The order of increasing energy is: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < ...

Can you imagine how the atoms physically interact to generate voltage and current? This is difficult to understand without visualizing 'invisible atomic movements.' As a first step towards that, let's ...