Which of the following semi-conducting devices is used as a voltage regulator?

$A$ Zener diode acts as a/an

Two Zener diodes ($A$ and $B$) having breakdown voltages of $6\, V$ and $4\, V$ respectively,are connected as shown in the circuit below. The output voltage $V_{0}$ variation with input voltage linearly increasing with time,is given by: $(V_{\text{input}} = 0\, V$ at $t = 0)$ (figures are qualitative).

If the current in the Zener diode is five times the current in $R_1$ and the breakdown voltage of the diode is $6 \text{ V}$,find the value of $R$.

For the given circuit,the power across the zener diode is ..... $mW$.

$A$ Zener diode of power rating $1.6\,W$ is to be used as a voltage regulator. If the Zener diode has a breakdown voltage of $8\,V$ and it has to regulate an input voltage fluctuating between $3\,V$ and $10\,V$, the value of series resistance $R_s$ for the safe operation of the diode will be $.........\Omega$.