Energy and Energy Conversions (Junior Cert Science): Revision Notes

Energy and Energy Conversions

What is energy?

Energy is the ability to do work. When you feel tired, you might say you have no energy. This means you feel unable to work or move around. Energy is what allows us to get things done.

The unit of energy is the joule ($\text{J}$), named in honour of the English scientist James Joule.

Forms of energy

Energy exists in many different forms. Understanding these different forms helps us recognize how energy moves and transforms in the world around us. Let's examine the main types:

Potential energy

Potential energy is the energy that an object has due to its position or shape. There are two main types:

Gravitational potential energy occurs when an object is held at a height. Water held behind a dam stores energy due to its position. A car at the top of a hill has more energy than it does at the bottom. The higher an object is positioned, the more potential energy it possesses.

Elastic potential energy is stored when an object is stretched or compressed. A stretched bow stores energy that can be released. When you pull back the string of a bow and arrow, energy is stored in the bow. This energy is released when the arrow is fired.

Kinetic energy

Kinetic energy is the energy of an object that is in motion. A moving lorry, the wind, and falling water all have kinetic energy. Sometimes kinetic energy is called mechanical energy. The faster an object moves, the more kinetic energy it has.

Chemical energy

Energy is stored in chemicals. Explosives release this energy very quickly when they react. Food is an example of a substance containing chemical energy - our bodies release this energy slowly over a period of time. Oil and other fuels, such as gas and coal, also have chemical energy that is released when they are burned. Batteries contain chemicals that react with each other to release electricity.

Light energy

Light from the sun provides the energy for plants to grow and make food and oxygen for us (a process called photosynthesis). We can use solar panels to get energy to heat water in our homes and generate electricity to run traffic safety signs. Most of the energy present on Earth originated from the sun.

Heat energy

When energy is given to an object, the temperature of the object may increase. The particles that make up the object move around more and move faster. The energy that is needed to heat our homes is usually supplied from gas, oil, coal, turf, or electricity. In many electrical devices, chemical energy is changed into heat energy, and this is then changed into electrical energy.

A burning fuel changes chemical energy in the fuel into heat energy connected to a generator. The generator then converts kinetic energy to electrical energy.

Electrical energy

A wire carrying electricity can be used to supply energy to machines and heaters. This is a convenient type of energy as it can be easily converted into many other forms. Electrical energy can be transformed into:

• Kinetic energy (in a washing machine)

• Heat energy (in a kettle)

• Light energy (in a light bulb)

• Sound energy (in a speaker or radio)

  

Sound energy

Sound is a form of kinetic energy created when objects vibrate. This makes the air around the object vibrate and causes our eardrums to vibrate, allowing us to hear. Sound can also travel through other substances, such as water and metal.

Nuclear energy

The nucleus of an atom has an enormous amount of stored energy. When the nucleus is smashed or changed, the atom can release this energy. This is used in nuclear-powered electricity power stations and nuclear weapons. Nuclear energy is converted into heat energy and then into electrical energy.

Energy conversions

Energy can be changed from one form to another. Think of all the ways you use electricity in your home. It can be converted to kinetic energy in the washing machine, heat in a heater, sound and light in a television, and chemical energy when batteries are recharged. We have many appliances that convert electrical energy from one form to another. Energy conversions happen all around us.

The principle of conservation of energy

When energy is converted from one form to another, the total amount of energy in one form (for example, electricity) will be equal to the total amount of energy converted in its new forms (for example, heat and sound). We say that energy is conserved. This is a fundamental principle in physics.

Understanding Sankey diagrams

The energy flow in appliances can be shown more clearly using a Sankey diagram. A Tipperary-born scientist, H. Riall Sankey, came up with the graphic diagram shown below.

Energy efficiency

Earlier we looked at the law of conservation of energy. We learned that energy cannot be created or destroyed, but can only change from one form to another. However, when converting energy from one form to another, some energy is wasted or dissipated.

Energy dissipated means that energy is lost in a form that is less useful.

Calculating energy efficiency

The energy efficiency of an appliance is often expressed as a percentage. It tells us what proportion of the input energy is converted into useful output energy.

Comparing different devices

A compact fluorescent lamp (CFL) is much more efficient than a filament lamp. If a CFL uses $100$ joules of electrical energy and produces $75$ joules of light energy, its efficiency would be:

$\% \text{ energy efficiency} = \frac{75}{100} \times 100 = 75\%$

The CFL is much more efficient, as 75% of the incoming energy is converted into light, compared to only 10% of the filament lamp. Much less energy is dissipated as heat in the CFL.

EU energy labels

To make it easier for the public to understand the energy efficiency of an appliance, the European Union has made it law that products must display an energy rating from A to G when being offered for sale. The higher the classification on the scale, the more efficient the appliance is. Certain other information can also be indicated depending on the appliance, such as annual expected electricity consumption, loudness, water consumption, etc.

Sources of energy

The sun is our most important source of energy. All green plants trap energy from sunlight. Plants use this energy during photosynthesis to make food and oxygen. Without photosynthesis, we would run out of food and oxygen.

Our sources of energy can be classified into two categories: renewable and non-renewable.

Renewable Energy Sources

Wind energy

Explanation	The kinetic energy of the wind is converted to kinetic energy of a wind turbine and then into electricity in a generator.
Advantages	- Free source of energy - No $\text{CO}_2$ emissions, which cause global warming
Disadvantages	- Some people think that turbines are unsightly - Cannot rely on wind blowing all the time

Solar energy

Explanation	- Photovoltaic solar cells convert light energy into electricity - Solar thermal collectors are used to heat water and convert light energy into heat
Advantages	- Can be useful in areas where mains electricity is not easily available, e.g. in outer space and weather buoys at sea
Disadvantages	- Energy is only available by day, so it needs to be stored if needed after darkness

Hydroelectricity

Explanation	Rivers are dammed. The potential energy of the water is converted to kinetic energy in a turbine and then into electrical energy in a generator.
Advantages	- Free source of energy - Can be switched on and off in minutes - The dam can control flooding and store water for public supply and recreation - No $\text{CO}_2$ emissions
Disadvantages	- Land may need to be flooded when the dam is built, damaging natural habitat for plants and animals and perhaps people's homes

Biomass

Explanation	- Fast-growing plants, manure, and waste plant material, e.g. brown bin waste, are known as biomass - Rotting biomass can be converted to natural gas (methane). Alternatively, fast-growing plants are burned to make heat - Chemical energy in the plants is converted to heat energy in natural gas, alcohol or heat energy by burning
Advantages	- An alternative fuel to fossil fuel for cars and jet engines - No net release of $\text{CO}_2$
Disadvantages	- Land that may have been used to grow food will now be used to grow fuel. This will make food more expensive - Some biofuels produce large amounts of smoke when burned

Wave energy

Explanation	The kinetic energy of the waves of the sea is used to generate electricity. As a wave approaches, it causes joined tubes to move up and down. This motion pushes on a hydraulic ram, which turns a turbine. The turbine is connected to a generator, which makes electricity.
Advantages	- No $\text{CO}_2$ emissions - Enormous amounts of energy are available
Disadvantages	- During storms, the equipment could be destroyed - The amount of energy produced depends on the weather

Tidal energy

Explanation	The tide is blocked from coming in and out of a bay by a dam. Just like hydroelectricity, the potential energy of the water is converted into electrical energy.
Advantages	- Tides are reliable and the energy supply will never run out - No $\text{CO}_2$ emissions
Disadvantages	- Prevents ships and fish moving into certain areas - Only available during certain times

Geothermal energy

Explanation	- Cold water is pumped deep into the Earth and returns as steam - The hot water or steam is used to heat our homes - The steam can be used to turn a turbine. The kinetic energy in a turbine and then into electrical energy in a generator
Advantages	- No $\text{CO}_2$ emissions - Hot water could also be used to heat homes and glasshouses
Disadvantages	- Only possible in certain volcanic parts of the world - May be expensive to build - Can release toxic gases

Non-renewable sources of energy

When non-renewable energy sources are used up, they will not be regenerated (made) again in our lifetime. These sources were formed over millions of years – for example, the coal and oil that we burn.

Fossil fuels

Explanation	Fossil fuels are made from the remains of plants and animals. They can form natural gas, oil, peat, and coal. When burned, their chemical energy is converted into heat energy.
Advantages	- Easy to transport and store - Relatively cheap
Disadvantages	- They produce $\text{CO}_2$, which causes global warming - When burned, they may cause acid rain

Nuclear fission

Explanation	Nuclear fission occurs when the nucleus of a large atom is split into two smaller nuclei with the release of large amounts of energy. Nuclear energy is converted into heat energy and then into electrical energy.
Advantages	- Vast amounts of energy can be produced from a small amount of fuel - No $\text{CO}_2$ or acid rain is produced, unlike fossil fuels
Disadvantages	- Reactors are expensive to build - Accidents at nuclear power stations, e.g. Chernobyl, have released radioactivity that has caused birth defects, cancer, and death

Sustainable energy usage

We need to replace burning fossil fuels with renewable energy sources. However, we cannot yet rely on renewable energy sources to supply the electricity needs of the country. There are times when we will not have enough wind energy, hydroelectricity, and sunlight to meet the energy needs of the country.

We need to use technology that is as energy efficient as possible. Over 50% of the population in the world has seen an increase in their standard of living with the building of many coal-fired power stations, which give out vast amounts of carbon dioxide into the atmosphere. This is not sustainable.

Things we can do to make our energy use more sustainable

Calculating energy use

The energy used to do a task is often referred to as work. The amount of work needed to push a box from place to place depends on the force needed to push the box and the distance the box is moved. Energy is measured in units called joules.