Electricity

B

Electrolytic Cells

Electrolytic cells use chemical energy to produce electricity. Chemical reactions within an electrolytic cell produce a potential difference between the cell’s terminals. An electric battery consists of a cell or group of cells connected together.

C

Other Sources

There are many sources of electric current other than generators and electrolytic cells. Fuel cells, for example, produce electricity through chemical reactions. Unlike electrolytic cells, however, fuel cells do not store chemicals and therefore must be constantly refilled.

Certain sources of electric current operate on the principle that some metals hold onto their electrons more strongly than other metals do. Platinum, for example, holds its electrons less strongly than aluminum does. If a strip of platinum and a strip of aluminum are pressed together under the proper conditions, some electrons will flow from the platinum to the aluminum. As the aluminum gains electrons and becomes negative, the platinum loses electrons and becomes positive.

The strength with which a metal holds its electrons varies with temperature. If two strips of different metals are joined and the joint heated, electrons will pass from one strip to the other. Electricity produced directly by heating is called thermoelectricity.

---

---

Some substances emit electrons when they are struck by light. Electricity produced in this way is called photoelectricity. When pressure is applied to certain crystals, a potential difference develops across them. Electricity thus produced is called piezoelectricity. Some microphones work on this principle.

VII

Electric Circuits

An electric circuit is an arrangement of electric current sources and conducting paths through which a current can continuously flow. In a simple circuit consisting of a small light bulb, a battery, and two pieces of wire, the electric current flows from the negative terminal of the battery, through one piece of connecting wire, through the bulb filament (also a type of wire), through the other piece of connecting wire, and back to the positive terminal of the battery. When the electric current flows through the filament, the filament heats up and the bulb lights.

A switch can be placed in one of the connecting wires. A flashlight is an example of such a circuit. When the switch is open, the connection is broken, electric current cannot flow through the circuit, and the bulb does not light. When the switch is closed, current flows and the bulb lights.

The bulb filament may burn out if too much electric current flows through it. To prevent this from happening, a fuse (circuit breaker) may be placed in the circuit. When too much current flows through the fuse, a wire in the fuse heats up and melts, thereby breaking the circuit and stopping the flow of current. The wire in the fuse is designed to melt before the filament would melt.

The part of an electric circuit other than the source of electric current is called the load. The load includes all appliances placed in the circuit, such as lights, radios, fans, buzzers, and toasters. It also includes the connecting wires, as well as switches, fuses, and other devices. The load forms a continuous conducting path between the terminals of the current source.

There are two basic ways in which the parts of a circuit are arranged. One arrangement is called a series circuit, and the other is called a parallel circuit.

A

Series Circuits

If various objects are arranged to form a single conducting path between the terminals of a source of electric current, the objects are said to be connected in series. The electron current first passes from the negative terminal of the source into the first object, then flows through the other objects one after another, and finally returns to the positive terminal of the source. The current is the same throughout the circuit. In the example of the light bulb, the wires, bulb, switch, and fuse are connected in series.

When objects are connected in series, the electric current flows through them against the resistance of the first object, then against the resistance of the next object, and so on. Therefore the total resistance to the current is equal to the sum of the individual resistances. If three objects with resistances R₁, R₂, and R₃ are connected in series, their total resistance is R₁ + R₂ + R₃. For example, if a motor with a resistance of 48 ohms is connected to the terminals of a current source by two wires, each with a resistance of 1 ohm, the total resistance of the motor and wires is 48 + 1 + 1 = 50 ohms. If the voltage is 100 volts, a current of 100/50 = 2 amp will flow through the circuit.

Voltage can be thought of as being used up by the objects in a circuit. The voltage that each object uses up is called the voltage drop across that object. Voltage drop can be calculated from the equation V = IR, where V is the voltage drop across the object, I is the amount of current, and R is the resistance of the object.

In the example of the motor, the voltage drop in each wire is V = IR = 2 × 1 = 2 volts, and the voltage drop in the motor is 2 × 48 = 96 volts. Adding up the voltage drops (2 + 2 + 96) gives a total drop of 100 volts. In a series circuit the sum of the voltage drops across the objects always equals the total voltage supplied by the source.

B

Parallel Circuits

If various objects are connected to form separate paths between the terminals of a source of electric current, they are said to be connected in parallel. Each separate path is called a branch of the circuit. Current from the source splits up and enters the various branches. After flowing through the separate branches, the current merges again before reentering the current source.

The total resistance of objects connected in parallel is less than that of any of the individual resistances. This is because a parallel circuit offers more than one branch (path) for the electric current, whereas a series circuit has only one path for all the current.

The electric current through a parallel circuit is distributed among the branches according to the resistances of the branches. If each branch has the same resistance, then the current in each will be equal. If the branches have different resistances, the current in each branch can be determined from the equation I = V/R, where I is the amount of current in the branch, V is the voltage, and R is the resistance of the branch.

The total resistance of a parallel circuit can be calculated from the equation

where R is the total resistance and R₁, R₂, ... are the resistances of the branches. For example, if a parallel circuit consists of three branches with resistances of 10, 15, and 30 ohms, then

Therefore, R = 5 ohms. In this circuit, a voltage of 150 volts would produce an electric current of I = V/R = 150/5 = 30 amp.

The greater the resistance of a given branch, the smaller the portion of the electric current flowing through that branch. If a parallel circuit of three branches, with resistances of 10, 15, and 30 ohms, is connected to a 150-volt source, the branch with a resistance of 10 ohms would receive a current of V/R = 150/10 = 15 amp. Similarly, the 15-ohm branch receives 10 amp, and the 30-ohm branch receives 5 amp. These branch currents add up to a total current of 30 amp, which is the value obtained by dividing the voltage by the total resistance.