Question: What do you understand by the term A.C.?

Alternating Current (A.C.):

An electric current whose magnitude changes with time and polarity reverses periodically is called alternating current (A.C.).

The instantaneous value of alternating current (a.c.) is expressed as:

$I = I_0 \sin \omega t \quad ...(1)$

where ($I_0$) is the maximum value or peak value or amplitude of a.c. and ( $\omega$) is the angular velocity, where:

$\omega = \frac{2\pi}{T} = 2\pi v$

Here, (T) is the time period and ($\nu$) is called the frequency of A.C.

The instantaneous value of voltage of an A.C. source is given by:

$V = V_0 \sin \omega t \quad ...(2)$

where ( $V_0$) is the maximum value or peak value or amplitude of voltage and ($\omega$) is driving angular velocity.

Note:

1. An alternating current and alternating voltage are preferably represented by small letters as ( $i = i_m \sin \omega t$) and ( $v = v_m \sin \omega t$) respectively, where ($ i_m$) and ( $v_m$) are the peak values of alternating current and alternating voltage respectively.  

2. In India, frequency for a.c. supply is 50 Hz.

What are the advantages and disadvantages of A.C.?

Advantages:

1. Generation, transmission and distribution of A.C. is more economical (less costly) than that of D.C.  

2. The alternating voltage can be stepped down or stepped up easily (using a transformer).  

3. A.C. travels on the surface of the conductor, so costly conducting material can be saved for long distance transmission by using inner core of cheaper material (e.g., steel). Example: ACSR (aluminium conductor steel reinforced) is widely used for electricity supply lines.  

4. A.C. can reach distant places without much loss of electric power (by using a transformer).  

5. Alternating voltage can be better controlled without any loss of electric power (say by using a choke coil).  

6. A.C. is easily convertible into D.C. (using rectifiers).  

7. A.C. machines are small sized, having longer life and easy to use.  

Disadvantages:

1. A.C. can be more dangerous than D.C. because maximum value of A.C. is ($\sqrt{2}$) times the effective value. Example: domestic supply of 220 V has a maximum value given by $(\sqrt{2} \times 220 = 311V)$.

2. A.C. cannot be used in electrolysis processes (such as electroplating, electrotyping and electro-refining etc.) where only D.C. is used.  

3. A.C. in a wire is not uniformly distributed throughout its cross-section. The A.C. density is much greater near the surface of the wire than inside the wire. This concentration of a.c. near the surface of the wire is called the skin effect. In fact, inner conducting material of a wire remains unused.  

4. Markings on the scales of A.C. meters are not uniform or equidistant for small measurements.