Current: A flow of electricity through a conductor. Current consists of moving electrically charged particles. It is the rate at which electrons flow. The faster they move, the higher the current. The equation for current is:
Ohm’s Law gives mathematical equations, which show the relationship between potential difference, current and resistance in an electrical circuit. The equations are:
V = I * R
R = V / I
I = V / R
Where V = Voltage
I = Current
R = Resistance
Ohm’s law states that the current through a metallic conductor is directly proportional to the potential difference across it, providing the temperature and other conditions are constant.
A high voltage produces a low current only if the resistance is high. A voltage across the ends of a conductor will produce a current through it, although you will not always get the same current for a particular voltage. Some conductors will allow a large current to flow, whereas other will only allow little. The amount of current flowing depends on resistance. A conductor with a high resistance resists the current and only allows a small current to flow.
Obstacle course of resistance
The flow of charge (electrons) through a conductor is similar to an obstacle course. As the electrons move from one part of a conductor to the other, they have to make their way past layers of cations which act as obstacles to the flow of electrons. Factors which increase the obstacles or make it difficult for the electrons to flow increase the resistance therefore decreasing the current.
Factors affecting resistance
There are four factors that affect resistance, they are:
- The length of the conductor
- The cross sectional area of the conductor
- The resistivity (type or nature) of the conductor
- The temperature of the conductor
The length of the conductor
Ohm’s law states that as the length of the conductor increases, the resistance increases too. Therefore, the length is proportional to the resistance. This is because as the length increases, the number of obstacles an electron may encounter increase and the rate of the flow of charge (current) decreases.
For any specific conductor at constant room temperature and thickness, the resistance is assumed to be the same for any equal length sections.
Each section will have the same resistance.
Total resistance = Sum of individual resistances
RT = R1 + R2 + R3 + R4 + R5 + R6 + R7 + R8 + R9 + R10
Book & Internet References Theory and diagrams taken from the following books and internet sites
Exercise book worksheets
BBC GCSE Bitesize revision
www.freestudentstuff.co.uk
Google definitions
www.courseworkbank.co.uk
Prediction
I predict that as the length of the wire increases, the resistance will increase too. This is due to the fact that the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions. Also as the resistance will increase, the current will decrease. Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions. Due to the fact that resistance is proportional to length, the graph should be a straight line.
Fair Testing
Definition
‘Fairness’ implies that the outcome of the activity truly depends on what is being investigated, and is not being distorted by other external factors. Therefore a fair test is one where all the variables are kept constant or the same, except the variable that is being investigated. A variable is anything that can change and which may influence the outcome of the investigation.
List of variables
- The length of the conductor
- The cross sectional area of the conductor
- The resistivity (type or nature) of the conductor
- The temperature of the conductor
Statement of fairness
We will try our utmost best on keeping the temperature constant, this means switching off any appliances which may cause heat or affect the temperature. However, it is impossible to keep the temperature constant throughout the whole experiment in a classroom science laboratory. All other factors which may affect resistance will too be kept constant as far as possible. This excludes the length of the conductor as this will be the variable being tested.
Experimental Procedure
Details of experiment:
A nickel wire will be used to test the resistance in the experiment. The wire gauge will be standard (30) and it will have a diameter of 0.315mm. Results will be taken in two decimal places so it will be to the nearest hundredth of an amp, volt or ohm.
Apparatus:
- Power supply
- Variable resistor
- Ammeter
- Voltmeter
- Metre ruler with a 100 cm nickel wire mounted onto it using sticky-tape
- Six connecting wires
Diagram:
Method:
1. Set up the apparatus as shown in the diagram above.
2. Place one of the free wires on 0 cm on the nickel wire mounted onto the ruler. (Press the crocodile clips [used to connect the wires] firmly so as to get a good connection.)
3. Place the other free wire at the length 10 cm on the nickel wire mounted onto the ruler. (Press the crocodile clips [used to connect the wires] firmly so as to get a good connection.)
4. Record the readings on the voltmeter and ammeter, if the value keeps changing then choose the value which appears for the longest period of time.
5. Repeat steps 1 to 4 but in step 3, increase the length of the wire by 10 cm each time until 100 cm ( 10 cm, 20 cm, 30 cm….100 cm).
6. Repeat the whole experiment until you have three sets of values for the current and potential difference for each length.
7. Calculate the resistance and average resistance.
Safety Precautions
Although there are no real safety implications with this investigation, the voltage will be kept low so that the wire will not get too hot and burn the fingers of the person carrying out the investigation.
Results
Compared to other results
In order to prove that my results are reasonably accurate, another results table is shown derived from another person who undertook the same investigation in the same environment.
The average resistance for the above results is 8.19. This is similar to my average resistance which is 8.37. This proves that the results I have obtained from my investigation are not abnormal.
Results (trends or patterns of observations)
There are some trend from my results, these are:
- As the length of the wire increase, the resistance increases
- It can be seen from the graph and table that if the length doubles, the resistance doubles too.
- All the readings are accurate to two decimal points; therefore it is accurate to one hundredth of an amp, volt or ohm.
Conclusion
Having performed the investigation, I drew the following:
As predicted, an increase in length resulted in an increased resistance. This can be clearly said for both wires tested. The wire shows a strong trend of a straight line, i.e. the length of the wire is shown to be directly proportional to the resistance – double the length and the resistance doubles. There is a slight anomaly and none of the results are exact when compared with a resistance of 0.153 ohms per centimetre. The reasons why this may have been are explained in the evaluation. 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 conductor, 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.
Calculation of the gradient
The gradient of the slope of the graph is a constant number that relates the resistance to the length. The gradient is:
Gradient = 100/15.4
Gradient = 6.493…
Gradient = 6.49 (to two decimal places)
Calculation of the percentage errors
Evaluation
I believe that the investigation was moderately accurate. My evidence to support this is my average percentage error which is 1.24%. This error may have occurred due to a number of reasons. Firstly, it is impossible to keep the temperature constant throughout the whole experiment in a classroom laboratory. So due to heating of different appliances, the temperature may have altered to cause the percentage error. Furthermore, to connect the wires we used crocodile clips. We may have made a human error whilst connecting the crocodile clips to the wire as we cannot be precise in measurements using only the naked eye.
In order to improve my investigation in the future I will use clean crocodile clips and try to be more precise when connecting them to the wire so my measurements are more accurate. Also I will try to use more means in order to keep the temperature constant. To make my investigation even more accurate I will repeat it many times and compare it with other results from people who did the same investigation. Furthermore, I would use a conductor that is stiff so that is does not bend and alter the length when measured on the ruler.