An Investigation Into Electrical Resistance

Background Knowledge

Resistance, in electricity, is a property of any object or substance to resist the flow of an electrical current. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit. The resistance of an object is determined by the nature of the substance of which it is composed, its resistivity, accounting for its dimensions and its temperature. Resistivity is expressed in terms of Ohms / cm3 at 20ºC. An electrical conductor is any material that offers little resistance to the flow of an electric current. The difference between a conductor and an insulator, which is a poor conductor of electricity, is one of degree rather than kind, because all substances conduct electricity to some extent. A good conductor of electricity, such as silver or copper, may have a conductivity billions of times as great, or more, as the conductivity of a good insulator, such as glass or mica.

Method

We were set the task of investigating the factors that come into play when determining the resistance of a piece of wire. We would be provided with the necessary apparatus needed to carry out the investigations. The basic setup would involve a circuit with a set of cells connected in series with an ammeter and the piece of wire being investigated, and a voltmeter connected in parallel with the wire. The ammeter is placed in series with the wire. An ammeter has a low resistance, so that it introduces as little extra resistance as possible into the circuit. The voltmeter is connected in parallel with the wire. Voltmeters have a high resistance, so the current they take is usually negligible.

We decided on the variables that could affect the resistance of a wire:

 Length of the wire

 Thickness of the wire

 The material used as the wire

 Temperature

We were to investigate as many of these factors as possible given a limited period of time. We would carry out a set of experiments where in each one we changed one variable while keeping the others constant. In each case we would have to measure the current passing through the circuit and the potential difference across the wire; given these the resistance could be calculated using the formula R=v/i, where R is the resistance in Ohms, v is the potential difference in Volts, and i is the current in Amperes.