What is energy?

http://www.eschooltoday.com/energy/kinds-of-energy/all-about-energy.html

Look around you. Is anything moving? Can you hear, see or feel anything? Sure… this is because something is making something happen, and most probably, there is some power at work. This power or ability to make things happen is what we can call energy. It makes things happen. It makes change possible.

Look at the sketch below to see an example of things working, moving, or happening… with energy.

Energy move cars along the roads and make aeroplanes fly. It plays our music on the radio and lights our homes. Energy is needed for our bodies, together with plants to grow and move about.

Scientists define ENERGY as the ability to do work. Energy can be neither created nor destroyed.

KINDS OF ENERGY

With the above explanation in mind, let us learn more. Energy can be (is) stored or transferred from place to place, or object to object in different ways. There are various kinds of energy. Let’s start by looking at kinetic energy.

Kinetic Energy

All moving things have kinetic energy. It is energy possessed by an object due to its motion or movement. These include very large things, like planets, and very small ones, like atoms. The heavier a thing is, and the faster it moves, the more kinetic energy it has.

Now let’s see this illustration below.

There is a small and large ball resting on a table.

Let us say both balls will fall into the bucket of water.

Let’s see what is going to happen.

You will notice that the smaller ball makes a little splash as it falls into the bucket. The heavier ball makes a very big splash. Why?

Note the following:

1. Both balls had potential energy as they rested on the table.

2. By resting up on a high table, they also had gravitational energy.

3. By moving and falling off the table (movement), potential and gravitational energy changed to

Kinetic Energy. Can you guess which of the balls had more kinetic energy?

(The big and heavier ball)

Let’s see another classic example.

If you are in a hot room and you turn on the fan, what do you begin to feel? Air (wind). The speedy movement of the fan’s blades has kinetic energy, which is then transferred into air (wind) that you now feel.

Other examples of Kinetic Energy include a moving car, moving wheel, and a moving arrow.

Mechanical Energy

Mechanical energy is often confused with Kinetic and Potential Energy. We will try to make it very easy to understand and know the difference. Before that we need to understand the word ‘Work’.

‘Work’ is done when a force acts on an object to cause it to move, change shape, displace, or do something physical. For, example, if I push a door open for my pet dog to walk in, work is done on the door (by causing it to open). But what kind of force caused the door to open? Here is where Mechanical Energy comes in.

Mechanical energy is the sum of kinetic and potential energy in an object that is used to do work. In other words, it is energy in an object due to its motion or position, or both. In the ‘open door’ example above, I posses potential chemical energy (energy stored in me), and by lifting my hands to push the door, my action also had kinetic energy (energy in the motion of my hands). By pushing the door, my potential and kinetic energy was transferred into mechanical energy, which caused work to be done (door opened). Here, the door gained mechanical energy, which caused the door to be displaced temporarily. Note that for work to be done, an object has to supply a force for another object to be displaced.

Here is another example of a boy with an iron hammer and nail. In the illustration below…

(1) The iron hammer on its own, has no kinetic energy, but it has some potential energy (because of its weight).

(2) To drive a nail into the piece of wood (which is work), he has to lift the iron hammer up, (this increases its potential energy because if its high position).

(3) And force it to move at great speed downwards (now has kinetic energy) to hit the nail.

The sum of the potential and kinetic energy that the hammer acquired to drive in the nail is called the Mechanical energy, which resulted in the work done.

Sound energy Sound is the movement of energy through substances in longitudinal (compression/rarefaction) waves.

Sound is produced when a force causes an object or substance to vibrate — the energy is transferred through the substance in a wave. Typically, the energy in sound is far less than other forms of energy.

Let’s see this illustration.

A vibrating drum in a disco transfers energy to the room as sound. Kinetic energy from the moving air molecules transfers the sound energy to the dancers eardrums. Notice that Kinetic (movement) energy in the sticks is being transferred into sound energy.

Sound vibrations create sound waves which move through mediums such as air and water before reaching our ears.

The diagram below shows how a sound wave is represented:

Sound energy is usually measured by its pressure and intensity, in special units called pascals and decibels. Sometimes, loud noise can cause pain to people. This is called the threshold of pain. This threshold is different from person to person. For example, teens can handle a lot higher sound pressure than elderly people, or people who work in factories tend to have a higher threshold pressure, because they get used to loud noise in the factories.

Heat (Thermal energy)

Matter is made up of particles or molecules. These molecules move (or vibrate) constantly. A rise in the temperature of matter makes the particles vibrate faster. Thermal energy is what we call energy that comes from the temperature of matter. The hotter the substance, the more its molecules vibrate, and the therefore the higher its thermal energy.

For example, a cup of hot tea has thermal energy in the form of kinetic energy from its vibrating particles. When you pour some milk into your hot tea, some of this energy is transferred from the hot tea to the particles in the cold milk. What happens next? The cup of tea is cooler because it lost thermal energy to the milk. The amount of thermal energy in an object is measured in Joules (J)

We cannot discuss thermal energy without touching on Temperature. Heat and Temperature are not the same thing.

Temperature The temperature of an object is to do with how hot or cold it is, measured in degrees Celsius (°C). Temperature can also be measured in a Fahrenheit scale, named after the German physicist called Daniel Gabriel Fahrenheit (1686 – 1736). It is denoted by the symbol ‘F’. In Fahrenheit scale, water freezes at 32 °F, and boils at 212 °F. In Celsius scale, water freezes at 0°C and boil at 100°C.

A thermometer is the instrument used to measure the temperature of an object.

Let’s look at this example to see how thermal energy and temperature are related:

A swimming pool at 40°C is at a lower temperature than a cup of tea at 90°C. However, the swimming pool contains a lot more water. Therefore the pool has more thermal energy than the cup of tea even though the tea is hotter than the water in the pool.

Let us see this example below:

If we want to boil the water in these two beakers, we must increase their temperatures to 100°C. You will notice that will take longer to boil the water in the large beaker than the water in the small beaker. This is because the large beaker contains more water and needs more heat energy to reach 100°C.

Conduction, Convection and Radiation. Heat can be tranferred from particle to particle or object to object in three different ways. These are Conduction, Convection and Radiation.

Chemical energy

Chemical Energy is energy stored in the bonds of chemical compounds (atoms and molecules). It is released in a chemical reaction, often producing heat as a by product (exothermic reaction). Batteries, biomass, petroleum, natural gas, and coal are examples of stored chemical energy. Usually, once chemical energy is released from a substance, that substance is transformed into an entirely new substance.

For example, when an explosive goes off, chemical energy stored in it is transferred to the surroundings as thermal energy, sound energy and kinetic energy.

Let’s see one good example in the fire-place illustration below.

The dry wood is a store of chemical energy. As it burns in the fireplace, chemical energy is released and converted to thermal energy (heat) and light energy. Notice that the wood now turns into ashes (a new substance)

Food is also a good example of stored chemical energy. This energy is released during digestion. Molecules in our food are broken down into smaller pieces. As the bonds between these atoms loosen or break, a chemical reaction will occur, and new compounds are created. When the bonds break or loosen, oxidation occurs almost instantly.

In the example above, notice that new compounds are formed from the breakdown of other molecules or atoms. Chemical reaction causes that.

A chemical reaction is involved in this breakdown. The energy produced keeps us warm, maintain and repair bodies, and makes us able to move about. Different foods store different amounts of energy. Energy in food is measured in kilocalories (or Calories). Can you think of some very good examples of chemical energy?

Electrical energy

Matter is made up of atoms. In these atoms, there are some even small stuff called electrons that are constantly moving. The movement of these electrons depend on how much energy is has. This means every object has potential energy, even though some have more than others.

Humans can force these moving electrons along a path from one place to the other. There are special mediums (materials) called conductors, that carry this energy. Some materials cannot carry energy in this form, and they are called insulators. We generate electrical energy whey we succeed to cause the these electrons to move from one atom to the other, with the use of magnetic forces.

Once we harness electrical energy, it can be used for work or stored.

How does an elctric current work?

A battery transfers stored chemical energy as charged particles called electrons, typically moving through a wire. For example, electrical energy is transferred to the surroundings by the lamp as light energy and thermal (heat) energy. Lightning is one good example of electrical energy in nature, so powerful that it is not confined to a wire. Thunderclouds build up large amounts of electrical energy. This is called static electricity. They are released during lightning when the clouds strike against each other.

Gravitational Energy, Potential Energy

It is important to know the difference between potential energy and gravitational energy.

Every object may have Potential energy but Gravitational energy is only stored in the height of the object. Any time that a heavy object is kept high up, a force or power is likely to be holding it up there. This is the reason why it stays up and does not fall. It is important to note that the heavier the object, the more its potential energy.

Let’s see the diagram below:

Consider Mr Green throwing a bag of gold coins to Mr Red, who is up a tower. As Mr Green throws the bag from A to B, potential energy is transferred from Mr Green into the bag, which now has kinetic energy.

As the bag moves upwards, kinetic energy decreases, and gravitational energy increases. At the highest point (B) Kinetic energy is zero, and Gravitational potential energy is highest.

As the bag did not get to Mr Red, it start falling from point B downwards due to gravity. It starts falling slowly (kinetic energy is low) and then speeds up downwards.

At point A, bag is at full speed and kinetic energy is highest, whiles gravitational energy is nearly lost.

When the bag of gold coins hits the ground, kinetic energy is converted into heat and sound by the impact.

It is worth noting that the higher the gravitational energy of an object moving downwards, the lower the kinetic energy, and the lower the kinetic energy of an object moving upwards, the higher its gravitational energy.

Radiant Energy

Radiant energy is energy of electromagnetic waves. It is a form of energy that can travel through space. For example, we receive the heat from the sun, which is located very far from the earth via radiation. The sun’s heat is not transmitted through any solid medium, but through a vacuum. This is possible by electromagnetic waves.

Before we go any further, let us understand what electromagnetic waves are.

Each time static energy from electric and magnetic force come together, they induce an electric field around them.

An example of electric static force is the shock you get when you hold a metal door knob.

An example of a magnetic force is the pull that attracts metals to the magnet. Now, the electrical field induced causes waves, called electromagnetic waves, and they can travel through vacuum (air), particles or solids. These waves resemble the ripple (mechanical) waves you see when you drop a rock into a swimming pool, but with electromagnetic waves, you do not see them, but you often can see the effect of it.

The energy in the electromagnetic waves is what we call radiant energy.

There are different kinds of electromagnetic waves and all of them have different wavelengths, properties, frequencies and power, and all interact with matter differently. The entire wave system from the lowest frequency to the highest frequency is known as the electromagnetic spectrum. The shorter the wavelength, the higher its frequency and vice versa. White light for example, is a form of radiant energy, and its frequency forms a tiny bit of the entire electromagnetic spectrum.

In the illustration above, you will see the different radiant energy levels represented by their wavelengths.

When radiant energy comes into contact with matter, it changes the properties of that matter. For example, when micro-waves (which forms part of the entire spectrum) are set off in a microwave oven, the water molecules in the food are charged and caused to vibrate billions of times per second, generating heat, that causes the food to cook. The microwave oven works with the concept of radiant energy (electromagnetic waves).

Energy Stored

Energy cannot be created or destroyed, but it can be saved in various forms. One way to store it is in the form of chemical energy in a battery. When connected in a circuit, energy stored in the battery is released to produce electricity.

If you look at a battery, it will have two ends: a positive terminal and a negative terminal. If you connect the two terminals with wire, a circuit is formed. Electrons will flow through the wire and a current of electricity is produced.

Energy can also be stored in many other ways. Batteries, gasoline, natural gas, food, water towers, a wound up alarm clock, a Thermos flask with hot water and even pooh are all stores of energy. They can be transferred into other kinds of energy.

Energy Transfer

In this diagram, we shall look at a bigger picture of how energy is transferred from one object to another, and from one state to the other. Take a close look and read the notes below:

A: Sun, source of solar energy. It transfers thermal (heat) and light energy to plants, humans and animals.

B: River, streams and waterfalls moving downstream are all a source or water energy (HydroPower). They are a store of kinetic energy by their movement downstream. Waterfalls also have gravitational energy.

C: Thermal and light energy from the sun is stored in plants as chemical and potential energy. When humans eat (plants) the stored energy is transfered to us. We use this energy to do work.

D: Heat energy from the sun is transfered to water bodies. This warms the water up. The result is stored thermal energy. The warm water heats the air over it. Warm air rises, so the air is now set into motion. The moving air now has kinetic enegy.

E: Kinetic energy in the moving air is a source of energy called Wind Energy. Wind can turn the blades and generate electrical energy, which we use in our homes.

F: Energy from the sun is captured as Solar energy by cells and pannels on top of our roofs. Solar energy is then transferred into electrical energy, which we use to warm our homes and turn the TV and computers on.

Energy Dissipation

When you slide a snooker ball on the board (table) it moves quickly, and then slowly, and gradually comes to a stop. This means that the kinetic energy (moving) of the ball decreases and eventually to zero when the balls come to rest. With zero external force on the pool table, the energy of the ball must be conserved. Then, what happened to the kinetic energy? Kinetic energy has been dissipated through friction.