All these factors must be kept constant to make the investigation fair. The same apparatus must be used throughout the investigation. It is also important to take three repeats and find the mean so if one result is very inaccurate, the others will average it out.
Research:
All materials, solid, liquid or gases are made up of atoms. The atoms themselves consist of a central bit, called the nucleus, made up of particles called protons (which have a +ve electrical charge) and neutrons (which have no charge) Orbiting around the nucleus are electrons which are very tiny and have a -ve electrical charge. One can think of the electrons orbiting in layers like the rings of an onion, and it's the ones in the very outside layer, the outer shell, that are the most important when thinking about conduction.
In metals, the outermost electrons are held only very weakly to the atom and often wander away from it and go to the nearby atom or one a bit further away. These wandering electrons are called conduction electrons and the more of these there are, for a given volume of metal, the better the metal will be as a conductor of electricity. When you connect a battery across a wire, one end becomes +ve and attracts the conduction electrons, which drift towards that end of the wire. But the electrons have obstacles to face because the metal atoms are jiggling about because of their thermal energy and so the electrons collide with them and are knocked all over. It’s this difficulty that the electrons have in moving along the wire that we call resistance.
Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors. A resistor is a material that makes it hard for electrons to go through a circuit. Without resistance, the amount from even one volt would be infinite. Resistance occurs when electrons travelling along the wire collide with the atoms of the wire.
The unit of resistance is Ohms and the symbol is:
The higher the resistance, the lower the current. If there is high resistance, to get the same current a higher voltage will be needed to provide an extra push for the electricity.
Some metals have less resistance than others. Wires are always made out of copper because copper has a low resistance and therefore it is a good conductor. The length and width of a wire also has an effect. In this investigation I will investigate how the diameter of a wire will affect the resistance in the circuit.
Resistance opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.
E.g. The filament an electric light bulb
Ohm's law
He discovered relationship that the amount of steady current through a large number of materials is directly proportional to the potential difference, or voltage, across the materials. Thus, if the voltage V (in units of volts) between two ends of a wire made from one of these materials is tripled, the current I (amperes) also triples; and the quotient V/I remains constant. The quotient V/I for a given piece of material is called its resistance, R, measured in units named ohms. The resistance of materials for which Ohm's law is valid does not change over enormous ranges of voltage and current. Ohm's law may be expressed mathematically as V/I = R. That the resistance, or the ratio of voltage to current, for all or part of an electric circuit at a fixed temperature is generally constant had been established by 1827 as a result of the investigations of the German physicist George Simon Ohm.
Alternate statements of Ohm's law are that the current I in a conductor equals the potential difference V across the conductor divided by the resistance of the conductor, or simply I = V/R, and that the potential difference across a conductor equals the product of the current in the conductor and its resistance, V = IR. In a circuit in which the potential difference, or voltage, is constant, the current may be decreased by adding more resistance or increased by removing some resistance. Ohm's law may also be expressed in terms of the electromotive force, or voltage, E, of the source of electric energy, such as a battery. For example, I = E/R.
With modifications, Ohm's law also applies to alternating-current circuits, in which the relation between the voltage and the current is more complicated than for direct currents. Precisely because the current is varying, besides resistance, other forms of opposition to the current arise, called reactance. The combination of resistance and reactance is called impedance, Z. When the impedance, equivalent to the ratio of voltage to current, in an alternating current circuit is constant, a common occurrence, and Ohm’s law is applicable. For example, V/I = Z.
With further modifications Ohm's law has been extended to the constant ratio of the magneto motive force to the magnetic flux in a magnetic circuit. Resistance values in electronic circuits vary from a few ohms, W, to values in kilo ohms, kW, (thousands of ohms) and mega ohms, MW, (millions of ohms). Electronic components designed to have particular resistance values are called resistors.
Theoretical KNOWLEDGE
What is resistance?
Electricity is conducted through a conductor, in this case wire, by means of free electrons. The number of free electrons depends on the material and more free electrons means a better conductor, i.e. it has less resistance. For example, gold has more free electrons than iron and, as a result, it is a better conductor. The free electrons are given energy and as a result move and collide with neighbouring free electrons. This happens across the length of the wire and thus electricity is conducted. Resistance is the result of energy loss as heat. It involves collisions between the free electrons and the fixed particles of the metal, other free electrons and impurities. These collisions convert some of the energy that the free electrons are carrying into heat.
How is it measured?
The resistance of a length of wire is calculated by measuring the current present in the circuit (in series) and the voltage across the wire (in parallel). These measurements are then applied to this formula:
V = I ´ R where V = Voltage, I = Current and R = Resistance
This can be rearranged to:
R = V
I
METHOD:
To ensure a fair test whilst carrying out my preliminary experiment I am going to be very careful. I am going to use a constant voltage of volts and a constant length of cm.
Prediction:
I predict that, as the length of the wire doubles, the resistance will also double, but as the cross-sectional area of the wire doubles, the resistance halves. This means that the length will affect the resistance more than the thickness will.
Apparatus:
Meter ruler To measure the wire being tested to ensure a fair test.
Selection of wires Different materials and widths but the same length.
Crocodile clips To connect the wire being investigated to the rest of the circuit. Voltmeter & Ammeter To measure the resistance.
Wires To connect the above items and to complete the circuit.
I will obtain the voltage and current readings from the voltmeter and ammeter.
Below is a circuit diagram for my experiment.
1 x Power Pack (to give varied voltage)
1 x Voltmeter
1 x Ammeter
5 x wires (with crocodile clips)
wire of varied length and thickness
Controlled variables:
Temperature (room temperature)
Wire material
Dependent variable:
Resistance
Independent variables:
Thickness of wire
Length of wire
To ensure a fair test I shall keep the power supply at __ volts and I shall keep the length of the wire at __ cm.
RESULTS:
Below is a table of results which I have collected from my preliminary experiment.
Before I start my main experiment I have chosen to do a risk assessment which is shown below.
SAFETY MEASURES:
I will handle the power supply carefully.
I am going to only use a voltage of __ volts.
I will be careful when handling live wires.
I will disconnect the main power supply before moving the wires around.
Safety
Whilst doing the investigation, it is important to keep safety into consideration. Before using the power pack, the pointer should point at 0 volts. It is important to be careful while using the power supply. While handling live wires, it is essential to be careful. The voltage should be kept low because of the safety factor and the wires heating up.
I will repeat the experiment another time to get a fair set of results which could help me to make a firm conclusion and to plot an accurate graph.
In the main experiment instead of using an ohmmeter I have chosen to use an ammeter and voltmeter ,
I have done this so that instead of relying on the ohmmeter to give the resistance I will calculate the resistance of the wire , I shall calculate the resistance of the wire using the equation below.
RESISTANCE = VOLTS
(AMPS)
I have chosen to use a meter ruler because the lengths that I will be measuring are to big for a smaller ruler and also the meter ruler can be accurate to +1mm or -1mm.
Analysis
From the graph on the previous page I can see that the resistance of the wire is proportional to the length of the wire. I know this because the Line of Best Fit is a straight line showing that if the length of the wire is increased then the resistance of the wire will also increase.
Conclusion
In my prediction I said that :
If the length increases than the resistance will also increase in proportion to the length. From my graph I have shown that my prediction was correct, as the Line of Best Fit is a straight line proving that the resistance of the wire is proportional to the length of the wire. The length of the wire affects the resistance of the wire because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion.
The resistance of a wire depends on the number of collisions the electrons have with the atoms of the material, so if there is a larger number of atoms there will be a larger number of collisions which will increase the resistance of the wire. If a length of a wire contains a certain number of atoms when that length is increased the number of atoms will also increase.
Evaluation
From my results table and graph I can see that my results that I collected are very reliable. I know this because my results table does not show any individual anomalous results this means that I did not have to leave any results out of my averages because they were anomalous. Also on the graph I can see that none of the averages plotted are anomalous because all the averages lie along the same straight line.
During my experiment I have noticed several modifications I could make to improve on the Investigation if I was to repeat it.
The first of these modifications would be the circuit that I would use. To be more accurate with my results I would use the circuit layout below:
POWER SUPPLY
__ VOLTS
AMMETER
VOLTMETER
WIRE
METRE RULER
Instead of connecting the voltmeter to the main circuit I would connect it to the wire which is being tested. I would do this so that the voltmeter is measuring the voltage of just the wire being tested and not the wires of the main circuit as well.
