Consider the following experiment. We accelerate electrons in an electrostatic accelerator with potential drop U and shoot them towards a sensitive detector. Between the accelerator and detector we set up a repulsive potential barrier V with help of an electrical condenser. In the plates there are holes along the path, so there is possibility that electron passes the barrier of the condenser and arrives at the detector on the other side. What will happen to the electrons and the detector if the potential barrier is higher than the accelerating potential? According to quantum theory, there will be particles arriving at the detector even if V > U (tunnel effect). 1) Does not this contradict conservation of energy? From the description it is clear that the potential energy of the electron at the detector is higher than potential energy at the source, so it seems like the detector receives more energy than the source loses by accelerating the electrons. 2) If not, where does the extra energy come from?