The silicon chip has been the cornerstone of the IT revolution for the past several decades. Over the years we have seen the size of devices such as microprocessors shrink and their speed increase considerably. However to enable miniaturisation to proceed as it has electronic device engineers have had to integrate discrete devices such as resistors, capacitors and inductors into the chips themselves. To achieve this they have had to make these components essentially out of the silicon itself. To enable them to do so they have had to understand what factors affect, for example, the resistance of the material that they used.
In a similar manner, see if you can discover what factors affect the resistance of a metal wire in the laboratory. Introduction To understand resistance and be able to calculate it many things must first be understood, such as: conduction in solids, current, charge, potential difference and how they connected to finding resistance. For conduction to take place there must be a supply of charge carriers in the solid and be able to move freely inside it. It is believed, in solids that the carriers of the supply is loosely held electrons (This experiment will be based on this theory).
Current is the amount of movement of negatively charged electrons. An ammeter measures current. Current is measured in amps (A). Potential difference is the amount of electrical energy transformed into other forms of energy when unit charge passes from one point to the other. A Voltmeter measures potential difference. The unit of potential difference is Volts (V). Resistance is the opposition to the current. The freely moving electrons that carry the charge have to move through the metal in-between the gaps between the ions.
The negative ions also collide with the ions in the metal and therefore cannot go through the ions because the negative electronic orbits repel the negative ions. The unit of resistance is ohm (? ). Conductors specially constructed to have resistance are called resistors. Ohm’s law – the resistance of a conductor is defined as the ratio of the potential difference across it to the current flowing through it. Resistance = Velocity Current Planning For this investigation it must be decided what factor of the material that acts as the resistance should be varied.
There are many factors of the material that will change the resistance of it. These are: ; The cross-sectional area – The particles in the resistor that resist or oppose the movement of the loose electrons are the charged ions and the electrons can therefore only pass through the metal resistor by travelling through the gaps between the ions. If the cross-sectional area is increased then more ions are present however there are also more gaps/channels for the electrons to flow through. Therefore the greater the cross-section the less the resistance. ; The length – The more the length the greater the resistance.
This is because there are more ions to resist the current of electrons and this time the number of channels for the electrons to flow through is the same. ; The material of the resistor – It is obvious that if the number of loose electrons that carry the charge increased then the resistance would be decreased and also that if the number of loose electrons was decreased. Therefore the material of the resistor will change the resistance because different materials will have different or even no loose electrons to carry the current, for example insulators have no loose electrons and are therefore very high resistors.
; Temperature – The hotter a material is the more energy a particle has and therefore increases in speed. This increases resistance because the higher temperature makes the ions in the solid vibrate more. If the ions vibrate the gaps between the ions decrease and it is then harder for the loose electrons to pass through the gaps between them i. e. decreases the chance of the electrons passing through the gaps. For this experiment the factor that will be investigation is the ‘length’.
This is because if the cross-sectional area was the factor under investigation different diameters of the wire would be needed and would need to be specially ordered by the school, the school has a limited supply of different types of wire which all have the same diameter. The length will also be much easier because by just moving the position of a crocodile clip can easily vary the variable. In the investigation the results that will be found will be the current, which is measured by an ammeter, and the potential deference, which is measured by a Voltmeter. The resistance can then be calculated by using ‘Ohm’s Law’:
V = I . R Or R = I V Preliminary work The aim of the preliminary work is to find a good range of current values. This is because if the current is too high then a lot of heat is produced which will change our results and make the experiment an unfair test. In order to find a good range of current values a suitable voltage must be found and a suitable range of the wires length must be found. A suitable voltage is one that is small enough to keep the temperature constant and a voltage large enough to be able to produce a large enough range of figures so that the ammeter can produce a difference in the results.
A suitable range for the length of wire would be that the ammeter is able to recognise a difference in the readings, and the length isn’t too small because it will increase the current and the temperature will rise and that the length isn’t too large so that it is recognised that an ammeter has been used. Previously it has been found that the temperature stays quite constant between 0. 1A and 1. 0A. This then means that the preliminary work should aim to find the velocity and a length so that the current range is between 0. 1A and 1. 0A. Preliminary Method 1.
Set-up the circuit as shown below. 2. Connect the circuit to a 1. 5V Battery. 3. Measure the current and voltage for every 10cm of wire. 4. Repeat, but measure the current for 90cm of wire. 5. Calculate the resistance using Ohm’s Law (V = IR). 6. If necessary, repeat steps 1-5 using 2 x 1. 5V batteries. 7. Repeat again, if necessary, to get a suitable range of currents, using wire lengths of 20cm/30cm/70cm/80cm. (It maybe necessary if the current is too high at a certain length. )