Look into any Physics book and you will see the definition for electrical resistance as shown above. In many books, voltage is stated as potential difference. For ohmic conductors, the voltage and current are directly proportional to each other provided the temperature and physical dimensions of the conductor remain constant. A graph of V against I will be as shown Fig 1. The gradient of the graph is constant and is equal to the ratio V/I for any value of the current. Now the gradient of the line is (change in V)/(change in I). In this particular case, the ratio V/I for different values of current is always equal to the gradient . Does this mean that resistance can be defined as V/I and also (change in V)/(change in I) ???By the way, can anyone tell me whether it is possible to type formulae with ratios etc when posting a blog? Right now I have to insert it as an image. Quite troublesome.
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