In Bohr's atomic model, the energy of the electron is 'E' in the second orbit of hydrogen atom. So the energy of the electron in the third orbit of helium atom $(Z = 2)$ will be

The activity of a radioactive sample is measured as $N_0$ counts per minute at time $t = 0$ and $\dfrac{N_0}{e}$ counts per minute at time $t = 3$ minute. The time (in minute) in which the activity reduces to half the value, is

In the following digital circuit, the output 'Y' will be '1' (one) for inputs 'A' and 'B' having values

In common emitter configuration of transistor amplifier, $r_i$, $R_L$ and $\beta$ represent the input resistance, load resistance and the a.c. current gain respectively. The voltage gain $A_V$ and power gain $A_P$ are represented in magnitude respectively by