What is the cause of resistance?

If three resistors of $5 \Omega$,$10 \Omega$,and $15 \Omega$ are connected in series,the equivalent resistance $R$ is equal to ........ (in $\Omega$)

What is meant by "electrical resistance" of a conductor? State how the resistance of a conductor is affected when $(i)$ a low current passes through it for a short duration; $(ii)$ a heavy current passes through it for about $30$ seconds.

An electric bulb draws a current of $0.2 A$ when the voltage is $220 V$. Calculate the amount of charge flowing through it in one hour.

Why is much less heat generated in long electric cables than in filaments of electric bulbs?

State with the help of a circuit diagram an activity to find the resistance $R$ of a given wire. $A$ current of $0.2 \, A$ passes through a resistor of $20 \, \Omega$. Find the potential difference across its ends.