A ray of light that hits the solar cells on the board frees an electron which is pulled into the n-type layer by the positive charge within it. Next, an electron from another adjoining atom in the p-type layer moves upward to fill the missing hole left by the freed electron. Finally, the electrons produce a current as light frees them. Returning electrons fill the holes they have left. When we covered the solar board, there was no light to free the electrons in the p-type layer causing no current flow in the system.
For the lemon battery, electron are pulled from the zinc bolt by the lemon acid and put on the penny. This creates a current that passes through the voltmeter.
The generator works with electromagnetic induction. The crank is attached to a coil of wire that is rotated through a magnetic field created by horseshoe magnets. A voltage difference results so current flows through the light bulb.
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