what is the resistance of earth(good ground)?

Earth is  a good  conductor, it has less than 5 ohms of resistance.

T Flip Flop used as frequency divider

Similarly To D FF,T FF also used as frequency divider.

D Flip Flop as a Frequency divider how?

D FF called as binary divider or frequency divider by small changes in circuit.
By connecting complement output Q(not Q) to input D,then its output is divided the input with a factor of 2.so it is called as frequency divider.

Inductor blocks AC why?

In regulator circuits, inductor is connected in series with rectifier circuits for blocking AC.
Inductor is allows Direct current(low frequency) and blocks alternating current(AC) this is because of inductive reactance.

Inductive reactance is XL=2pif.

If frequency f is high,inductive reactance also high so it blocks high frequency signal.it means high frequency alternating current(AC) is blocked by inductor.

similarly,if frequency f is low,inductive reactance is low so it allows low frequency signal.it means low frequency direct current(DC) is allowed by inductor.

capacitor blocks dc why?

capacitors are connected in series in amplifier circuits for blocking dc from input.this means capacitor blocks direct current(low frequency) and allows alternating current(high frequency).
this is because of capacitive reactance Xc=1/2pif

 if input frequency f is high,the reactance of a capacitor becomes low so it allows a signal passing through it.it means high frequency alternating current(AC) is allowed by a capacitor.

similarly,if input frequency f is low,the reactance of a capacitor becomes high so it not allows low signal passing through it.it means low frequency direct current(DC) is not allowed by a capacitor.

negative resistance property

In resistor,when input applied voltage increases it causes electric current also increases through in it because of ohms law,this property is called positive resistance of element or circuit.but here is negative resistance property,if applied voltage increases it causes decreases electric current through in it,it is called negative resistance of a element or circuit.

EX: uni junction transistor(UJT),tunnel diode,gunn diode, thyristors, IMPATT, magnetrons etc.

*Advantages of negative resistance:
 
1.used in oscillators. UJT oscillator,gunn diode oscillator.
2.mainly used in high frequency oscillators in microwave applications.

linear and non linear circuits

LINEAR ELEMENT AND LINEAR CIRCUIT:

If the input signal is passed through the linear elements in the circuit,the circuit in which the circuit parameters(resistance,inductance,capacitance,waveform,frequency etc) are constant, that is the output is following input signal, that circuit is called linear circuit and that device called linear device.
simply circuit parameters are not changed by applied input signal is called linear circuit.
the output response of circuit is directly proportional to input signal.

Ex: resistors and resistive circuits.
 inductors and inductive circuits.
 capacitor and capacitive circuits.

NON LINEAR ELEMENT AND NON LINEAR CIRCUITS:

If the input signal is passed through the non linear elements in the circuit,the circuit in which the circuit parameters (resistance,capacitance,inductance,waveform,frequency etc) are not constant, that is the output is not following the input,that circuit is called non linear circuit and that device is called non linear device.
circuit parameters are changed by applied input signal is called non linear circuit.
the output response of circuit is not directly proportion to input signal.

Ex:diodes,transistors,transformers etc.