Today we experimented with a slightly different physical discipline: electricity. To illustrate principles of voltage difference and magnetic induction, we examined three scenarios: a "lemon battery," a solar-powered battery, and a hand-crank power generator.
The "lemon battery" demonstrates the the concept of charge polarization due to a chemical reaction. Basically, the reaction consists of the lemon's acid "pulling electrons" from the valence shells of the copper penny. The zinc screw, which is a better conductor than the copper penny, then "accepts" the copper's electrons. This polarization creates a voltage difference similar to that of a battery, in which current flows from a positive voltage, here about 0.2 V, to a negative voltage, here about zero. The natural and fairly low resistance of the voltmeter keeps the system from short-circuiting.
The solar panel exhibit demonstrates, like the "lemon battery," a battery involving a voltage drop. Basically, the solar panel consists of impure silicon in both P and N forms together to create a semiconductor. Energy from sunlight in the form of heat creates "holes" of positivity in the silicon crystal. In an attempt to fill these holes, electrons rush from the N-type region of the silicon to the P-type, where the predominance of positivity "holes" exist. This electric surge creates a natural charge barrier between the N-types and P-types, as the electrons rushing to the positive side exert a repulsive electric force on the rest of the electrons in the N-type region. This polarization effect creates a small voltage difference similar to a battery.
The third and final demonstration was a hand-cranked induction generator. The basic form consisted of a coiled wire attached to a hand-crank surrounded by magnets. By the equation for magnetic inductance, emf = -N(d"flux"/dt), we know that a change in magnetic flux generates a voltage difference. In this scenario, a change in flux is generated by altering the angle, through the movement of the hand-crank, of the wire with respect to the magnetic field. The voltage difference generated induces a current powering the light bulb. When the circuit is incomplete, or switched to "off," the crank is easier to turn because there is no resistor. However, switching the circuit "off" disables the light bulb by removing the resistance. This concept of voltage induction through a magnetic field's influence on a coiled wire is similar to the situation in a solenoid.
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