Is energy given off or absorbed when an electron and a hole of semiconductor crystal recombine

Explain why electric current cannot exist if a current doesn't have a voltage source.

An ideal otto cycle has a compression ratio of 10.5, takes in air at 90 kpa and 40°c, and is repeated 2500 times per minute. using constant specific heats at room temperature, determine the thermal efficiency of this cycle and the rate of heat input if the cycle is to produce 90 kw of power.

How do you know how many valence electrons an atom has?