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Beginners Notes

Arnold's Mechanisms & Inventions
Orbit of the Earth around a room

Solar cells produce direct current electricity when the Sun is shining on them (or if they are close to a bright light).

They do not have to be “charged up”. Nor do they hold a charge. When light photons strike the solar cell material, a flow of electrons is produced instantaneously, and that electricity can be used to power electrical devices such as motors, lights, and buzzers. When the light stops, the electricity stops.

Electrical energy is measured in watts, with amperes of current flowing between voltage potential differences. So voltage times amperes equals watts. Volts x Amps = watts V x A = Energy

A watt is an S.I. unit of energy named after James Watt, Scottish inventor of the steam engine. In those days watts were derived from burning coal. Now we understand the power of the Sun.

Each cell of a solar panel has a roughly 0.5 volt potential. A 1.0 volt panel will consist of two cells, wired in series. A 1.5 volt solar panel has 3 cells, and a 2.0 volt solar panel has 4 cells wired in series.

These values of 1.0, 1.5, and 2.0 volts are approximations – what we call “nominal” values. The actual voltage of each cell is somewhat higher, around 0.54 volts.

Our 1.5 volt x 500 milliamp pv panel in direct sunlight produces about the same amount of electricity as an AA, C, or D-cell battery produces. Our 2.0 volt x 500 milliamp panel produces ONE WATT of electricity.

Alligator clip test leads allow for clipping into battery cases and experimenting. The 1.5 volt panel and 2 volt panels can also charge a re-chargeable battery.

[SAFETY NOTE: Ni-Cad batteries are a bit tricky introducing into a classroom, because they can dump their load so fast, quickly heating short-circuited wires to burn flesh or set fire to flammables.]
Solar cell electricity is direct current (DC) electricity – it flows in one direction. When a solar panel is attached to a motor, the motor can be made to spin in the reverse direction by reversing the connections at the back of the motor.

Some devices, such as LED’s, or piezo-electric buzzers, will work only with the current flowing in a particular direction, and not in the reverse direction. On LED’s the longer leg is positive.

No harm will result if the clips are “shorted” on a single solar panel. While electricity will flow, according to the amount of sunlight, no damage results to the cell, nor will the wires wear out. These solar panels will not harm each other if hooked together in any combination.

Solar cells and panels can be hooked up in series (positive to negative – adding voltages together); or in parallel (positive to positive, negative to negative – adding amperages together) circuits.

The solar cell reaction has an electrical potential of approximately 0.5 volts. If we add two cells together in series, we get 1 volt. Three cells in series gives us 1.5 volts. Higher voltages are obtained by linking many cells in series. Solar panels are made with solar cells connected in series, and sometimes in series and parallel circuits both, to build up useful amounts of electrical current.

Voltage is the measure of potential energy each electron has in the electric field formed within an electrical circuit. Electrical current is measured in amperes. Amperage is a gross measurement of how many electrons are flowing in the circuit.

To understand the difference between voltage and amperes, a waterfall is a helpful analogy. (Another is the water hose, where voltage is water pressure, and amperage the amount of water coming out of the hose.) With the waterfall analogy, voltage is like the gravitational potential energy at the top of the water fall, and amperage is like the amount of water flowing over the edge. At the bottom, where the water splashes down, we have power according to how far (voltage) the water fell, and how much (amperage) water fell each moment of time.
volts x amperes = watts (power)

V x A = W

an example: 1.5 volts x 500 milliAmps = 0.75 watts

Normally we must turn an electromagnetic generator with physical force to obtain electrical energy. Or discharge a chemical battery. Most power plants run on steam turbines turning massive generators. The steam is produced by water being heated by burning coal or gas, or by nuclear fission. The expanding steam turns the steam turbine, and the turbine shaft turns the generator shaft, which holds coils of wire whirling in a magnetic field.

Note that your DC motors can also be used as DC generators, simply by spinning the shaft with enough force to produce electricity at the electrical contacts at the back of the motor.

Solar-generated electricity is different – the energy comes from light, and the reaction in the cells is not chemical, but electrical only. The atoms remain the same, and are not involved in chemical reactions.

What are affected are the electron clouds of the silicon crystals, as photons of light energy are absorbed.

When two layers of differently doped purified silicon are joined, immediately an electrostatic field with an electric potential of ~0.5 volts is formed at their junction.

Because it is rather tricky to achieve in our three-dimensional world – having a photon strike an electron on the P-side of the electrical junction formed by two layers of differently doped purified silicon – and very close to the junction to boot – solar cells are only about 12 – 15 % efficient in changing light energy into electrical energy. That is, only about 1/8 to 1/6 of the solar energy striking the panel gets converted to electricity.

If cells, or panels, are wired in parallel, their amperes are added together. The amperage of a solar cell is determined by the cell quality and area of the cell, and the amount of sunlight striking the cell. A cell that faces the Sun directly receives more energy than a cell tilted away from the Sun.

For a solar panel, unfortunately, it is the cell that receives the least amount of light that limits the amount of current flowing through a string of cells. This is why avoiding even slight shading of solar panels is important.

If two of our 1.0 x 500 mAmp panels are connected in parallel, 1000 mAmps of 1 volt electricity could flow through a connected device, which might be useful if the device needs some low-end power.

Try shading the panels in different ways and observe what sort of shading lets the motor run, and what stops it completely. Clue: if the voltage of one cell is brought to zero, can the voltage of the other cell pass through it?

On the other hand, sometimes a solar car runs into the shade, and stops. One way to get it going again without moving it directly is to reflect some sunlight with a mirror onto the solar panel. The car will move forward – light IS energy.

Arnold's Mechanisms & Inventions
Orbit of the Earth around a room