Electricity

Physical Science

Static Electricity Static electricity is an excess or lack of

electrons Lightning, a spark jumping from your

fingertip to a metal doorknob, and the clinging together of articles of clothing when they are removed from a clothes dryer are all forms of static electricity

Normally all atoms are electrically neutral Each atom has the same number of

negative electrons and positive protons Because electrons are found whirling

about the nucleus at high speeds they may be removed from the atom by friction

Static Electricity

When certain materials are rubbed together, electrons are transferred from one surface to another

Excess electrons on the surface of a substance produces a negative charge

A lack of electrons produce a positive charge One surface gains electrons and acquires a negative

charge The other surface has lost electrons and is left with a

positive charge Unlike electric charges attract, and like charges repel

Static Electricity When a surface has acquired a

strong negative charge, the extra electrons may jump to a neutral or positive object

You see this jump of electrons when you see a spark

A spark is a rapid movement of a number of electrons through the air

Lightning is a giant spark that sometimes occurs when clouds that have acquired a charge suddenly discharge electrons

Electric Current

Normally, electrons move about atoms at random However, under certain conditions, some of the

free electrons may be made to migrate in the same general direction

When this happens, electrons within a conductor are migrating in the same direction, an electric current is flowing in the conductor

Electric Current

In a copper wire, some electrons will move freely and randomly among the copper atoms making up the wire

But suppose that in some way many electrons are brought close to one end of the wire

Repulsive forces will be exerted upon the electrons in the wire

The electrons in the wire will tend to migrate away from the area of electron concentration

Electric Current

If a path is provided on the other end of the copper wire, permitting the electrons to move to an area that absorbs them, electrons will continue to migrate along the wire

The movement is away from the source of electrons

The movement of the electrons through the wire constitutes an electric current

Electric Current

Electric current is the movement of electrons through a conductor

The rate at which the electrons move through the conductor is called current

To produce an electric current requires a source of electrons, a conducting path, and a place for the electrons to go-a ground or sink

Conductors Conductor is any

material that electrons can move through easily

Such a material offers little resistance to the flow of electrons

Examples of materials that are good conductors include copper, silver, gold, and aluminum

Insulators

Insulators are substances that do not carry electrical energy well

Because these materials are poor conductors, they offer high resistance to the flow of electrons

Wood, rubber, plastic, and dry air are examples of insulators Some substances-germanium, silicon, and selenium- are neither

conductors nor insulators and are called semiconductors They can be used to make tiny electrical devices to control the

flow of electrons Semiconductors are widely used in the making of computer

chips

Chemicals Can Produce Electricity

The tendency to yield electrons varies from one metal to another

The electromotive series lists metals in descending order, according to this tendency

When any two metals are connected by a wire and immersed in an electrolyte (a conducting liquid), electrons will flow through the wire from the metal that is higher in the electromotive series to the metal that is lower

Chemicals Can Produce Electricity

This principle is the basis for the construction of dry cells, voltaic cells, lead storage cells, flashlight cells, and other common portable methods of producing electric current

The electrolytes used in storage batteries are commonly acids such as sulfuric acid

However, common salts such as table salt or ammonium chloride will also work in an electric cell

Magnetism Can Produce Electricity

Each electron in an atom is a tiny magnet and will tend to align itself with external magnetic fields

When the external magnetic field changes direction or magnitude, force is exerted upon the electrons

In metals that are subjected to a changing magnetic field, the electrons that are loosely held by atoms will tend to migrate in the direction that reduces the stress placed on them

Since metals tend to have an abundance of loosely held electrons moving about among their atoms, a changing magnetic field will result in a flow of electrons within the metal

Magnetism Can Produce Electricity

When a current is caused to flow in a conductor by means of a fluctuating magnetic field, the process is called electromagnetic induction

Commonly coils of copper wire are rotated within a magnetic field to produce an electric current

Example: generator

Commercial Generators

The commercial electricity used in homes, businesses, and industries is produced by generators

A generator is any device that permits a magnet to be rotated rapidly inside a coil of copper wire and so produces an electric current

The motion of generators is produced by a variety of means- water power; steam engines that burn oil, gas, or coal; nuclear power; wind power; etc.

Commercial Generators

In most commercial generators, the electrons flow in one direction through the copper wires during one half of a rotation and in the opposite direction during the other half of the rotation

This produces alternating current (AC), as opposed to current that flows always in the same direction, direct current (DC)- ex. electricity from dry cells (batteries)

In most commercial distributing systems, the direction of the current changes 60 times per second

Light Can Produce Electricity

The atoms of the metal oxides of potassium and cesium contain electrons that, when struck by light energy, become excited and jump from the surface of the material

A photoelectric cell may be constructed by placing the electron-yielding substance in a vacuum and providing a place for the excited electrons to go

The stronger the light source, the more electrons will be given off by the photosensitive coating of the cell

Much of this same process takes place in the electric tubes of radio and television sets

Heat Can Produce Electricity

In these applications, the electron-yielding materials are heated

The energy provided causes electrons to be emitted from the surface of the material in a process called thermionic emission

A thermocouple is a device in which 2 wires of unlike metals are joined and heat is applied to the junction

Electrons will migrate from one metal to the other to produce a small current

Electric Circuits Think of an electric circuit as a path provided by a conductor that

enables electrons with a great deal of energy to leave a source of energy (generator, battery, etc.), move through the conductor (a wire), lose some energy due to resistance (a light bulb), and end in an electron sink or an acceptor of electrons

As long as the switch is closed (the wires are connected), energy is transmitted through the circuit

The light bulb- the resistance- converts some of the electrical energy to light energy

Any time a region with excess electrons is joined by a conductor to a region with fewer electrons, the electrons will flow from the region of higher concentration to the region of lower concentration

The magnitude of the current in a circuit is defined by the number of electrons that flow through the circuit in a given period of time

Electric Circuits

Ampere-the unit used to express the number of electrons passing by a given point in a conductor

Voltage- the force or pressure exerted upon the electrons in the conductor

The magnitude of the voltage depends on the difference in the concentration of electrons at the opposite end of the conductor

The difference in concentration affects the number of electrons that move and the energy that they expend as they pass through the circuit

Volt-the unit used to express the pressure on the electrons in a conductor, or the energy that they expend in the circuit

Resistance- is the opposition to the flow of current in the circuit

Electric Circuits

The atoms of different materials vary in the ease with which they permit their electrons to migrate

Atoms of high-resistance materials hold on to their outer electrons more strongly than do atoms of low-resistance materials

Thus, even with identical voltages, materials vary in the number of electrons they will pass among their atoms

The unit that defines the opposition to the flow of current (resistance) in a circuit is the ohm

Ohm’s Law

The relationships that exist among voltage, resistance, and current:

The magnitude of the current in a conductor is directly related to the voltage (current will increase when voltage increases); current is inversely related to the resistance (current will decrease when resistance increases)

This is known as Ohm’s law I=V/R I=current V=voltage R=resistance

Water System Analogy of the Electric Circuit

Often for simplicity’s sake, electric circuits are compared to a water system

A water pump is analogous to the generator in that the former produces water pressure and the latter electrical pressure

The pipes of the water system represent conducting wires in the electrical circuit

Just as the size of the water pipes affect the amount of water that flows through a system, so the size of the conducting wires affects the number of electrons that flow through the electric circuit

Water System Analogy of the Electric Circuit

The constricting effect of the pipes or wires that oppose the flow of water or electricity is resistance

The amount of water that flows through the water system depends on the pressure exerted by the pump and the resistance offered by the pipes

Likewise, in the electric circuit, the number of electrons (current) that flow through an electric circuit depends on the pressure (volts) exerted by the generator and the resistance (ohms) offered by the wires

Electricity Flow Electricity may flow in series

or parallel circuits Series circuits- When

electrons follow a single path from their source to their destination

The circuit provides only one path for current to flow through

The electrons must pass through every bulb

A break in any one bulb shuts off the current for all other bulbs

Electricity Flow Parallel Circuit- When more than

one path is provided for the passage of electrons, the electrons divide themselves among the separate conductors, and current flows in each of the paths

A break in the filament of one bulb will shut off that bulb only

All other bulbs will still receive electrons and pass through them

These bulbs will remain in operation

Characteristics of Series and Parallel Circuits

Series Circuit One light goes out, all go out Bulbs becomes dimmer as

lights are added Current is reduced with

addition of bulbs Total resistance is increased

with addition of bulbs Current follows the same

path Voltage of the power source

is divided among ALL bulbs

Parallel Circuit One light goes out, the rest

remain on Bulbs remain same

brightness as more are added Current is increased with

addition of bulbs Total resistance is decreased

with addition of bulbs Current is divided among

several paths ALL bulbs have same

voltage, which is equal to the voltage of the power source

Continuity of the Circuit

An electric lamp (light bulb) is simply a wire called a filament encased in a glass envelope containing very little oxygen

For a light bulb to burn, a current must pass through its filament

As the electrons pass through the filament of the bulb, this special wire (usually made of tungsten metal) will heat up and give off heat and light energy

If the filament is broken, or if for any other reason the current cannot pass through, the bulb will not light

Amount of Resistance

When an electric current must pass through a high-resistance material, such as the filament of a light bulb, the number of electrons that pass through is reduced

In a series circuit, the current must pass through ALL resistors

Therefore, adding a resistor, such as an additional light bulb, increases the total resistance of the circuit

Amount of Resistance In a parallel circuit, just the reverse is true When an additional resistor is added, this provides a new path

for electrons The number of electrons that flow through the new resistor

adds to the number already passing through the established resistors

The total current in the circuit is increased even though the current through each of the established resistors remains the same

Since the current was increased, according to Ohm’s law the resistance must have been reduced

Adding resistors in a parallel circuit decreases the total resistance to the flow of electricity

Brightness of Bulbs

In the circuit, the brightness of the bulbs depends on the amount of current flowing through them

The total resistance of a series circuit increases with the addition of resistors

A greater resistance means a reduced current flow and dimmer light bulbs

In parallel circuits, the addition of a resistor should not affect the brightness of the bulbs

An exception occurs when a parallel circuit is powered by a limited source of power, such as a weak dry cell

The available electrons flowing out of the cell will be divided among all of the bulbs and the bulbs may grow dimmer

Magnitude of Current

The current in a series circuit is the same at any point in the circuit

The addition of a resistor any place in the circuit adds to the total resistance of the circuit and reduces the current that flows through it

In parallel circuits the addition of a bulb provides a new path for the flow of electricity and adds to the total current of the circuit

Voltage

The total voltage available to a circuit is determined by the source of power- a generator, a battery, etc.

The voltage is an expression of the energy that is available from the source

In a series circuit, the voltage of the power source is divided among the resistors

In a sample series circuit that has 3 bulbs of equal wattage and a 6 volt battery as a power source, each bulb would account for 2 volts of power or energy

If a 4th bulb is added, each bulb in the circuit will then receive ¼ of the available voltage, or 1.5 volts

Voltage

In parallel circuits, each bulb has available the full voltage of the power source

3 bulbs in a parallel circuit with a 6 volt power source each have the full 6 volts of power

If a 4th bulb were added, each bulb would still have 6 volts of available power

Electricity Produces Heat and Light

Electrons in electrical conductors sometimes yield energy in forms of heat and light

Thus, 2 very practical applications of electrical energy are the production of heat and light

Heat and light energy are given off from a conducting wire when the motion of the electrons that are being forced through the wire is opposed by the wire’s natural resistance

The opposition causes the electrons to give up energy to their surroundings

When the production of heat or light is desired, as in heating elements, stoves, and incandescent bulbs, high resistance iron, tungsten, or nichrome wires are used in the circuit

Electricity Produces Motion Each moving electron produces a

magnetic field about itself Many electrons moving through a

conductor will produce a strong magnetic field about the conductor

The magnetic field about the conductor may repel and/ or attract other magnetic substances causing them to move

This is the basis for the construction of electric motors

Electricity Produces Chemical Activity

Metal atoms often lose 1 or 2 electrons to nonmetallic atoms in the formation of salts

Metal atoms are then said to be metal ions

Metal ions can become metal atoms again when they replace their lost electrons

Metal atoms may be deposited from a salt solution upon the surface of a substance that provides the needed electrons

This is the basis for the electrolytic or electroplating cell