Ohmic and Non-Ohmic Conductors

Ohmic and Non-Ohmic Conductors

Conductors which obey Ohm's Law have a constant resistance when the voltage is varied across them or the current through them is increased. These conductors are called ohmic conductors. A graph of the current vs. the voltage across these conductors will be a straight-line. Some examples of ohmic conductors are circuit resistors and nichrome wire.

As you have seen, there is a mention of constant temperature when we talk about Ohm's Law. This is because the resistance of some conductors changes as their temperature changes. These types of conductors are called non-ohmic conductors, because they do not obey Ohm's Law. A light bulb is a common example of a non-ohmic conductor. Other examples of non-ohmic conductors are diodes and transistors.

In a light bulb, the resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we increase the supply voltage in a real lamp circuit, the resulting increase in current causes the filament to increase in temperature, which increases its resistance. This effectively limits the increase in current. In this case, voltage and current do not obey Ohm's Law.

The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, which increase a lot in temperature (up to about \(\text{1 000}\) \(\text{℃}\), and starting from room temperature) the change is quite large.

In general, for non-ohmic conductors, a graph of voltage against current will not be a straight-line, indicating that the resistance is not constant over all values of voltage and current.

Optional Experiment: Ohmic and Non-Ohmic Conductors

Aim

To determine whether two circuit elements (a resistor and a lightbulb) obey Ohm's Law

Apparatus

4 cells, a resistor, a lightbulb, connecting wires, a voltmeter, an ammeter

Method

The two circuits shown in the diagrams above are the same, except in the first there is a resistor and in the second there is a lightbulb. Set up both the circuits above, starting with 1 cell. For each circuit:

1. Measure the voltage across the circuit element (either the resistor or lightbulb) using the voltmeter.

2. Measure the current in the circuit using the ammeter.

3. Add another cell and repeat your measurements until you have 4 cells in your circuit.

Results

Draw two tables which look like the following in your book. You should have one table for the first circuit measurements with the resistor and another table for the second circuit measurements with the lightbulb.

Number of cells	Voltage, V (\(\text{V}\))	Current, I (\(\text{A}\))
\(\text{1}\)
\(\text{2}\)
\(\text{3}\)
\(\text{4}\)

Analysis

Using the data in your tables, draw two graphs of \(I\) (\(y\)-axis) vs. \(V\) (\(x\)-axis), one for the resistor and one for the lightbulb.

Questions and Discussion

Examine your graphs closely and answer the following questions:

1. What should the graph of \(I\) vs. \(V\) look like for a conductor which obeys Ohm's Law?

2. Do either or both your graphs look like this?

3. What can you conclude about whether or not the resistor and/or the lightbulb obey Ohm's Law?

4. Is the lightbulb an ohmic or non-ohmic conductor?