Generation and transmission of electricity Notes

How Is Electrical Energy Produced From Energy Resources?

1. Electricity is not a primary source of energy, it is an energy carrier.
2. Before it reaches a phone charger or a light bulb, energy must be converted from some source (often chemical energy in fuels, or kinetic energy of wind and water) into electrical energy, then transmitted efficiently to where it is needed.

What energy resources are used to generate electricity?

1. Electricity can be generated using both non-renewable and renewable energy resources.
2. The choice of energy resource affects how sustainable the electricity supply is.
3. Regardless of the resource used, most systems are designed to make a turbine rotate.

Non-renewable energy resources

1. Fossil fuels such as coal, oil, and natural gas store chemical energy.
2. Nuclear fuel stores energy in the nucleus of atoms.
3. These resources exist in limited quantities and will eventually run out.
4. They can release large amounts of energy, making them useful for large power stations.

Renewable energy resources

1. Renewable resources are replenished naturally over short timescales.
2. They rely on natural processes such as wind movement, flowing water, or sunlight.
3. Many renewable systems still use turbines and generators.
4. Some renewable systems convert energy directly into electrical energy.

Electricity Comes From Energy Transformations

1. A central idea in physics is the conservation of energy: the electrical energy we use must come from transforming energy from somewhere else.
2. On Earth, many energy resources can ultimately be traced back to the Sun.
3. Sunlight drives photosynthesis, storing energy as chemical energy in plants. Burning wood releases that stored energy.
4. Over millions of years, buried plant and animal matter can be transformed (by pressure, temperature, and chemical change) into coal, oil, and natural gas.
5. These are fossil fuels, and burning them releases energy that originally came from sunlight long ago.
6. Sunlight also drives the water cycle. Evaporation and rainfall can place water at higher altitude, giving it gravitational potential energy.
7. As it flows back downhill, this can be converted to electrical energy in hydroelectric systems.

The common energy transfer pathway

1. Energy from a resource is first converted into thermal energy or kinetic energy.
2. This energy is used to turn a turbine.
3. The turbine transfers energy to a generator.
4. The generator converts mechanical energy into electrical energy.

How Is Electricity Generated Using Fossil Fuels?

1. Fossil fuel is burned in a boiler.
2. Burning releases chemical energy as thermal energy.
3. The thermal energy heats water stored in pipes.
4. The water turns into high-pressure steam.
5. The steam flows over turbine blades.
6. The turbine begins to rotate.
7. The rotating turbine drives a generator.
8. The generator produces electrical energy.

Energy transfers in a fossil fuel power station

1. Chemical energy in the fuel is transferred to thermal energy in the boiler.
2. Thermal energy is transferred to kinetic energy of moving steam.
3. Kinetic energy causes mechanical energy in the rotating turbine.
4. Mechanical energy is converted into electrical energy in the generator.

Limitations of fossil fuel electricity generation

1. Burning fossil fuels releases carbon dioxide.
2. Carbon dioxide contributes to the greenhouse effect.
3. Fossil fuels are finite and will eventually be depleted.
4. Waste heat is released to the surroundings during generation.

How Is Electricity Generated Using Nuclear Energy?

1. Nuclear reactions release large amounts of thermal energy.
2. This thermal energy is used to heat water.
3. Steam is produced at high pressure.
4. Steam turns a turbine.
5. The turbine drives a generator.
6. Electrical energy is produced.

Key features of nuclear electricity generation

1. A very small amount of nuclear fuel releases a very large amount of energy.
2. No carbon dioxide is released during the electricity-generation stage.
3. Radioactive waste is produced and must be stored safely.
4. The turbine-generator system is similar to fossil fuel power stations.

How do renewable resources generate electricity?

1. Wind power

1. Wind is caused by uneven heating of Earth’s surface by the Sun.
2. Moving air has kinetic energy.
3. Wind turns turbine blades.
4. The turbine drives a generator to produce electricity.

2. Hydroelectric power

1. Water stored at height has gravitational potential energy.
2. When water flows downhill, this energy becomes kinetic energy.
3. Flowing water turns turbines.
4. Turbines drive generators.

3. Tidal power

1. Tides are caused by the gravitational pull of the Moon.
2. Rising and falling seawater moves through turbines.
3. Electricity generation follows predictable tidal patterns.

4. Solar power

1. Solar panels absorb light energy from the Sun.
2. Light energy is converted directly into electrical energy.
3. No turbines or generators are needed.

5. Geothermal power

1. Thermal energy from inside Earth heats underground water.
2. Steam is produced naturally.
3. Steam turns turbines connected to generators.
4. This method is only possible in suitable geological areas.

Why Must Electricity Be Transmitted Over Long Distances?

1. Power stations are often located far from cities and towns.
2. Electricity must be transported from where it is generated to where it is used.
3. Transmission allows one power station to supply electricity to many locations.
4. Efficient transmission is important to reduce energy waste.

Why is energy lost in transmission cables?

1. Transmission cables have electrical resistance.
2. When current flows through a cable with resistance, the cable heats up.
3. This heating represents energy loss to the surroundings.
4. The lost energy is mainly transferred as thermal energy.

What factors increase energy loss?

1. Larger electric current causes more heating.
2. Longer cables increase resistance.
3. Thinner cables have higher resistance.

What Is A Transformer?

1. A transformer is used to change the voltage of an AC supply.
2. Transformers only work with alternating current.
3. Voltage is increased for transmission and decreased for safe use.

Step-up and step-down transformers

1. A step-up transformer increases voltage.
2. Step-up transformers are used before long-distance transmission.
3. A step-down transformer decreases voltage.
4. Step-down transformers are used before electricity enters homes and buildings.

Why transformers reduce energy loss

1. Step-up transformers increase voltage.
2. Increasing voltage reduces current.
3. Lower current causes less heating in cables.
4. This makes transmission more efficient.

Understanding efficiency in transformers

1. Not all input electrical energy is transferred to the output.
2. Some energy is lost as heat in the coils.
3. Some energy is lost as sound or heating of the core.

Efficiency is a measure of how much useful output energy is obtained compared to the input energy.

Efficiency of a transformer

1. A highly efficient transformer transfers most of the input energy to the output.
2. Modern transformers are very efficient but never 100%.
3. Efficiency is improved by reducing energy losses.

$$\text{efficiency} = \frac{\text{useful output power}}{\text{total input power}}$$

Why transformers are not 100% efficient

1. Electrical resistance in coils causes heating.
2. Energy is lost as sound due to vibrations.
3. Magnetic energy can be lost in the core.

How Is Electricity Distributed To Users?

1. After long-distance transmission, voltage must be reduced.
2. Step-down transformers lower voltage to safe levels.
3. Electricity is then distributed through local networks.
4. Homes, schools, and factories receive electricity at suitable voltages.

Putting it all together: the journey of electricity

1. Energy resources provide the initial energy.
2. Power stations convert this energy into electrical energy.
3. Electricity is transmitted at high voltage to reduce losses.
4. Transformers change voltage levels when needed.
5. Electrical energy is finally delivered safely to users.

Generators Produce Alternating Current By Electromagnetic Induction

1. A generator is closely related to a motor.
2. If a motor is operated "in reverse", a force turns the coil and a voltage is induced across it.
3. This happens because the coil (or magnets) move relative to each other so that conductors cut through magnetic field lines.
4. A generator does not require the coil to move specifically, the essential condition is relative motion between coil and magnetic field.
5. In some designs, magnets spin inside a fixed coil (like a simple dynamo).

How A Transformer Works (A.C. Only)

1. A transformer uses electromagnetic induction:
   1. The primary coil is connected to an a.c. supply.
   2. The changing current creates a changing magnetic field in an iron core.
   3. The changing magnetic field induces a voltage in the secondary coil.
2. If the secondary coil has more turns than the primary coil, the output voltage is higher.

Electricity Demand Changes, So Storage Helps Balance Supply

1. Electricity use varies with time:
   1. Winter often has higher demand (more heating and lighting).
   2. Evenings and weekends often have higher household demand than weekday daytime.
2. Power stations try to match supply to demand, but when excess electricity is generated, it may be wasted because electricity is difficult to store directly.
3. Two storage methods are:
   1. Pumped hydroelectric storage
   2. Compressed air energy storage

Cost And Efficiency Show Up In Everyday Electricity Use

1. Efficiency affects how much electrical power is needed for the same useful output.
2. Lighting is a clear example: different bulb types provide similar brightness with very different efficiencies and lifetimes.
3. The cost of electricity can be estimated using the idea that supplying $1\ \text{W}$ continuously for a year costs about \$1.40.