An electric current which changes its direction i.e. (polarity) after a certain fixed interval of time is called ________ current.

Hint: Electric current is defined as the rate of flow of electric charge through a region. The charged particles carry electric charge. These charged particles are known as carriers. Alternating current is the form in which electric power is supplied in the form of electrical energy.

Complete step by step solution:
Step I:
Two types of currents are known
Alternating current or a.c. - The current which changes its direction after fixed intervals of time.
Direct current or d.c. – The electrical current which flows continuously in only one direction.

Step II:
Alternating current is used when the power that is produced at the power station is to be transmitted to distant users. Alternating current can be produced by using a device known as an alternator.

Step III:
The waveform of alternating current is a sinusoidal wave. If the voltage and current are varying or different, many waveforms of alternating current can be formed.

Step IV:
Since the alternating current has a sinusoidal waveform, it depends on three factors: Amplitude, frequency and the phase. The equation of sine wave can be written as:
\[V(t) = {V_p}\sin (2\pi ft + \phi )\]---(i)
Where \[V(t)\] is the voltage.
\[{V_p}\] is the amplitude, describing the maximum value of voltage that can be obtained.
\[\sin \] function means that voltage has a sinusoidal waveform.
\[2\pi \] is a constant
\[f\] is the frequency of the sine wave
\[\phi \] shows the phase of the sine wave.

Step V:
Therefore, a current which changes its direction after a certain fixed interval of time is called alternating current.

Note: The terms mentioned on the R.H.S. of equation (i) are helpful in determining how voltage is changing with time. It is to be remembered that the constant \[2\pi \] is used to convert the frequency from cycles in Hertz to the cycles in angular frequency.