CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 29(a)

(a) In a series circuit, if one appliance fails or is switched off, the circuit is broken and all the other appliances stop working, so the appliances cannot be operated independently. Also, the same current flows through every appliance and the $220$ V supply is shared among them, so appliances that need different currents, such as a bulb and a heater, cannot each get the current and voltage they need. (b) The three paths from A to B are in parallel: $6+4=10\Omega$, $15\Omega$, and $3+3=6\Omega$. So $\frac{1}{R_{AB}}=\frac{1}{10}+\frac{1}{15}+\frac{1}{6}=\frac{1}{3}$, and $R_{AB}=3\Omega$.