Advantages of using a large number of poles in a DC machine

*length of magnetic circuit is reduced and resulting in reduction of field ampere turns.

*smaller air gap is required.

*Length of armature end connections are reduced. 

*The flux per pole is reduced. This reduces the thickness of armature core and yoke and hence the weight of the machine.

* Current per brush arm is reduced.

Advantages and disadvantages of carbon brushes

Advantages

    *They provide good commutation. 

    *If sparking occurs they damage the commutator less than with the copper brushes. 

    *They lubricate and polish the commutator.

Disadvantages

    *Contact resistance is high and cause a loss of about 2 V.Hence they are not suited for low voltage machines.

    *Due to high voltage drop , the commutator must be made larger than  that for copper brushes. 

    *Due to the low current density large brush holders are required.

Importance of D.C machines

        D.C. generators are not much useful because direct current can be easley obtained with the help of rectifiers.However DC motors are widely used because of the following reasons

*Speed of the motor can be widely change.

 
*Good speed regulation. 

*Torque speed characteristics of DC motors can be varied over a wide range while retaining high        efficiency.
     

      Because of these reasons DC motors are used to drive devices such as fans, pumps, hoists, punch presses etc .

Why does glow of lamps become weaker when a heavy load is switched on?

      

  Electrical wiring is done in parallel.When heavy load is applied total resistance in the system decreased since the load connected in parallel manner.This will increase the line current and cause a large voltage drop in the line.Therefore voltage available across the lamp decreases and their glow also..

Why are domestic appliances connected in parallel?

    * Appliances are rated for different powers but for same voltage (230v).When connected in parallel performance of each appliances becomes independent of each other.
   

    *If any of the branch circuit breaks it will not affect the other branches

    * Since connection is in parallel net resistance will be smaller than that connected in series.

Why does an operational amplifier have high input impedance and low output impedance

   Operational amplifiers amplify the input signal they got.In order to receive the input voltage signal that should be dropped across the operational amplifier.
   As we know by Ohms law voltage drop V=IR .If resistance is more voltage drop across that device also more.Hence an operational amplifier should have high input impedance in order to drop the input voltage signal across it.
   If operational amplifier has low input impedance the input voltage signal does not drop across it and operational amplifier may not receive the signal.
  Similarity an operational amplifier should have very low output impedance so that the amplified voltage signal drop across the device which is connected to the output terminals of operational amplifier and not in the operational amplifier itself .
If the operational amplifier has high output impedance then the amplified voltage will drop across the operational amplifier itself and it will not transferred to the device which connected to the output terminals of operational amplifier.That's why operational amplifier needs low output impedance .
  Another reason for high input impedance is because of loading effect .If operational amplifier has low input impedance then heavy current will drown from the supply it may Leeds to the burning of the operational amplifier .So by using high input impedance we can avoid loading effect of operational amplifier .

High pass R-C circuit

     In an R-C series circuit the reactance of capacitor is          Xc=1/(2*pi*f*c)
Here the reactance of capacitor is inversely proportional to the frequency .ie,Reactance of capacitor is increase with decrease in frequency and decrease with increase in frequency.So at low frequency the frequency the capacitor has considerable reactance and it block the low frequency signals.But for higher frequency signals it offers very small reactance and pass through it.The output is taken across the resistance.Since this circuit block the low frequency signals and pass high frequency signals through it,the circuit is called High pass R-C circuit.
       Due to the blocking property of this circuit this circuit is widely used as a coupling circuit to provide isolation between input and output.The capacitor used in this circuit is also called blocking capacitor.