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What is alternating current and direct current?

A direct current flows in one direction only in a circuit.
The magnitude Of a direct current may be
(a) constant
(b) changes with time
Figure shows the graph of current against time for some direct currents.
An alternating current flows to and fro in two opposite directions in a circuit.
Figure shows an alternating current with a constant magnitude of 3 A.
Figure shows an alternating current with a magnitude that changes with time.
(a) The amplitude of the current or the peak current is I_p = 5 A
(b) The time for one cycle is, T = 0.04 s
(c) The frequency of the alternating current is given by:
Figure gives more examples of alternating currents.
An alternating current can flow through both a resistor and a capacitor whereas a direct current can flow through a resistor only.

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Alternating Current and Direct Current Experiment

Aim: To compare the output generated by a direct current source and an alternating current source.
Materials: Two 2.5 V bulbs, bulb holders
Apparatus: Battery holder with two dry cells, alternating current power supply, plug key switch, cathode ray oscilloscope (C.R.O.), connecting wires
Method:
alternating current and direct current 6

The two circuits are set up as shown in Figure.
The settings of the cathode ray oscilloscope are adjusted to obtain a clear trace on the screen.
The traces formed by the output from the battery and the alternating current supply are observed and compared.
The trace displayed on the screen of the oscilloscopes are sketched.

Observation:Discussion:

The battery made up of two dry cells is a direct current source. The power supply connected to the mains supply delivers alternating current.
Both the bulbs are lighted up. Therefore direct current and alternating current can flow through the filament of a bulb and produce a heating effect.
The direct current flows in one direction only. The direction of the alternating current changes with time.
The magnitude of the direct current remains constant. The magnitude of the alternating current changes between zero and a maximum value.

Conclusion:
A direct current flows in one direction only but the direction of an alternating current changes with time.

What is the Principle of DC Motor

1. Principle : It works on the principle of motion of a current carrying conductor in a magnetic field, according to Fleming’s Left Hand Rule.
It is a device which converts electrical energy into mechanical energy of rotation.

2. Diagram :
Principle of DC Motor 1

3. Construction : It has four mains parts, named below.
A field magnet having concave magnetic poles N–S, which provides a strong magnetic field.
An armature, which is moving part of the motor. It has two parts :
(i) Laminated shaft X–Y.
(ii) Copper coil ABCD wrapped on end X of the shaft inside the field.
A pair of metallic split rings R₁ and R₂ (commutator).
A pair of metallic carbon brushes B₁ and B₂.

4. Working : A direct current (D.C.) source is connected between metallic brushes B1 and B2. When current passes through the coil, it flows in arms CB and AD in a direction perpendicular to the magnetic field. Equal and opposite forces act on these arms (in a direction according to Fleming’s Left hand Rule) and they form a couple. The coil rotates in clockwise direction (Fig.). After half rotation, split parts of ring change brushes. Current becomes reverse in the arms but couple acts in same direction as before. The coil continues rotating the shaft on which it is wrapped. Thus, rotatory motion (motor action) becomes available.
The rotation is not smooth with single coil. It is made smooth by increasing number of coils (multiphase motor) or preferably by having a continuous winding.

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