^Generating EM waves

Maxwell suggested an accelerating charge produces EM waves. An electric charge oscillating harmonically with time produces an oscillating electric field in its neighbourhood & this field in turn produces an oscillating magnetic in the neighbourhood. The process continues because the oscillating electric and magnetic fields act as sources of each other & hence an EM wave originates from the oscillating charge. The frequency of the EM wave is equal to the frequency of oscillation of the charge. The energy carried by the wave comes from the source which makes the charge oscillating. To generate an EM wave of frequency f, we need to set up an ac circuit in which the current oscillates at the frequency f. It is easier to generate low frequency EM waves such as a radio wave than to produce high frequency EM waves. e.g. the generation of yellow light requires an oscillator of frequency 6 x 10¹⁴ Hz, while the modern oscillators have frequency not above 10¹¹ Hz.

^Advantages of ac

1. economical than dc.

2. can be easily transformed from low voltage to high voltage or vice versa using transformers.

3. can be transmitted to distant places with comparatively smaller energy losses.

4. can be controlled using choke coils without having appreciable energy losses.

5. can be converted to dc using rectifiers.

Limitations of ac

6. can’t be used in electrolysis.

7. Shock of 220 V ac is more dangerous than that of 220 V dc.

8. Special wires (a combination of large number of thin wires insulated from each other) are used to carry ac due to skin effect.

^Choke coil

A coil having large inductance & small resistance connecting it in series with a fluorescent tube in order to control current in it  without appreciable power loss.

Application of chock coil A resistor due to larger power dissipation is not used. For a normal inductor consumes power smaller than consumed by a resistor by a factor of

As thus for a given emf to reduce low frequency alternating current the choke coils with high inductance i.e. inductors with soft iron cores are used & for reducing high frequency alternating currents the choke coils with low inductance i.e. inductors with air as core are used .

^dc motor

Slip ring arrangement used in ac generator is replaced by split ring arrangement in dc motor. Current flowing in the motor is given by .  It is maximum when the motor is just switched on as then induced emf is zero. This strong current may even burn the coil. As the motor picks up the speed, emf induced in the coil increases & as a result current in it starts decreases.

^LCR series or acceptor circuit

If the generator oscillates at a frequency equal to natural frequency of LC oscillator then current in the circuit becomes maximum & the stage is called resonance. Following points are of interest at resonance.

1. X_L= X_C, Z = min. = R

2.

3. Φ = 0

4. Quality factor or Q factor or voltage multiplication factor is defined as the ratio of voltage across inductor (or capacitor) V_L (or V_C) to ratio of voltage across a resistor at resonance. i.e.

5. Also

Here ω₁ & ω₂ are called half power side band frequency.

6. Value of Q describes the sharpness of I – ω curve at resonance.

If band width is low, the quality factor will be high & the I – ω graph will be thin & sharp & vice – versa as shown in the following plots. Higher the value of Q-factor, the narrower & sharper is the I – ω curve at resonance.

The larger the value of Q – factor, the smaller is the value of 2 Δω or the bandwidth and sharper is the resonance.