Question

Which of the following graphs indicates direct current?
  

Answer 1

Hint: We know that direct current is indicated as DC current. A direct current is a form of non-oscillatory current which flows in only one direction. It is kept in mind that with time the polarity of DC current does not change.

Complete step by step solution:
We will begin by looking at voltage-time and current-time graphs for both DC current and AC current.

Now let’s take a circuit diagram to understand the graph of the direct current,
 

When a sinusoidal wave or an AC input is given, it passes through the diode and AC input is transformed into an intermittent DC flow. The conversion of AC to DC can be efficiently observed in a bridge rectifier that uses four diodes to reverse both AC input directions into a clean DC output. However, in the above circuit at the output, we get a DC output.
In a DC current, the polarity remains the same. When we observe the above options, A, B, and C have changing polarities.
Hence, option D is the correct answer.
Additional information: AC current which stands for alternating current is an oscillatory current which changes its direction, i.e., changes the flow of electrons as shown in the graph above, back and forth at regular intervals of time.

Note: Current coming from the power stations will be AC current; however, through the use of rectifiers this current is converted to DC current, which becomes handy and makes the electrical appliances at home functional. However, AC current is beneficial as it results in minimum power loss during the transmission process.