You are given following current-time graphs from two different sources:

(i)    Name the type of current in two cases.
(ii)   Identify any one source for each type of these currents.
(iii)   What is the frequency of current in case II in India?
(iv)   Use above graphs to write two difference between the current in two cases.

(i) Graph I represents a direct current.
    Graph II represents an alternating current. 

(ii) A dry cell is a source of direct current.
     An a.c. generator is a source of alternating current. 

(iii) Time period of a.c., T = 0.02 s
      Frequency of a.c., $\mathrm{f} = \frac{1}{\mathrm{T}} = \frac{1}{0.02 \mathrm{s}} = 50 \mathrm{Hz}.$

(iv) Direct current:
      1. Direction of current does not vary with time and remains constant. 
      2. The frequency for direct current is zero.
     Alternating current:
      1. Both the magnitude and direction of current varies within equal intervals of time. 
      2. The frequency of ac is 50 Hz or 60 Hz depending upon the country.