The size of the electric current flowing through a conductor depends on the voltage across it, and the resistance of the conductor. When the voltage is increased, the current increases. For a particular metallic conductor, the current is directly proportional to the voltage, as long as the temperature stays the same.
I also found out from reading scientific textbooks that resistance is caused by the free electrons colliding with the ions in the metal lattice. Every time the electron collides with an ion, it loses energy in the form of heat and the result of this decreases the speed.
Metallic conductors
Metallic conductors have free electrons, which form a gas, which then can fill the entire volume of the metal. In conductor which is neautral, the negative charge of the free electrons are balanced by the positive charge of the ions. This makes up the lattice of the metal.
Collision theory
= Free electrons moving
= Atoms vibrating
Electrons move around the lattice in a random movement, with different speeds and different directions. They collide with each
Other and the atoms. As they hit into each other the movement is passed on and the vibration increases. More collisions mean greater resistance and more energy exchange.
Variables
Length: The longer the piece of wire the more collisions there is, and therefore the greater the resistance for a given current.
Cross-sectional area: The narrower the conductor the greater the number of collisions for a given current.
Temperature: the higher the temperature the more vibration in the lattice, the more collisions for a given current.
Material: the material from which the wire is made.
Resistively: resistively is related to the number of factors, which
affect resistance:
- Number of free electrons
- Crystallinity in lattice
By reducing crystallinity, it quickens up the flow of electrons and increases collisions, for a given current.
Preliminary work
To help me understand more I have decided to do some preliminary work. This will help me to research the experiment and explore faults in the experiment. This will make it more understanding for myself and for my own hypothesis. I will change my resistance through my variable resistor, which will then alternate the current travelling through the wire.
I will then record my results of my current and voltage.
Method
I will use the following apparatus:
- 5 1meter constantan 32swg wires.
- 1 voltmeter
- 1 variable resistor
- 1 ammeter
- 2 crocodile clips
- 1 d.c power supply
- 6 connecting wires
Results
Using one wire
Using five wires
Diagram of circuit
Step by step
- Set up my apparatus for my circuit as shown above
- Then the piece of constantan wire is fixed to 2 crocodile clips
- The power supply is then switched on
- The current is altered using a variable resistor.
- Results are then recorded from the ammeter and voltmeter
- This will be repeated until there is a good range of results.
- Results are then displayed in a graph.
Fair test
To make this experiment a fair test I must keep things the same. I will make it a fair test to ensure that my recorded results will be accurate and correct.
Independent variable
My independent variable is the cross-sectional area of the wire and the resistance of the wire. This is what I will measure.
Dependant variable
My dependant variable is the voltage and the current. This is what I will change.
Control variable
My control variable will be the length and the temperature. These are the factors I will keep the same.
Safety precautions
It is important to observe the safety precautions needed:
- Make sure apparatus is set up correct.
- Ensure that you use the proper equipment.
- Do not turn your D.C power supply to high.
- Do not let your circuit heat up.
- Keep your circuit away from water.
- Only switch power supply on when needed.
Hypothesis
I predict that more collisions will take place in a thin piece of wire. I think this because I know the narrower the lattice ions the more difficult it will be for the electrons to get through, therefore making more collisions. I predict prior to my previous knowledge that the thicker wire will have the least resistance, because the thicker the wire, the easier it is for the electrons to pass through the lattice of ions, resulting in less collision and less resistance. So the more collisions there are the higher the resistance so in result of this the thinner wire will have the most resistance. I predict that the longer the length of wire, the longer the electrons must travel. So the electrons will hit into the ions more and loose more energy because of this, which means there will be more resistance made.
Conclusion
My graphs show that there is a relationship between my voltage and current. I can see that my results form a straight-line graph, which shows that my results are correct. I can also say that my choice of equipment was correct as I obtained good results. I controlled the variables well throughout the experiment, particularly the temperature. I did this by switching off the D.C power supply after each result, to keep the temperature down. I have discovered that the smaller the CSA of the constantan wires, the lower the resistance. I have observed the relationship between voltage, current, and gradient. The higher the gradient the greater the resistance, which means there must be an increasing voltage and current. My graph shows that it obeys ohms law, and did not over heat. Overall it showed to be a success because I obtained a straight-line graph.
