Alternating Current

Direct Current (DC) : An electric current whose magnitude is either constant or variable but the direction of flow in a conductor remains the same is called direct current. It is denoted by DC. Sources of DC are voltaic cell, a dry cell, battery, DC generator, etc.

Alternating Current (AC): An electric current whose magnitude changes with time -and direction reverses periodically is called alternating current. It is denoted by AC.

Sources of AC are hydro-electric generators, thermal power generators and nuclear power generators, etc.

Suppose you connect a bulb to an AC source such as the AC supply of your house (Figure 5.23a),

The potential difference V_A - V_B across the bulb keeps changing with time. When the current flows from A to B in the bulb, V_A > V_B . When the current flows from B to A in the bulb, V_A < V_B. When the current becomes momentarily zero before changing direction, V_A = V_B.

In fact, the potential difference across the bulb changes gradually from a maximum value V_o, to zero and then to the minimum value -V_o From -V_oit increases to zero before reaching V_o. once again. The way the potential difference (voltage) across the bulb changes with time is shown in figure.

A plot of the alternating current through the bulb will also be similar. The number of cycles completed by the AC in one second is called the frequency of AC. Starting from a particular value (say, maximum or minimum), when the current or voltage reaches the same value next time, it is said to complete one cycle. If it completes 1 cycle in one second, we say that its frequency is 1 hertz (Hz). The frequency of our household AC supply is 50 Hz, i.e., there are 50 cycles per second, which means that AC changes its polarity after every 100th part of a second as it changes its polarity twice in one cycle. or we can say it changes direction 100 times in 1 second.

The major difference between AC and DC is that DC always flows in one direction, while AC reverses its direction periodically. The advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Using AC the electric power can be transmitted over________________distances .
A. Long	B. Short	C. Both (a) and (b)	D. None of these
Right Option : A
View Explanation

Question : 2

When AC current flow it becomes momentarily _______________ before changing direction
A. Constant		B. Zero
C. Changing		D. None of these
Right Option : B
View Explanation