Forces and energy: Kinetic, potential, conservation and transformation

Forms of energy

The two broad forms of energy are potential and kinetic and each have different types, which we outline in more detail below. Others energy forms include sound and thermal energy. We will focus on potential and kinetic here. Light could also be considered a form of energy, but it gets interesting because is has both particle and wave-like properties. In the context of electromagnetic radiation, which ranges from radio to gamma waves, it is all just light with different amounts of energy.  For an in-depth look at light, See FLEET Schools teacher resource, Light: reflection, refraction, diffraction

Potential energy: This is the energy associated with either the position of an object and the forces being exerted on it (e.g., a skateboarder stationary at the top of a ramp), or its structure (e.g., the chemical bonds in different molecules). The many types of potential energy include gravitational, chemical and elastic. Each can be defined in different ways. Collectively, however, potential energy represents the potential that something has to do work.

The skateboarder positioned at the top of the ramp (stationary) has potential gravitational energy. When they lean forward, however, and start hurtling down the ramp, that potential energy is transformed into kinetic energy – movement.

Get students to do Activity 4. Potential energy, to take a deeper dive into this topic and learn how to calculate how much potential energy something has. See also the Powerpoint presentation.

Kinetic energy: Kinetic energy is movement, or the energy of a moving object. To get an object to move we must apply a force. The amount of kinetic energy something has is dependent on its mass and velocity (how fast it is moving). The greater the mass and velocity, the greater the kinetic energy. If something is not moving then it has no kinetic energy.

We can express this relationship between energy, mass and velocity mathematically:

Kinetic energy = ½mv²

Where m=mass (kilograms) and v=velocity (m/sec).

Students can explore this relationship in more detail in Activity 5 Kinetic energy. See also the Powerpoint presentation.

Question: What other famous equation describes a relationship between energy, mass and speed?

Way back in 1907, Einstein published his paper that described the now famous equation, E=mc2, to describe the relationship between energy, mass and the speed of light (velocity). This equation changed how we think of energy and inertia (matter and movement – or lack of, as the case may be). We will examine this and another not so famous equation in the Quantum energy topic.

Transfer of energy and Conservation of energy

There are two really important points to make here: Energy has many forms, but we cannot create or destroy energy – it is conserved. And, for energy to do work it needs to transform from one form to another. These points are probably more important than understanding what energy is because how energy changes allows us to predict how things will behave in the system we are studying, whether that system is at the atomic scale or the universe itself.

Conservation of energy

The conservation of energy is underpinned by the 1^st law of thermodynamics, which simply means that the entire energy of the universe is conserved or remains constant. We can only use the energy that already exists. If we use energy to do work such as move a box or skate down a ramp, the energy does not disappear; it is not used up. The energy used to do the work is just transformed into other forms of energy. We will examine how this happens in some of the activities coming up.

A cool thought: From the moment of the Big Bang when our universe was created, all the energy generated at that moment has remained constant since then. No more energy created; none lost.

Transformation of energy

Let us use potential energy as the starting point here, though it could be any form of energy. For example, living things such as humans and plants have potential energy in the chemical bonds in the fats, carbohydrates and sugars stored in the body. Batteries have a different form of potential chemical energy, in the form of their charged particles stored in the two halves of the battery (the positive and negative).

To do work, we need to transform that potential energy into another form. But remember the total amount of energy in the system remains unchanged. It is conserved.

Unfortunately, not all the energy in for example the chemical bonds in fats or carbohydrates, or the battery, is used to do the work we want. There are inefficiencies in the systems. Think about when you use your laptop or mobile phone. They need electrical energy to do work (compute stuff, give off light, sound and heat). You will notice your laptop and phone getting hot if you use it for more than a few minutes. This is caused by electrical resistance, which happens when electrons flowing through a circuit (kinetic energy) interact with the atoms in the circuit. Some of the kinetic energy in electrons transfers to the atoms, which makes them jiggle a lot more and give off heat. This heat energy cannot do useful work for us and it is wasted energy. For an in-depth look at electricity and circuits check the FLEET Schools resource, Electricity, conductors and insulators.

Students can test out the impact of the transfer of energy in the following hands-on activities:

Activity 6. Double bounce

Activity 7. Catapults

Activity 8. Energy transfer. How far does chocolate get you?

See also Powerpoint presentation.