resistance and temperature effect

A sample of conductor is said to obey Ohms law if its resistance R remain constant that is , the graph of its V verses I is exactly a straight line.

Conductor which strictly obeys the ohms law is called ohmic.Howe ever,there are devices,which do not obeys the ohms law they are non-ohmic devices.

Lets us apply certain potential difference across terminals of a filament lamp and measure the resulting current passing through it.if we repeat the measurements for different values of potential difference and draw a graph of voltage V verses current I it will be seen that a graph is not straight line. it means that graph is non ohmic device.The deviation of I-V graph from straight line is due to increase in the resistance of the filament with temperature.A current is passing through filament is increase from zero,the graph is straight line in initial stage because due to increase in resistance of the filament with the temperature due to small current is not appreciable. As the current is further increased,the resistance of the filament continues to increase due to rise in temperature.

Practical Units

The three forms of ohms law can be used to practical units current,potential difference,and resistance as follows:
1 ampere = 1 volt/1 ohm

1volt = 1ampere x1 ohms

1 ohm = 1volt /1ampere
One ampere is the amount of current through a one-ohm resistance that has one volt of potential difference applied acrossit.
One volt is the potential difference across a one-ohm resistance that has one ampere of current through it.
One ohm is the amount of opposition in a resistance that has a V/I ratio of 1, allowing one ampere of current with one volt applied.

A triangle diagram to help in memorizing the ohms law formula V=IR, and R=V/I. The V is always at the top.

The Current

Definition,

ohms law shows that the current I is directly proportional to voltage and inversely proportional to resistance.

Ohm's law show the relationship between Current,voltage and resistance.

I=V/R

Explanations:

One ohm is the unit of resistance.When this the amount of current through the resistance R connected across the source the potential difference V.With volts unit for V and ohms for R,The amount of current Is in amperes. Therefore,

Amperes = volts/ohms

I = V/R

This formula says to simply divide the voltage across R the ohms of resistance between two points of potential difference to calculate the amperes of current through R.

If we keep the same resistance in a circuit but vary the voltage,the current will vary.

High voltage and low current

It is important to realize that with high voltage, the current can have a low value when there is very high resistance in the circuit. For example, 1000V applied across 1,000,000Ω result in the circuit of only 1/1000 A By ohms law,

I = V/R

=1000V/1,000,000Ω

= 1/1000

I = 0.001A

Low voltage but high current

At the opposite extreme, a low value of voltage in very low resistance circuit can produce a very high current. A 6-v battery connected across Resistance of 0.01Ω produce 600A of current:

I=V/R

= 6V/0.01Ω

I = 600A