There for it can also be written as:
Potential difference = resistance
Current
In other words the resistance of a conductor is the ratio of the potential difference (pd) across it the current flowing through it.
The units for the p.d are volts and current as amperes. The definition above helps to find the unit of electric resistance. This is ohms law and is defined as:
The ohm (Ω) is the resistance of a conductor in which the current is
1 ampere when a potential difference is 1 volt is applied across it.
It follows that: Volts = Ohms
Amperes
Or in symbols: V=R
I
Ohms work was only carried out using metal conductors and so the law only applies to certain materials. It was to be obeyed that all physical conditions must remain constant. For example, the resistance of some conductors will alter if they are bent, placed under tension or if they are put under right angles to a strong magnetic field.
Ohms law does not apply to semi-conducts or recitations, or to the conduction of electricity through gases.
Resistance
Electrons move more easily through some conductors than others when a p.d is applied. The opposition of a conductor is called its resistance.
A good conductor has a low resistance and a poor conductor has a high resistance. The resistance of a wire of a certain material
(i) Increases as the length increases
(ii) Increases as the cross-section area decreases.
(iii) Depends on the material.
A wire with a resistance is called a resistor and its circuit symbol is
When the value of voltage/current is the largest for the conductors with the largest resistance (when the current is the smallest). For this reason voltage/current is called the resistance the devise.
The formula:
Resistance= potential difference
Current
Or R= V = Voltmeter reading
I = Ammeter reading
Variables
The variables that will affect my investigation will be the length, width, temperature, voltage, resistance, material.
What I will be investigating is the length and the thickness of a wire. The current will hold as a constant because it can increase or decrease the resistance of a wire. For example if the current was decreased then according to the formula R=V/I the resistance will increase.
I will also change the different lengths of the ruler to measure the amps; this will also have an affect on the resistance.
Preliminary
In my preliminary test I will be using a variation of wires, copper a thick wire. I will be finding the resistance of each of these wires in different lengths and voltages.
To make certain that my preliminary work is a fair test I will be careful when picking out my variables checking the thickness is the same. I will be using different lengths of wire and will be using a meter ruler to measure the length of the wire and to make sure that the test is fair. I predict that the shorter the length of wire the greater the resistance.
Preliminary Results
Copper wire
Thick wire
My results are not accurate therefore I will not be able to find out the resistance so I will not be able to find out the resistance so I will not be able to draw a graph from these results.
I could only do the experiment till 2v as it kept on going off scale and so I could not carry on my experiment to 4v and 6v for either copper or thick wire.
The thick wire kept on going off scale so I was not able to get any resistance.
To find better and more accurate results with better readings for resistance I will carry out my investigation using Nichrome wire with the same method. I believe that the Nichrome wire will give me better results.
Apparatus
The following apparatus will be what I will be using for my investigation:
Voltmeter
Ammeter
Ruler
Crocodile clips (to connect from the ammeter and voltmeter to the wire.)
Masking tape (to hold the wire to the ruler)
Power pack
Nichrome wire
Thick wire
Copper wire
This is how my apparatus will be set up.
Fair test.
To make it a fair test I will repeat my investigation again. Just to make sure I have accurate results. I will also make sure that I will use the same lengths of space between the wires for three different voltages. I will also repeat the experiment again to make it a fair test.
Safe test
To make the experiment safe I will keep the wire away from the power pack as it will burn the wire if it kept touching. I will also make sure that I am not working on a damp surface and that I will not touch the wire when it has a current through in case I get a short circuit
Method
I will first set up my equipment. Using the ruler I place the wire on the ruler and tape it at each end. I will use one of the crocodile clips to clip at one end of the wire to give voltage and the other to measure the current, by placing it at 10cm on the wire each time for 2 volts. I will do this for 4 volts and 6 volts. I will record my results and draw them on a graph. I will work out the average from this.
Results
2 volts
4 volts
6 volts
Conclusion
My results backed up my prediction that the longer the wire the higher the resistance will be. My graph shows a straight line which means that the resistance is proportional to the length meaning if the length of the wire is 40cm and the resistance is 40 Ohms then if the length is doubled to 80cm the resistance will be 80 Ohms. This links with the scientific theory that if you double the length you double the number of atoms in it to double causing the electrons to ‘jump’ causes the resistance.
Evaluation.
For this investigation I have gained reliable results as seen on my graph and tables. I also know that my results are accurate and reliable because there are no an anomalous results also because of the precautions I took to make it a fair test.
The crocodile clips and the connecting leads could have affected the fairness of the experiment. They are a different type of metal from the Nichrome wire and may have different properties and therefore different resistance. Therefore the resistance of the Nichrome wire showed up on the multimeter was slightly more than it actually was.
For further investigation I could investigate the other factors/variables that would make a difference in the resistance of a wire. From my own scientific knowledge I believe that the temperature, voltage, resistance, material would change the resistance of a wire.