I also have to ensure that the wires do not touch and short out, as this would render my results useless. This can be done by keeping the loose wires neat and by not letting them touch each other.
Apparatus List
- A 1.5V power supply
- A digital 0-20V Voltmeter
- An Analogue 0-1A Ammeter
- 5 pieces of approximately 1 metre long constantan wire with various specific diameters.
- Meter ruler
- Leads including crocodile clips
- Insulation tape.
Diagram of apparatus set up as in investigation.
Method
- Set up apparatus as in diagram above, making sure that the electricity is turned off to prevent an electric shock.
- Tape length of wire over the Metre Rule, making sure that there are no kinks.
- Switch the power on and adjust the crocodile clips accordingly.
- Switch the power off and record the Voltage and Ampere reading in a table.
Reliability and Accuracy
Because of my choice to get readings for all 5 wires, I will have not have time to do repeats during the double period we have to carry out this investigation. This means that I have no way of knowing how reliable my results are. I will spot any anomalies on my graph though, so it shouldn’t be too much of a problem.
To ensure that my results are accurate, I will make sure the wire is taught and that I avoid the error of parallax while reading measurements off the ruler.
Ohms Law
To record and measure the resistance I will use an Ammeter to measure the current in amps and a voltmeter to measure the voltage in volts. I will work out resistance in ohms by using Ohms Law which is the formula shown below:
R = V ÷ I
Resistance (in Ω) = Voltage (in Volts) ÷ Current (in Amps)
Range of Results
I will take 5 readings from each wire; therefore giving me 25 results that I think is suitable. I will take a reading every 20cm’s so I should get a clear indication of whether or not my results are accurate and reliable.
My Prediction
Before I make my prediction, I will explain about free electrons, electricity, atoms and resistance:
In a metal atom's structure on the very outer shell the electrons become free electrons. This means they can move around freely in the solid metal and so the metal can conducts electricity. When a voltage is passed through a metal object the electrons (which are negatively charged) are attracted to the positively charged end of the wire, which is towards the positive inlet of the battery or power source. This makes all the free electrons line up and move in the same direction. This is an electrical current.
Electricity travels through a conductor, in this case a length of wire, by means of free electrons. Electrons are the smallest parts of an atom that is situated in. An atom has protons that are positively charged and neutrons that have a neutral charge in the centre. The neutrons and protons are like the core of the atom. There are then electrons that are negatively charged circulating round outside the core. There are as many electrons as protons in an atom so they cancel each other out and the atom has no overall charge.
Resistance involves collisions between the free electrons and the fixed particles of the atoms in the metal. These collisions loose energy and time and therefore resistance occurs. As the resistance of a material increases so must the force required to push the same amount of current through the metal. Ohms law is defined by this equation: Resistance, in ohms (Ω), equal to the Voltage, in volts (V) divided by the current, in amperes (A).
I predict that the resistance will increase proportionally as the length of wire increases. If I double the length, the resistance will double. I say this because in a certain length of wire, electrons have to negotiate their way through a collection of ions. If you then double that certain length of wire, the electrons have to negotiate their way through twice as many ions therefore doubling the resistance of the length of wire. This is shown in my free-electron model below. To quantify my prediction, I will give a simple example. If I have a length of wire 50cm, with a resistance of 2 ohms, and I increase the length of the wire to 100cm, I predict that the resistance will increase to 4 ohms.
I also predict that the overall resistance will be consecutively higher starting from wire 5 to wire 1, because the wires get consecutively smaller in diameter thus increasing the resistance of the wire.
Free-Electron Model
Obtaining Evidence
Analysing
Conclusion
From the graph you can see a very clear trend, which is, as the length of the wire increases so does the resistance of the wire. A more important trend is that it the increase of resistance of the wire is constant to increase of length of the wire, which makes the line or curve of best fit a straight line, which follows on through the origin of the graph.
The line of best fit on my graph shows that the results have followed the expected pattern. The points are very close to the line of best fit or even touching it. This showed that the results were directly proportional throughout the experiment, as the line does not curve suddenly or curve or bend at all in any way, as the gradient of the line stayed the same throughout.
After looking at the graph and the results I conclude that my prediction was correct. The resistance of the wire did change in proportion to the length of the wire. This is because when the length of the wire increased the electrons that made up the current had to travel through more of the fixed particles in the wire, which caused more collisions and therefore a higher resistance. This is again shown in my free-electron model.
Free-Electron Model
Evaluating
Anomalous Result
The one point that was not that close to the line was an anomaly. The reason for this could have been due to a number of different variables:
1. The heat of the wire.
2. The heat of the surrounding area.
3. The equipment used wasn't accurate enough. e.g. the ammeter could have been more accurate and digital or different ways to connect the resistance wire to the circuit because the crocodile clips slipped and came off easily.
4. Kinks in the wire.
All the reasons stated above are acceptable but 4 is the most likely one because all the other results in the other wires were close or on the line of best fit, which would mean that the problem would have to have been within that certain section of the investigation.
Accuracy
I think that my evidence is accurate because nearly all of the points on my graph tough their respective lines of best fit. Heat, apparatus or kinks could have affected the points that strayed off the line, but they were only minor deviances, not enough to be considered anomalies.
Reliability
Most errors in my experiment were encountered in the measuring of the wire. This is because it was hard to keep the piece of wire straight and measure it with a 1m ruler and then fixing crocodile clips to the designated part on the wire. Also I do not feel that the crocodile clips were always fixed securely to the wire with a good connection.
My Method and Improvements
I think that my method was completely suitable for this investigation. The only improvements I can think of is to give us some more time to allow us to do some repeats to ensure reliable results and prevent anomalies and also to give us a device that we can use to hold the wire taught to ensure accuracy.
