rg m3 3.4 generation of electricity

If light falls upon certain semiconductors, such as silicon or germanium, it may cause

charge separation and therefore generate electricity.

the model for conduction of electricity in a semiconductor is very different from that of a metal. Where metals are pictured as crystals with a framework of fixed ions and free electrons, semiconductors are pictured as

a framework of fixed atoms joined by covalent bonds. These covalent bonds allow some conduction when they contain extra electrons, or spaces where electrons should be known as holes

holes behave as

positive charges. As electrons shuffle from one covalent bond to the next to fill the hole, the hole effectively moves the other way.

A semiconductor’s conductivity depends on the

rate at which the crystal bonds are broken

The breaking of bonds of a semi conductor releases an

electron and a hole

the conductivity i.e The rate at which the bonds break in a semiconductor depends on the

rate of vibration of the atoms i.e., on the amount of thermal energy stored in the material.

Energy input, such as that from light, increases

the vibration of the atoms and therefore increases the conductivity of semiconductors.

By adding some impurities (a process called doping), we can create semiconductors that tend to produce either

more electrons (negative or n-type) or more holes (positive or p-type)

A photovoltaic cell (solar cell) is made by

placing a thin slice of n-type silicon over a thicker slice of p-type silicon.

When light falls upon the region where P-N silicon meet, called the

P-N junction

both electrons and holes flow across the

junction and an Electromotive Force (EMF) is generated.

Strips of metal on the top and bottom surfaces of the photovoltaic cell act as

terminals which may be connected to an external circuit, providing a source of electrical power.

Thermal energy stored in a material may be transferred directly to electrical energy in a circuit by making use of the

thermoelectric effect.

operation of a thermocouple, where two different metals are twisted or welded together to form a junction. A small

EMF is produced across this junction due to the difference in how closely the electrons are bound to their atoms in each material This EMF increases when temperature increases in a very predictable way, and the small current generated may be used to measure temperature differences very accurately.

thermocouple sensors are used to measure the

Exhaust Gas Temperature (EGT) of an aircraft's turbine engine.

Larger and more efficient thermoelectric generators may be constructed from semiconductor materials, similar to the construction of a photovoltaic cell but making use of

thermal energy rather than light energy. These may be used to generate moderate amounts of electricity but are not efficient enough for high-power applications.

Rubbing an insulator causes

electrical charges to separate and is a source of EMF. The voltages generated can reach very high values – up to thousands of volts – and can present a hazard in some situations.

Personnel working with static sensitive electronic components usually wear

a protective bonding braid around the wrist, which is connected to earth in order to prevent the build-up of such static charge.

Static electricity can be generated whenever

two materials contact or separate and at least one is a poor conductor of electricity.

Static electricity also occurs when insulating liquids and gases

flow

Rubbing a balloon on an item of clothing leaves the balloon with an electric charge, causing it to

produce small sparks and be attracted to the ceiling or wall of a room.

Two bodies of different charge have a voltage between them, meaning

there is an electrical field between these two bodies.

an electric charge would feel the attractive or repulsive force due to

Coulomb's law.

The arrows show the direction of the force on a positive charge, and the force is higher where

the lines are closer together.

If the electric field becomes too strong, electrons move through

the normally insulating air towards the positively charged body.

The electrical field tends to be stronger at

points and corners, which is where sparking occurs.

the primary reason for static dischargers on aircraft. These features are designed to

dispel the static charges that have built up on the aircraft, created by friction as the aircraft flies through the air. They are usually attached to the trailing edges of the wings, elevators, and fins, where the electric field is more concentrated (and the field lines are closer together).

Using pressure to deform a piezo crystal generates a

voltage This is referred to as the “piezoelectric effect”.

Exerting mechanical stress on certain solid materials such as crystals separates charges within the crystal and is therefore a source of EMF. The action is reversible. if a voltage is applied to this material

internal mechanical strain is generated.

The charges are collected using certain materials, e.g., aluminium foil across the surfaces of the crystal, to form

terminals

The piezoelectric effect is applied in

crystal pick-ups of record players, crystal microphones, crystal loudspeakers, lighters, gas lighters, pressure gauges, and dynamometers.

in a chemical action Voltage is generated by the separation of charge. Thus, a difference in charge between two points occurs, i.e.,

a potential difference.

immersing two different pieces of metal (e.g., copper and zinc) into an electrically conducting liquid (e.g., water with added sulphuric acid) generates

a voltage between the two electrodes (metals).

Certain metals, such as zinc, tend to

force positive metal ions into the solution.

Other metals, e.g., copper, tend to extract

positive metal ions from the solution.

Compared to the solution, the metals are

positively charged. Thus, there is a charge difference between the two metals, i.e., a voltage.

Voltage generators based on these chemical principles are referred to as

“galvanic cells” (primary and secondary cells).

All batteries work on the principle of

transferring energy from the chemical energy store in the cell to electrical energy in the circuit.

If a conductor moves through a magnetic field, and that motion is in some part at right angles to the direction of the magnetic field, then

an EMF is induced across the conductor. This is known as the “generator effect” and is the principle behind the operation of dynamos and alternators.

the wire containing free electrons is moving towards you (out of the page), which is perpendicular to the magnetic field, which is directed vertically downwards. This causes a force to act on the electrons, deflecting them to the left. This force is perpendicular to both the magnetic field direction and the direction of movement of the wire. This results

in a charge difference at either end of the conductor – an electron deficiency on one side and an excess on the other – thereby generating an electrical voltage and source of EMF. Energy is transferred from the kinetic energy store of the moving conductor to electrical energy in the circuit.

An EMF is induced when

either the coil or the magnetic field move relative to one another.

Electromagnetic induction is the most important form of voltage generation in

engineering

As the operating principle of all generators, it is referred to as the

generator principle

in a generator The induced voltage increases in proportion to

the magnetic field strength, the movement speed, and the effective length of the conductor.

Electromagnetic induction is the principle behind

electrical generators, magnetic pick-up devices, moving-coil microphones, and field telephones.

Most electrical generators use

electromagnets (rather than permanent magnets) arranged in a static ring around the outside, called the stator

rotating set of coils on an axle running through the stator, called the

rotor

Most generators provide an alternating voltage and are often referred to as

alternators