OHM's LAW | Resistance and Unit of Resistance | Characteristics of Ohmic and Non Ohmic Conductors

OHM's LAW

 OHM's LAW: Take a monochrome wire of about 50 cm length and apply a potential difference of 1.5 V from a battery. Measure the current flowing through the wire using an ammeter connected to it in series. Also measure the potential difference across the wire using a voltmeter connected across it. Obtain a set of readings of I and V, by increasing the number of cells. Plot a graph between I and V. This will be a straight line as shown bellow.

If V is the potential difference across the two ends of any conductor, then current i will flow through it the value of current is changes with the changes in potential difference and is explained by ohm's law, started as:

The amount of current passing through a conductor is directly proportional to potential difference applied across its end, provide the temperature and the physical state of the conductor does not change.

                i.e.,         I ∝ V  or V ∝ I
             or      V = IR

Where R is the constant of proportionality, and is the resistance of the conductors. Its SI unit is ohm, denoted by a symbol Ω. if a graph is plotted between I and the potential difference V, a straight line will be obtain. 

Resistance:

 The property of a substance which offer opposition to the flow current through its called Resistance.

This opposition comes from the collision of moving electrons with atoms of substance. 

Unit of Resistance: ohm

  The Si unit of resistance R is ohm. If we put V =1 V, and I = 1 A, the value of R will be 1 Ω. Thus

When a potential difference of one volt is applied across the ends of a conductor and one ampere of current passes through it, then its resistance will be one ohm.

Characteristic of Ohmic and non Ohmic conductor 

 Ohm's law is valid only for certain materials.

Materials that obey ohm's law, and hence a have a constant resistance over a wide range of voltages, are said to be ohmic Materials having resistance that changes with voltages or current are non-ohmic.

Ohmic Conductor have a liner voltages-current relationship over a large range of applied voltages. The straight line shows the constant ratio between the voltages and current. Ohm's law is obeyed. For example, most metal shows Ohmic behavior.

Non ohmic materials have a non liner voltage-current relationship. For example, filament lamp, and thermistor. The resistance of filament rises as it get hotter, which is shown by the gradient getting steeper. A Thermistor ( a heat sensitive resistor) behave in the opposite way. its resistance decrease (current increase) as it gets hotter. This is because on heating, more free electrons become available or conduction of current.