There is a frill of $20$ bulbs connected in series in a room. One bulb gets fused. The remaining $19$ are again joined in series and connected to the same supply. Will the light increase or decrease in the room?

$(a)$ What is meant by saying that the potential difference between two points is $1$ volt?
$(b)$ Why does the connecting cord of an electrical heater not glow while the heating element does?
$(c)$ Electrical resistivities of some substances at $20^{\circ}C$ are given below:

Silver	$1.60 \times 10^{-8} \, \Omega m$
Copper	$1.62 \times 10^{-8} \, \Omega m$
Tungsten	$5.2 \times 10^{-8} \, \Omega m$
Iron	$10.0 \times 10^{-8} \, \Omega m$
Mercury	$94.0 \times 10^{-8} \, \Omega m$
Nichrome	$100 \times 10^{-6} \, \Omega m$

Answer the following questions in relation to them:
$(i)$ Among silver and copper, which one is a better conductor? Why?
$(ii)$ Which material would you advise to use in electrical heating devices? Why?

In a circuit containing two unequal resistors connected in parallel:

If three bulbs of $100 \; W$, $60 \; W$, and $40 \; W$ are used for $2 \; \text{hours}$ daily in a house, how many units of electrical energy are consumed in $30 \; \text{days}$?

Which of the following substances does not contain free electrons?

What is the equivalent resistance of a circuit when three resistors of $5 \Omega$,$10 \Omega$,and $30 \Omega$ are connected in parallel (in $\Omega$)?