What is the difference between resistance and resistivity?

A potentiometer wire of length 100 cm has resistance of 20 ohm. It is connected in series with an external resistance and a cell of emf 2 volt and of negligible internal resistance. A source of emf 20 mV is balance against a length of 40 cm of the potentiometer wire. What is the value of external resistance?

A battery of emf, 20 V is connected to resistances as shown in figure. The potential at the point C will be

Ohm’s law is valid if

An object 1 cm tall is placed at 4 cm infront of a mirror, In order to produce an upright image of 3 cm height one needs a :-

In a step up transformer, if ratio of turns of primary to secondary is 1 : 10 and primary voltage is 230 V. If the load current is 2A, then the current in primary is :-