resistance in a unit is called an ohm () The graph below shows the relation between the current and the voltage of a conductor, showing that as the current increases the voltage increases. This is what is termed as Ohm’s Law.
In order to find the resistance of a conductor, the gradient of the graph is found out using the formula: Gradient of the graph = I/V Whereby R = 1/gradient FACTORS AFFECTING THE RESISTANCE OF A WIRE: There are four external factors that influence the resistance in a conductor. Thickness (cross sectional area of the wire), length, the material of the wire being used and temperature all have some effect on the amount of resistance created in a conductor. LENGTH: The length of a conductor is similar to the length of a hallway.
A shorter hallway would allow people to move through at a higher rate than a longer one. When an atom has more energy it begins to vibrate and as it receives more energy, it vibrates more. As it vibrates it is hitting other atoms around it and passes on its energy. This has a knock on effect and energises all atoms around until this energy is lost through heat. As the atoms vibrate they are more likely to hit electrons because the electrons now have less space to move through. If this is over a shorter distance there will be less atoms for the electrons to hit.
If this is carried out over a larger distance there will be more atoms for the electrons to hit. Therefore resistance is proportional to the length of the conductor, providing the temperature, cross sectional area and material of the conductor are kept constant. The graph below shows this: CROSS SECTIONAL AREA: The cross-sectional area of a conductor (thickness) is similar to the cross section of a hallway. If the hall is very wide, it will allow more people through it, while a narrow hall would be difficult to get through due to it’s restriction to a high rate of flow.
If you consider this situation in terms of electrons, a wire with a small cross sectional area would have a smaller number of electrons flowing through it while a wire with a larger area would have a greater amount of electrons flowing through it thus allowing a higher current to flow through it. Thus showing that the resistance is less in a wire with a larger cross sectional area, which would mean that the resistance of the conductor will be inversely proportional to the cross sectional area of it, providing the temperature, length and material of the conductor are kept constant. The graph below shows this theory:
TEMPERATURE: The temperature of a conductor has a less obvious effect on the resistance of the conductor. It would be as hard to apply the hallway analogy as it is hard to say whether a hot hallway would make us move faster or slower than a cold hallway. To truly understand the effect you must picture what happens in a conductor as it is heated. Higher temperature means more vibrations. Imagine a hallway full of people. Half of the people (the electrons) are trying to move in the same direction you are and the other half (the protons) are evenly spaced but stationary in the hallway.
This would represent a cold wire. Since the wire is cold the protons are not vibrating much so the electrons can run between them fairly rapidly. As the conductor (hallway) heats up, the protons start vibrating and moving slightly out of position. As their motion becomes more erratic they are more likely to get in the way and disrupt the flow of the electrons. As a result, the higher the temperature, the higher the resistance and we can say that Ohm’s law holds true unless temperature changes. Which leads us to conclude that the resistance of the wire is proportional to the temperature.
The graph below shows this: MATERIAL: The fourth factor is the conductivity of the material we are using. Some metals are just more electrically conductive than others. This however, is considered an internal factor rather than an external one. The resistivity is a value that only depends on the material being used. For example, gold would have a lower value than lead or zinc, because it is a better conductor than they are as it has more free electrons flowing within it thus has a lower resistance.