A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately

The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux $$\phi $$ linked with the primary coil is given by $$\phi $$ = $$\phi $$₀ + 4t, where $$\phi $$ is webers, t is time in seconds and $$\phi $$₀ is a constant, the output voltage across the secondary coil is

What is the value of inductance L for which the current is maximum in a series LCR circuit with C = 10 $$\mu $$F and $$\omega $$ = 1000 s^$$-$$1 ?

A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produce oscillations of frequency $$f$$. If L is doubled and C is changed to 4C, the frequency will be