the temperature of a heat engine is 500 k. some of the heat generated by the engine flows to the surroundings, which are
at a temperature of 125 k.
the efficiency of the engine is

The efficiency of the engine is 75%  under given conditions.

Explanation:

Efficiency is defined as the ratio of work done by the engine to the total heat supplied to a heat engine. We know work done by the heat engine be difference between total heat supplied by total heat utilized . We can derive formula for efficiency by considering the above factors,  

Now consider a heat engine is at a temperature of 500K and the heat produced by the engine released to the surroundings at a temperature of 125K. The efficiency is calculated by using the above formula,

∴ Efficiency of the engine is 75%

when it gains or loses electrons