Energy Transfers

Change in Storage

Energy is the potential for a system to do work. Energy can exist in several forms:

Thermal (heat)

Gravitational Potential (energy due to height)

Kinetic (energy due to movement)

Electromagnetic including visible light

Sound

Electrical

Stretch or strain

Nuclear

Chemical (energy stored in chemical bonds)

Energy is measured in Joules (J) which is equal to a Newton-meter.

Energy is transferred from one form to other forms when an object (or system) moves or changes in any way.

Image you are placing a book on a shelf, the chemical energy stored in your food is converted to the kinetic energy of your muscles as you move, this in turn is converted to the kinetic energy of the book as you move it, then once upon the shelf the kinetic energy has been converted to gravitational potential energy. It remains stored in this form until someone moves the book or it falls off the shelf.

Objects do not have to move to change and, therefore, for there to be a transfer of energy. Your phone is a good example of this sort of change. When you charge the phone, the electrical energy may have come from burning gas. The chemical energy stored in the gas is converted to electrical energy, this then charges your phone battery and is stored as chemical energy in the battery. When you use the phone that chemical energy is converted to electrical and then into the light, sound and electromagnetic energy for the call, data (microwaves) or wifi signals (radio waves).

Energy is also transferred when an object is heated. A kettle takes in electrical energy and transfers it heat in the element. This in turn heats up the water. Whenever an object is heated energy is being stored in the molecules atoms of the object. The particles in objects move faster as they are heated, so they have more kinetic energy.

Energy transfers occur when

An object is moved by a force.

An electrical device is in use.

An object is heated (or cooled).

Transfer Diagrams

The changes in energy type as objects move or change can be represented in a diagram known as a Sankey Diagram. With these diagrams it is important to recall that all the energy going into a system must come out as some other type of energy. Energy is converted not created or destroyed.

The diagram represents a typical energy transfer for a light bulb. Electrical energy is transferred to Heat and Light energies. The size of the arrows can be used to represent the amount of energy and the values for each energy type should be added.

In this example 80% of the electrical energy is converted to light in this low energy light bulb. In an LED the light would represent 95% of the energy transfer.

Energy Conservation

The Law of Energy Conservation or the First Law of Thermodynamics, states that energy cannot be created nor destroyed. So in a closed system, where energy is not added from or removed to the surroundings, the total energy of the system is constant.

For any system:

Total Energy In = Total of all Energy Out

A Plasma TV will take in approximately 120 KJ of electrical energy in an evening, this is converted to light, sounds and some heat. If the amount of light, sound and heat were measured they would add up to the same 120 KJ of energy. No energy has been created or destroyed it has been converted into other types, but the total in equals the total out.

[Extension: The First Law of Thermodynamics is the classical way of thinking about conservation of energy and in simple situations like a TV or light bulb etc, it holds true. However, it is possible to convert mass into energy and energy into mass. This happens in a nuclear reactor for example. Einstein’s famous equation __E = mc2_ shows this is possible. (E is energy. m is mass and c the speed of light). To account for these situations the law has been changed to the Law of Conservation of Mass-Energy. The total of the mass and energy in a closed system is conserved.] _

Examples of Energy Transfers

Example 1: Cooking on a gas stove.

The initial energy is the chemical energy in the bonds of the gas. This is burnt to release some of that energy as heat. The heat is transferred to the food via the pan. Some of course escapes into the air or heats up other parts of the stove. Importantly the total energy in is equal to the total energy out.

Example 2: A Roller Coaster.

All roller coasters work on the same simple set of energy transfers.

At the start electrical energy is used to pull the cars to the highest point in the ride. The electrical energy is converted to gravitational potential energy (GPE). As the cars run down the slope this GPE is converted to kinetic energy (KE) and some heat lost due to friction. As the car run up the next slope the kinetic energy is converted back to GPE. This exchange from GPE to KE continues throughout the ride until too much energy is lost due to friction and the ride will come to an end.

Waste Energy

During any energy transfer some energy is wasted, normally as heat. Eventually all energy is transferred to heat and is dissipated into the surroundings. The amount of energy that is used to do a useful job in an energy transfer is a measure of the efficiency of the system or device.

This low energy light bulb, produces 80 J of useful energy but 20 J is wasted as heat which will spread out into the air and dissipate.

This heat is not in a useful form and can not be used to perform a useful function in this situation.

Which type of energy is considered waste energy depends upon the function. In a heating element that works in a similar way to a conventional light bulb, the heat is the useful energy, the light produced as it glows is, in this case, the waste energy and it will dissipate into the air.

Mechanical systems with moving parts always have some friction, this rubbing together of moving surfaces generates heat energy which is lost to the surroundings making the system less efficient. In most situations the amount of waste heat energy due to friction can be reduced by adding a lubricating material, like oil, to the moving parts.

In all systems as energy changes occur the energy is eventually converted to forms that are less useful. This is known as the Second Law of Thermodynamics which tells us that in any system the amount of energy in a less useful form will increases, (this is properly known as entropy - a measure of the amount of thermal energy in a system that cannot be used to do work).

This means that eventually all energy is converted to heat. For example, the chemical energy in petrol is converted to the kinetic energy of the car, but when the brakes are applied all that kinetic energy is converted to heat by the friction between the brake pads and the brake disc. The heat then dissipates into the air. All the energy that was in the form of the chemical energy was eventually ended up as heat.

[Fact: Eventually all energy in the universe will be in this form of heat and no more work will be possible, this is known as the Heat Death of the Universe.]

In the energy transfer about which of the energies are wasted energy in this system and what will happen to this energy?