Electricity has a polarity, a "positive" and a "negative" property. A small "solid state" component called a diode, allows electricity to only pass through it in one direction. The diode, a small one of which looks like a toothpick with a bulge in the middle, has a positive and a negative side. Using hook-up wire or insulated alligator clip jumpers, a dual "D" cell battery holder with two "D" cell batteries (to create a 3-volt electrical source), and a light-emitting diode (LED), make a circuit by connecting wire from the positive "+" battery holder terminal to one "leg" (lead) of the LED and connect another wire from the negative terminal to the other LED lead. The LED will only allow electricity to flow in one direction, and when it does, it will light up. Note as to whether the LED is illuminated. Reverse the positive and negative wires on the battery, and again note whether the LED is illuminated. The LED will light when the negative battery wire is connected to the negative lead on the LED, which can usually be determined by the LED having a flat side or a dot on its plastic case, but it will not light if a positive voltage is present at its negative lead.
When components are placed in a line, they are referred to as being in "series," and when they are connected across each other they are in "parallel." Christmas tree lights whose bulbs are in series will not light if one bulb is out (it makes an "open" circuit). Think of the flow of electricity as the flow of water through pipes. Connect two "D" cell batteries to make a 3-volt source. Connect the negative terminal to a switch, such as a light switch used in a home's wall. Connect the other terminal of the switch to the negative lead on an LED. Hook the positive LED lead to the positive battery terminal. The switch is said to be in a "series" circuit. The position of the switch, on or off, controls the electrical flow through the circuit. Now connect the terminals of a second switch "across" the terminals of the first switch, placing them in "parallel." If either switch is in the "on" position, the LED will light.
Make a circuit where an LED will blink on and off. An "integrated circuit" or "IC" contains many tiny components in one small case. You will need a Sylvania ECG-876 integrated circuit. Connect the positive terminal of a 1.5-volt battery to pin 4 on the IC, and the negative terminal to pin 8. Connect the positive lead on a 100 microfarad, 35-volt electrolytic capacitor to pin 6, and the negative lead to pin 5. Connect a wire from pin 1 to 5. Connect the negative lead of an LED to pin 1 and the positive lead to pin 2. The LED will blink on and off. Change the rate of blinking by substituting a 220-microfarad or a 470-microfarad capacitor.
The electricity supplied to your home from the electric utility company has its polarity changing 60 times a second (in the U.S.). This AC (alternating current) frequency is in a range that we can actually hear, though it is a very low-pitched tone. Connect a 6- or 9-volt AC transformer to a wall outlet in your home. This will lower the voltage from 120 volts to 6 (or 9). Connect a 1,000 ohm, one-half watt resistor to one leg of the transform's output. Connect the other end of the resister to one terminal of an old speaker. Connect the other terminal of the speaker to the other transformer output terminal. You will see and feel the speaker vibrate. The tone will be a "sine" wave, which is a very pure sound lacking overtone harmonics, and it will have a very low pitch.