One way that an electric current can be created is by a dry cell. A dry cell consists of a zinc can, a carbon rod in the center of this can, and a chemical paste of potassium hydroxide. Potassium hydroxide is an electrolyte.
When an element loses electrons, it is said to be oxidized. When an element gains electrons, it is reduced. An electrolyte is a compound which has an electrical charge. In the dry cells, the electrolyte oxidizes the zinc and the electrolyte becomes reduced. These removed electrons collect at the negative terminal. If connected to a light bulb, for example, the electrons will flow out of the negative terminal, through the light bulb, and return to the dry cell by way of the positive terminal. This flow of electrons goes through the tungsten filament producing the heat that causes it to glow.
A single dry cell can only generate 1.5 volts of electricity. If several dry cells are put together, you have what is commonly called a battery. A two dry cell battery produces three volts, a four dry cell battery six volts, an six dry cell battery nine volts and an eight dry cell battery 12 volts. These are the common batteries used to power such appliances as: flashlights, toys, radios, or a Discman or Walkman. The dry cell is called an electrochemical cell because chemicals called electrolytes oxidize zinc and become reduced to produce a flow of electrons. Potassium hydroxide is today the most commonly used electrolyte. Potassium hydroxide is a base or an alkaline substance, therefore, these are called "alkaline" batteries.
Another kind of electrochemical cell is the wet cell. The typical automobile battery is an example of a series of wet cells. The wet cell has two different metal plates called electrodes which are in a liquid called an electrolyte. The two metals are usually lead and lead dioxide. The electrolyte is usually sulfuric acid which is diluted in water. The sulfuric acid electrolyte oxidizes the lead and reduces the lead dioxide. Each cell can produce two volts of electric current. Therefore, a six volt car battery has three wet cells. A twelve volt car battery has six wet cells.
Dry cells and wet cells produce only direct current (D.C.). In direct current, the electrons move only in one direction. Alternating current (A.C.), has electrons which move back and forth in a regular, repeating cycle. Household current in the United States of America is A.C. This current is 60 cycles per second or 60 hertz. This means that the electrons move back and forth 60 times in a second.
The advantage of A.C. over D.C. is that A.C. can be transported over longer distances with less energy loss than D.C. This is because A.C. can be transformed up or down, and D.C. cannot. The energy of electric current is converted to heat, light and motion by electric appliances.
If you connect just one wire to an electric appliance, it will not work. In order for electrons to flow, electrons need a closed path to travel. This path which electrons travel is called a circuit.
An electric circuit consists of: 1. A source of electrons, such as a wet cell, dry cell or a generator. 2. A load. A load is the device that uses the electrons, such as a light bulb, radio, microwave oven, or TV. 3. Wire--usually made of metal or graphite. Wires must be made of substances which are good conductors of electrons. They must also be ductile--meaning, capable of being made into wires. 4. Switch--a device which opens and closes the circuit. The "on" position closes the circuit so that electrons can flow. The "off" position opens the circuit so that electrons can't flow.
If all loads are connected one after another the circuit is a series circuit. The disadvantage of a series circuit is that if there is a break in any part of the circuit, the entire circuit is open and electrons can't flow. Some old Christmas tree lights used a series circuit. If one bulb went out, all bulbs went out. You had to find and replace the bad bulb before the string of light would work again.
The other type of circuit is the parallel circuit. In the parallel circuit the different loads are on separate branches so there are several paths for the electrons to take. If there is a break in any part of the circuit, the electrons have alternate paths to travel, so that a break does not open the entire circuit. All modern electrical wiring use parallel circuits.
The load is the part of the circuit which produces resistance. The SI unit for measuring resistance is the ohm. If there is too much resistance on a circuit, the increased resistance produces excess heat which can burn up the wire and start a fire. To protect against circuit overload, most electric circuits have either fuses or circuit breakers.
A fuse has a thin strip of metal through which the electric current flows. If the resistance becomes great, the heat causes this metal strip to melt. This breaks the flow of electricity by opening the circuit.
A more modern method to protect from circuit overload is the
circuit breaker. Circuit breakers have a switch which opens the
circuit if resistance becomes too high. When you correct the
overload, the breaker can be reset. Circuit breakers are preferred
over fuses because they are easy to use and do not have to be
replaced after each overload. However, circuit breakers are more
expensive than fuses.