4. TYPES OF CURRENT

4.1.Direct current

Between the terminals of a battery there is a continuous stable flow of energy. This is called direct current. In the same way if we connect a light bulb to a battery the electrons always flow in the same direction with the same current.

4.2.Alternating current

The flow of electrons changes direction 50 times every second as if the positive and negative poles of the socket were constantly changing places. In addition the electric current is not always the same.
The variation of any electrical parameter over a period of time is an electrical signal.
The tension or voltage of domestic electricity is an alternating signal because it alternates between positive and negative values. Its waveform is also sinusoidal with a smooth regular shape.



4.3.The efficiency of alternating current

The average power of alternating current is equal to the direct current that is needed to produce the same effect. In the case of an alternating sinusoidal current the average power would be as follows:

V ef =V max / square root of 2

4.4. Transformers

There is an important difference between alternating and direct current. Alternating can be increased or decreased by a transformer. this allows us to transfer electrical energy at very high voltages and over hundred kilometres, without losing any of the energy as heat.

THEIR SYMBOL IS:

3. TYPES OF CIRCUITS

3.1. Series circuits

Two or more elements form a series circuit when the output of one element provides the input for the next element.

To calculate the total resistance of a circuit, we add the resistance values of each load:

RT = R1 + R2 + R3 ...

3.2. Parallel circuit

In a parallel circuit, the various components share the same input and output. In other words, the wires from both sides are joined together.

If identical batteries are connected in parallel, the voltage of the circuit won't increase. 

3.3. Combination circuit

A combination circuit has some elements connected in series, and some other in parallel.

In this type of circuit, the current remains constant between elements that are connected in series. At the same time, the voltage remains constant between elements that are connected in parallel.

2. ELECTRICAL QUANTITIES

2.1. Voltage or potential difference

A light bulb uses energy from the electrons to produce light and heat.

The amount of energy that a generator can transfer to electrons depends on its voltage. This is measured in volts.

2.2. Measuring electric current

Electric current is the charge of electrons that flow through the cross-sections of a conductor every second.

Electric current is measured in amperes or amps in the International System or SI.

2.3. Electrical resistance: Ohm's Law

The Resistance of a material is equal to the voltage divided by the intensity of the electric current which travels throgh the material. This ratio, whic is called Ohm's Law, can be expressed as follows:

R = V / I

V = R x I

I = V / R

2.4. Electrical energy and power

Electrical energy

Electric energy is the energy that keeps the electrons of a circuit in motion. This energy is provided by a generator and consumed by one or more loads.

Electric power

The electric power of a load is the amount of energy that it can transform over a certain amount of time. Electric power is measured in watts or kilowatts. One Kilowatts equals 1000 watts.

P = V x I

1. AN ELECTRIC CIRCUIT

An electric circuit is a pathway for the flow of electrons. It consists of different parts. Electric current is a continuous flow of electrons through a circuit.

1.1. Parts of an electric circuit

Electric circuits consists of various parts:

- Generators provide the energy that electrons need in order to move.

- Loads are devices that transform electrical energy into other types of energy that we can use.

1.2. Diagrams and symbols

We use a system of conventional symbols to simplify the visual representation of electric and electronics circuits, making them easier to understand.

UNIT 7: ELECTRIC CIRCUITS AND ELECTRONICS

1.- An electric ciruit

2.- Electrical quantities

3.- Types of circuits

4.- Types of current

5.- Efects of electric current

6.- Electromagnetic mechanisms

7.- Electromagnetics control systems

8.- Electronics

7. ENVIRONMENTAL IMPACT

Power station have an important impact on local ecosystems.

7.1.Environmental impact assessment

Any proposed new technological project should include an environmental impact assessment.There should also be an assessment of the economic and social repercussions of the project before any important decisions are made. 

7.2. Effects on the environment

Extracting natural resources

Fossil fuels and radioactive elements , such as uranium, must be extracted from underground deposits.
Large areas of forest have been destroyed to provide wood for fuel.

Transporting fuel

Most oil is transported over land through pipelines and by sea in large ships called oil tankers.
Natural gas is transported over land through gas pipelines or bu sea in tankers as liquid natural gas (LNG).These tankers are call LNG carriers.

Generating electricity
Hydroelectric power stations require large amounts of water which must be stored behind dams in reservoirs.

Waste treatment

Filters can reduce pollutants , such as nitrous oxide and sulphur.
Low-sulphur coal can also be used to reduce sulphur emissions.
Large forests should be protected because they remove CO2 from the air.

Nuclear waste is kept in special containers with thick walls that block radiation.These containers are stored in nuclear graveyards,which are usually located underground or in deep ocean trenches.

7.3. CLIMATE CHANGE

Most of our electricity comes from power stations that use non renewable-energy sources.This situation has negative effects on the environment and the most serious is climate change a problem that is associated specifically with fossil fuels.

6.POWER STATIONS THAT USE RENEWABLE RESOURCES

6.1.Wind farms

Wind farms use the kinetic energy of the wind to generate electricity.The wind turns the blades of a turbine at the top of a tower.The blades are connected to a gearbox which increases the rotational speed of the generator.

The output and efficiency of a wind farm depend on two factors:
-the location of the farm,which determines the speed and strength of the winds.
-the number of turbines that can be installed there.

6.2.Hydroelectric power stations

Hydroelectric power stations use the energy of falling water to produce electricity.
There are two main types of hydroelectric power stations:

-In conventional hydroelectric stations,the water flows from the reservoir to the turbines through a high-pressure conduit.Then it flows out, usually into a river.

-In pumped-storage hydroelectric stations,the water flows from the turbines to a second reservoir.Then it is pumped back up to the higher reservoir and stored for later.These stations are often built in areas where there isn't enough rain or river water to keep the upper reservoir full, so the water must be reused.
 

Solar power stations use energy from sunlight to generate electricity.There are two types of solar power stations:solar thermal and photovoltaic.

6.3. Solar thermal stations

Solar thermal stations can use sunlight in two ways:
-With solar collectors that absorb sunlight in order to produce heat.
-With mirrors called heliostats that reflect and concentrate sunlight in one place.

Photovoltaic stations
In photovoltaic stations, solar pannels convert sunlight directly into electricity.Each panel contains many photovoltaic cells.
Small solar power installations can provide energy for homes and rural areas.Excess power can be stored in batteries or accumulators and used at night.

6.4.Biomass power stations 

Biomass is any organic material that is produced by natural processes.

There are many types of biomass which can be used to produce energy.
Plants materials can be composted and fermented in tanks called digesters.These processes convert biomass into fuel.

In a biomass power station the fuel used to produce energy comes from biomass.The steam produced from burning the biomass moves a turbine that is connected to a generator.

6.5.Marine power stations

Marine power stations use the movement of ocean water to generate elctricity.
There are three general types of marine power stations:

-Tidal power stations,which use the energy of tides.



-Wave power stations,which use the energy of waves.


-Ocean thermal conversion stations,which use the difference in water temperature between the surface of the ocean and deeper areas to produce energy.

6.6.Geothermal power stations 

Geothermal power stations use natural heat from the deepest underground layers of our planet.
Geothermal energy can be used in two ways:
-It can be use directly to provide hot water for heating and industrial uses.
-It can be used indirectly to drive generators and produce electricity.