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Prediction: I believe that I will discover that the resistance of a wire increases proportionally with the length. I think that this is due to the way resistance occurs in a typical wire. Resistance is the result of negatively charged electrons (the actual current) colliding with the positively charged ions that make up the wire. The collisions cause the energy in the electron to be lost, and when they occur on a larger scale there is a noticeable difference between the start and end voltage of a circuit. As the length of the wire increases, so must the number of particles.
As the number of particles increases, so will the number of collisions, and therefore the amount of resistance encountered. George Ohm discovered that the voltage of a circuit is directly proportional to the current flowing through the circuit, meaning that if you triple one, you triple the other. He then came up with a rule for working out the resistance of a circuit (rearranged from his original equation): Resistance = Current / Voltage This is the formula I will use to calculate the resistance of the wire. A wire, showing collisions occurring If you double the length of the wire, I believe you will double the resistance.
Background: The flow of charge in a wire is called the current. It is expressed in terms of the number of “coulombs” per second going past a given point on a wire. One coulomb/sec equals 1 ampere (symbol A), a unit of electric current named after a French physicist. I have explained how resistance occurs above, and volts are an expression of the amount of energy being carried by the electrons flowing through a circuit. The electrons are negatively charged, and move round the circuit (from the negative side to the positive side of the battery) as a result of attraction to the protons.
Plan: I will carry out the experiment by first affixing the 100cm length of the chosen wire to a ruler, using sellotape. I will then set the power supply to approximately the right voltage, and then use the variable resistor to set the exact current (measured on the voltmeter). I have to use this method because the PSU itself is not accurate enough for our purposes. Once this preparation is complete, I will attach the first crocodile clip to one end of the ruler, and the second clip at the first distance, completing the circuit drawn below.
The power supply will then be turned on long enough for the measurement to be taken, to prevent the wire heating up. Circuit diagram: Variables and Constants: o The length of the wire, obviously, will change to give a series of readings. o I will also vary the voltage used, to see if the same relationship exists at differing voltages. o The equipment used (see below for list) will all be kept the same, to ensure a fair experiment. o The temperature of the wire I will attempt to keep the same by turning off equipment when not in use. The wire will heat up (as a result of resistance), and so I will give it a chance to cool.
Apparatus used: o Meter rule o 1M length of chosen wire, attached with sellotape to above rule. o Crocodile clips o Voltmeter and Ammeter o Standard power supply o Variable resistor o Connecting cables Safety Considerations: o The wire will heat up during the experiment, so care must be taken to ensure it does not burn. The current will only be left on for the minimum amount of time, to prevent dangerous heating. o Some wires available are very thin but strong, and can cut easily into flesh. o Trailing cables are a hazard, and I must make sure no cables drape onto the floor or similar.