George Simon fixed the measurement of resistance, and he published the detail of experiment in 1826. And History of Georg Simon Ohm according to Wikipedia. Easiest way he said in his experiment about relation between the current which passing through any wire at the end with potential difference. It is known as called the Ohm’s law, which says when pass the current through any conductor there is also under constant values of temperature is proportional to potential difference of across the any conductor.

**Ampere:**

If volt is 1. And if we put 1Ω resistance against that in parallel. So if any current will pass with that area called 1 ampere. Ampere is a unit of current. We know that current actually name to the revolve of electron.

So in the case of above that’s current is 1 ampere that’s way in this period electron will pass negative points to positive points in (6300000000000000000/Sec), So this value is not a easy for calculate. It’s very hard. So that’s way fix the basic unit of current is ampere. Now for example we increase the value 2 ohm of resistance in above diagram. So we have increased the value of resistance. Just as it voltage will drop according to value of resistance. More increased the value as it voltage will drop. If we increased the voltage in 2 volt so current will pass 2 ampere. Again for example if we increased the voltage more like 4 volt then 4 ampere will pass. So now proved if we increase the voltage then current will increase directly.

- So now if we see as the mathematically method as:

V (Volt) = I × R (Volt)

- When do we transpose this formula, as we have above formula?

I = V ⁄ R (Amp)

- And finally we get:

R = V ⁄ I (Ohm Ω)

It’s is very easy formula for understand about the relation between, current, voltage and resistance, Now I am showing in more understandable and very easy diagram. For specially student they can easily remember formula after one picture seeing. And they can easily find out in mind the calculation formula of ohm’s law.

So after seeing above formula you can identify how is it very easy to find out, there are three boxes in the triangle, Top Volt, below right Resistance and below left Current. Let suppose if you want to find Volt then hide above Box and see .

- Just as it you can find more two values “current” and “resistance” just hides the finding value as your requirement.
- For find current, how can you find “I” from ohm’s law equation? Now see image below.

- Now again more image how to find Resistance formula easy to remember.

- Above all derive and equation define easy way with sample of images. If you want to get pdf file here is ohm’s law pdf download from here:

## Resistivity relation with ohm’s law:

Now resistance or opposition to current flow varies for different materials, each having a particular constant value. If we know the resistance of 1 metre of a material, then if we find resistance of 5 metres then it will be five times the resistance of 1 metre. The resistivity (symbol *–* the Greek letter ‘rho’) of a material is defined as the resistance of a sample of unit length and unit cross-section. Using the constants for a particular material we can calculate the resistance of any length and thickness of that material from the equation.

Where:

* **Р =* the resistivity constant for the material (m).

*l * = the length of the material (m).

a = the cross-sectional area of the material (m2).

According to chart the resistivity of silver as which means we can say that a sample of silver 1 metre long and 1 metre in cross-section area will have a resistance of 16.4 × 10^{-9} Ωm.

- According to all above resistivity equation below are example 1 Numerical:

- Example 2 Resistivity Numerical:

The above all examples finally show that’s the resistance of aluminium cable is almost greater 60% then copper conductor lengths are same and cross-section. Therefore, if aluminium cable need to replace a copper cable, then must be we need to conductor size increased according to table and rule and regulation. And now we know that’s other factor which affects the resistance in any material with temperature. So now I am consider to discuss about temperature coefficient.

**According to ohm’s law Temperature Coefficient:**

Resistance of many materials change with temperature. Normal Conductor increase their resistance when temperature increase and just as it insulator decrease with a temperature increase. In term of electrical it’s bad effect of increasing temperature in materials. Each material has different resistance according to their quality, and it’s also respond different way in temperature. In the electrical laws scientist have constant value of material for calculate which are called temperature coefficient of resistance.

By using the constants value for a particular material and substituting these values into the following formula to the resistance of a material and different temperatures may be calculated. For the temperature increase from 0°C:

So where :

Rt = The resistance of the new temperature t°C.

R0 = The resistance at on 0°C

* ∝ = *The temperature coefficient of the particular material.

- For a temperature increase among two intermediate temperatures above 0°C:

Rt = The resistance on the authentic temperature.

R2 = The resistance at the final temperature.

∝ = The temperature coefficient for the Specific material.

If we take a 1 resistor of, say, copper, and lift its temperature by using 1°C, the resistance will increase via 0.004 to at least 1.004. This growth of 0.004 is the temperature coefficient of the material.

- Now let see Example 1 of temperature Coefficient Numerical:

- Now let see Example 2 of temperature Coefficient Numerical:

It is clear from the last two sections that the resistance of a cable is affected by length, thickness, temperature and type of material. Since Ohm’s law tells us that current is inversely proportional to resistance, these factors must also influence the current carrying capacity of a cable.