In which of the following activities is an induction coil not useful?

In a series R-L-C circuit, R = 10\(\Omega\), \(X_{c}\)= 4\(\Omega\) and \(X_{L}\) = 9\(\Omega\). The impedance of the circuit is

A student uses two compass needles to investigate the magnetic field around a bar magnet. Which of the following diagrams show the correct directions of the needles?

Which of the following is emitted in a process of natural radioactivity?

In which of the following situations is photoelectric effect employed?

I. Automatic devices for switching on light at dusk

II. Automatic doors in departmental stores

III. Automatic devices for switching on air conditioners

IV. Automatic voltage stabilizers