Ohmic and Non-Ohmic ConductorsJohn McCarthy

Physics Practical Investigation:
Ohmic and Non-Ohmic Conductors
John McCarthy

Introduction:
Resistors are substances that prohibit the flow of current. One property that all resistors share is that they act as conductors under certain conditions. The inverse of this is also true, hence the terms conductor and resistor are partially interchangeable, and the resistance through a conductor can be measured in the same way as a resistor.

There are two types of conductors, Ohmic and Non-Ohmic. Ohmic conductors are conductors that adhere strictly to Ohm’s law of , where is the current, the potential difference and the resistance, at least over a certain temperature range, and hence have a direct relationship between the current flowing through the conductor or resistor and the corresponding resistance. Non-Ohmic conductorsare conductors which do not obey Ohm’s law over varying temperatures, and thus have varying resistances.

According to Ohm’s Law, , in an Ohmic conductor plotted on a graph in the form of , the resistance would be given by the slope of the graph, whilst in a Non-Ohmic conductor, the resistance at any point would be given by . This is demonstrated in the graphs above, where the relationship between voltage and current is linear in the Ohmic graph, but non-linear in the Non-Ohmic graph, instead increasing as temperature increases due to the higher voltage.

As the resistivity of an Ohmic conductor is not dependent on temperature, its resistivity can be represented as a function of the substance the conductor is made of, in the form , where is the intrinsic resistivity of the substance the conductor is made from, is the length of the conductive wire and is the area of the cross section of said wire.

In this practical investigation, these concepts will be used to determine the nature, be it Ohmic or Non-Ohmic, of two different conductors, and hence determine other factors about their resistivity.

Aim:
The aim of this practical investigation was to investigate the resistive properties of two conductors. These conductors were a 12V Globe and a Nichrome Wire.

Independent Variable:
The independent variable is the variable that is changed throughout the investigation. In this investigation there were two independent variables:

  • The conductor attached to the circuit was changed during the practical to understand the nature of Ohmic and Non-Ohmic conductors
  • The current flowing through the circuit was changed throughout the investigation for each conductor.

Dependent Variable:
The dependent variable is the variable that is measured throughout the investigation. The dependent variable in this investigation was the voltage across the conductor.

Controlled Variable:
The controlled variables are variables that remained constant throughout the practical investigation. It is of note that the independent variable, the conductor, also acts as a controlled variable throughout this practical investigation as the conductor used must remain constant as the voltage across it is being measured. Other controlledvariables include:

  • The temperature of the room
  • The temperature of the conductor
  • The ammeters and voltmeters used
  • The rheostat used
  • The wiring used to connect the circuit
  • The power-pack used

Uncontrolled Variables:
Uncontrolled variables were variables that could have an effect on the results of the practical investigation that couldn’t or weren’t controlled. In this practical investigation there were no uncontrolled variables of significance great enough to effect the results.

Method:

  1. Connect the non-variable (DC) terminals of the power supply to the DC terminals on the rheostat.
  2. Record the power rating of the globe, the diameter of the Nichrome wire and the length of the Nichrome wire.
  3. In parallel with the rheostat, connect an ammeter, and in series to the ammeter, connect, in parallel, the globe, acting as the conductor, and a voltmeter.
  4. Connect this part of the circuit through the rheostat back into the circuit, as per the diagram:
  1. Powering on the power supply, adjust the Rheostat until the current flowing through the circuit according to the Ammeter is .
  2. Record the voltage drop across the conductor at , and, adjusting the Rheostat and voltage output of the power supply to increase the current through the conductor in steps of , record each subsequent voltage up to a current of .
  3. Remove the globe from the circuit and instead connect a long Nichrome wire.
  4. Repeat steps 4-6 for the Nichrome wire, recording your results in a suitable table. Take care to allow time for the Nichrome wire to cool after each recording, as the high voltage and current causes significant heating which effects the resistivity of the wire.

Page 1