The aim of my investigation is to find whether the length of a wire affects its resistance. The battery provides the energy that is transferred to the wire; it also provides the force, which drives the charges round the circuit. The size of the current depends on the size of the battery’s voltage, but the circuit itself also affects the size of the current. The ammeter records the current passing round the circuit. The more voltage that is supplied, the bigger the current that flows in the circuit. Voltage is not the only factor controlling the size of the current in a circuit.
The current also depends on the wires and components through which it passes. In circuits containing metallic conductors, the only particles that are free to move are electrons. Metals contain positive ions, neutral atoms that have lost one or two of their outermost electrons. These electrons are free to move about in the metal and are affected by electric forces. Any movement of charge is an electric current, but the flow rate depends on more than just the speed of movement. Each electron in a metal carries a tiny amount of charge, 1. 6 X 10-19 coulomb (C).
The flow of charge in a metal is due to vast numbers of charge-carrying electrons flowing at low speeds, typically a few millimetres per second. The size of an electric current is a measure of the charge flow past any point each second. In the 1820s, George Ohm, a German schoolmaster, set out to find a relationship between the current through a wire and the voltage. Ohm established a law that said: Voltage = constant resistance for a wire, where the temperature does not change. Current Factors that affect the resistance of a wire are the type of wire, thickness, length and temperature.
Thickness: Imagine charge moving from a connecting wire through the lamp filament… The filament has fewer conducting electrons than the same length of connecting wire and so the charge has to move faster to maintain the same rate of flow, or current. This leads to the filament being heated as the electrons transfer their energy in collisions with the metal particles. The greater the number of collisions, the more energy is transferred as an electron passes through, and the greater the resistance to charge flow. Temperature:
Resistance of a metal to electric current is due to the conducting electrons colliding with the particles of the metal. The more frequent the collisions, the greater the resistance. Increasing the temperature of a metal causes an increase in the energy and amplitude of the vibrations of the metal particles. This results in more frequent collisions between the electrons and the metal particles, hence the increase in resistance. The factor that I am going to investigate will be the length of wire, and how this affects resistance. Using my background knowledge, I have come up with a hypothesis for my investigation.
Hypothesis: the longer the wire, the higher the resistance of the wire. This is due to the idea of the free moving electrons being resisted by the atoms in the wire. In a longer piece of wire, there would be more atoms for the electrons to collide with and so the resistance would be greater. The relationship between the wire length and the resistance should be directly proportional. For example, in a wire twice the length of another wire, there would be double the amount of atoms causing the resistance. – In a 20cm wire, there should be twice the resistance of that in a 10cm wire.