Two coils require $20 \ min$ and $60 \ min$ respectively to produce the same amount of heat energy when connected separately to the same source. If they are connected in parallel to the same source,the time required to produce the same amount of heat by the combination of coils will be . . . . . . $min$.

An inductor coil takes a current of $8 \, A$ when connected to a $100 \, V$ and $50 \, Hz$ $AC$ source. $A$ pure resistor under the same condition takes a current of $10 \, A$. If the inductor coil and resistor are connected in series to a $100 \, V$ and $40 \, Hz$ $AC$ supply, then the current in the series combination of the above resistor and inductor is:

In an $LCR$ circuit,the capacitance is changed from $C$ to $4C$. For the same resonant frequency,the inductance should be changed from $L$ to:

An inductor and a resistor are connected in series to an $AC$ source. If the power factor of the circuit is $0.5$, the ratio of the resistance of the resistor and the reactance of the inductor is:

An oscillating circuit consists of a capacitor with capacitance $C = 10 \, \mu F$, a coil with inductance $L = 6.0 \, \mu H$, and active resistance $R = 10 \, \Omega$. The mean power that should be fed to the circuit to maintain undamped harmonic oscillations with an external driving source of frequency $f = 50 \, Hz$ and peak voltage $V_m = 280 \, V$ is:

For the given figures,choose the correct option: