Welcome to our first Lascells Blog which will become a regular feature each month and will cover topics of interest to the physics student and educator. In this inaugural blog we will look at an “Introduction to Energy and Power”, indeed Power & Energy will be our focus for the first quarter of 2019.
Energy is one of the most important concepts of physics, but more than just a theoretical concept energy plays an essential role in everyday events. For example, you can no doubt name many forms of energy, from that provided by the food that we eat, to the energy we use to run our cars, to the sunlight that warms us on the beach. Indeed, the world use of energy resources, especially oil and gas continue to grow, with potentially dangerous consequences economically, socially, politically, and environmentally.
What makes it even more important is that the total amount of energy in the universe is constant. Energy can change forms, but it cannot appear from nothing or disappear without a trace. Energy is thus one of a handful of physical quantities that we say is conserved.
We are all aware of the principle that energy can neither be created nor destroyed and this is confirmed by experiment. Even as scientists discovered new forms of energy, conservation of energy has always been found to apply. Perhaps the most dramatic example of this was provided by Einstein when he suggested that mass is equivalent to energy (his famous equation E= mc2)
We can loosely define energy as the ability to do work, admitting that in some circumstances not all energy is available to do work. So, let’s take a close look at the relationship between work and energy.
Work and energy
Whenever a force is applied to an object, causing the object to move, work is done by the force. If a force is applied but the object doesn’t move, no work is done; if a force is applied and the object moves a distance d in a direction other than the direction of the force, less work is done than if the object moves a distance d in the direction of the applied force.
The physics definition of “work” is:
The unit of work is the unit of energy, the joule (J). 1 J = 1 N m.
Or in simple terms work is energy transferred during a transformation involving movement.
Work can be either positive or negative: if the force has a component in the same direction as the displacement of the object, the force is doing positive work. If the force has a component in the direction opposite to the displacement, the force does negative work.
If you pick a book off the floor and put it on a table, for example, you’re doing positive work on the book, because you supplied an upward force and the book went up. If you pick the book up and place it gently back on the floor again, though, you’re doing negative work, because the book is going down but you’re exerting an upward force, acting against gravity, If you hold a book at a constant height you don’t do any work on it, despite the fact that you have to exert an upward force to counteract gravity.
There are different types of energy, but the two main ones are Kinetic energy and Potential energy
Kinetic Energy
Kinetic Energy has 5 forms
- Mechanical energy –is the movement of objects or substances from one place to another
- Electrical energy – the energy from flow of electric charge (movement of electrons in one direction)
- Thermal energy – or heat energy, the internal energy of a substance due to the vibration of atoms and molecules making up the substance
- Radiant energy – or light energy, or electromagnetic energy that travels in transverse waves
- Sound energy – the movement of energy through substances in the form of compression waves
An object has kinetic energy if it has mass and if it is moving. It is energy associated with a moving object. For example, when you are walking or running your body has kinetic energy.
Potential Energy
Potential Energy has 4 forms:
- Gravitational energy – the energy an object has because of its position or height
- Chemical energy – the energy stored in the bonds between atoms that holds molecules together
- Nuclear energy – the energy stored in the nucleus of the atom that holds the nucleus together
- Elastic energy – or stored mechanical energy, is energy stored in an object by the application of force
Potential energy is defined as mechanical energy, stored energy, or energy caused by its position. The energy that a ball has when perched at a top of a steep hill while it is about to roll down is an example of potential energy.
The work-energy principle
There is a strong connection between work and energy, in a sense that when there is a net force doing work on an object, the object’s kinetic energy will change by an amount equal to the work done:
Work-Energy Connection – a more technical look
There is a relationship between work and total mechanical energy. The relationship is best expressed by the equation
TMEi + Wnc = TMEf
This equation can be described in words as:
“the initial amount of total mechanical energy (TMEi) of a system is altered by the work which is done to it by non-conservative forces (Wnc). The final amount of total mechanical energy (TMEf) possessed by the system is equivalent to the initial amount of energy (TMEi) plus the work done by these non-conservative forces (Wnc).”
The mechanical energy possessed by a system is the sum of the kinetic energy and the potential energy.
What do we mean by non-conservative forces?
Forces that do not store energy are called non conservative or dissipative forces. Friction is a non conservative force, and there are others. Any friction-type force, like air resistance, is a non conservative force.
The work done to a system by non-conservative forces (Wnc) can be described as either positive work or negative work. Positive work is done on a system when the force doing the work acts in the direction of the motion of the object. Negative work is done when the force doing the work opposes the motion of the object. When a positive value for work is substituted into the work-energy equation above, the final amount of energy will be greater than the initial amount of energy; the system is said to have gained mechanical energy. When a negative value for work is substituted into the work-energy equation above, the final amount of energy will be less than the initial amount of energy; the system is said to have lost mechanical energy. There are occasions in which the only forces doing work are conservative forces (sometimes referred to as internal forces). Typically, such conservative forces include gravitational forces, elastic or spring forces, electrical forces and magnetic forces. When the only forces doing work are conservative forces, then the Wnc term in the equation above is zero. In such instances, the system is said to have conserved its mechanical energy.
Some Practical Examples of Energy in Action
Let’s take the example of a dam which illustrates really well energy conversion not only from potential to kinetic, but from energy in which gravity provides the force component to energy based in electromagnetic force. A dam big enough to be used for generating hydroelectric power forms a vast steel-and-concrete curtain that holds back millions of tons of water from a river or other body. The water nearest the top thus has enormous potential energy.
Other examples include sound and chemical energy. Sound, which is essentially nothing more than the series of pressure fluctuations within a medium such as air, possesses enormous energy. Let’s consider the example of a singer hitting a certain note and shattering a glass.
Contrary to popular belief, the note does not have to be particularly high: rather, the note should be on the same wavelength as the glass’s own resonant frequency. When this occurs, sound energy is transferred directly to the glass and the amplitude of the vibrations increases until it shatters by this net intake of energy.
As for chemical energy, it is associated with the pull that binds together atoms within larger molecular structures. The formation of water molecules, for instance, depends on the chemical bond between hydrogen and oxygen atoms. The combustion of materials is another example of chemical energy in action.
Lascells and Energy
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