If a wheel with 24 metallic spokes each 40 cm long is rotated with a speed of $180 \mathrm{rev} / \mathrm{min}$ in a plane normal to the horizontal component of Earth's magnetic field, the emf induced between the axle and the rim of the wheel is $E$. If the number of spokes is made 12 and the wheel is rotated with a speed of $90 \mathrm{rev} / \mathrm{min}$ in the same field, the induced emf is

If a resistor of resistance $4 \Omega$, a capacitor of capacitive reactance $6 \Omega$ and an inductor of inductive reactance $9 \Omega$ are connected in series with an AC source, then the impedance of the circuit is

The ratio of the magnitudes of the electric field and $10^8$ times the magnetic field of a plane electromagnetic wave is