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

A coil of inductive reactance 31 $$\Omega $$ has a resistance of 8 $$\Omega $$. It is placed in series with a condenser of capacitative reactance 25 $$\Omega $$. The combination is connected to an a.c. source of 110 V. The power factor of the circuit is

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

The core of a transformer is laminated because