Science Notes: Fundamental Forces of Nature

All forces in nature may be classified into four types. The gravitational force holds together the universe at large, plus the atmosphere, water, and us to the planet Earth. The electromagnetic force governs atomic level phenomena, binding electrons to atoms, and atoms to one another to form molecules and compounds. The strong nuclear force holds the nucleus together. The fourth force, the weak nuclear force, is responsible for certain types of nuclear reactions and has little bearing on energy sources today.

Table 7 shows the four forces, the property on which each acts, and examples of each force. Gravitation and electromagnetism are the two forces with which we will be primarily concerned, as these are the two forces that operate at the macroscopic level of environmental systems. These also currently form the basis of our most prevalent sources for energy technologies. The strong nuclear force is the strongest of the forces. Nuclear fusion reactions on the surface of the sun are the result of the nuclear strong force.

FORCE

RELEVANT PROPERTY

EXAMPLES


Gravitational


Mass

Weight of object near a planet;  force that keeps planets in their orbits around the sun


Electromagnetic


Electric charge

Force that keeps an electron in its orbit around the atomic nucleus; (i.e., attraction or repulsion between a “charged” plastic comb and a strand of hair)

Strong Nuclear

Isotopic spin*

Force that keeps protons and neutrons together in a nucleus

Weak Nuclear

Spin*

Force responsible for certain types of nuclear reactions

Table 7: The four fundamental forces (or, interactions) and the properties on which they act.  *Spin and isotopic spin are properties of elementary particles that we will not define here.


Exercise:
Find four “sources” of energy and identify the force responsible for the energy transformation. For example, a hydroelectric plant has a waterfall for its source, and the force responsible is gravitation.

What fundamental force is responsible for (a) wind, (b) solar, (c) tidal, and (d) nuclear energy? Explain your reasons.

Every force that we experience belongs to one of these four categories, even when the connection is hard to see. The force of friction, for example, is an electromagnetic force. The force of the explosion of a chemical explosive is also electromagnetic in origin. The energy of an atomic bomb (more correctly, a nuclear bomb) is released as a result of the action of strong nuclear forces.

Energy changes occur when matter changes position or matter changes state in the presence of these forces. What we call energy "production" is really energy transformation -- that is, energy is converted from a potential form to a form available to us for use.

Each type of force acts on a specific property of an object. The specific property refers to the aspect of an object that is necessary for that object to be influenced by the force -- or "to feel" the force. For example, gravitation acts on the mass of an object. Strictly speaking, as an object fall towards the Earth, the Earth is falling towards the object. Because of the mass difference, the total effect of the Earth on the object is much more than that of the object on the Earth. However, the gravitational force on each kg of Earth is the same as on each kg in the object.

In the case of the electromagnetic force, the object must have an electric charge. But if that object has no net electric charge, then an outside source of electromagnetic force cannot exert a force on that object. The space in which a force is felt is called the "field" of the force. Thus all objects on Earth are in Earth's gravitational field, the planets and moons are in the sun's gravitational field, as well as in their mutual gravitational field.

Potential and Kinetic Energy
Whenever we say that we are producing energy, what we really mean is that we are transforming energy from one form to another that is more usable. For example, water at the top of a waterfall has more gravitational potential energy than when is at the bottom of the waterfall, because the water at the top is further from the center of the Earth than at the bottom. So, if the water is allowed to fall from the top to the bottom, (that is, the Earth's gravitational force does work on the water moving it), then the energy stored as potential energy at the top becomes transformed into the kinetic energy of this water and we can use it to do work. This is the principle behind the production of hydroelectric power.

Exercise:
In each of the following cases, trace the chain of energy transformations from the sun to the energy in its final form:
a. A pot of water is boiled on an electric stove.
b. A 100 nt automobile accelerates from rest on a level road, climbs a hill at constant speed, and comes to stop at a traffic light.
c. A windmill pumps water out of a flooded field.
     

Potential energy, therefore, is the energy associated with different positions in the force field. The water at the top of a waterfall has higher gravitational potential energy than at the bottom because of the different positions in the gravitational field. Consider two points (A and B) in the Earth's gravitational field (g) where B is h meters higher than A. Then a mass (m) has a potential energy mgh higher than its potential energy at A. At a point 2h above A, the mass has a potential energy of 2mgh. So height is a measure of the potential energy.

Thus, an analogy with water and gravitational potential energy gives us a way to represent energy levels showing the potential energy state of a system in terms of horizontal lines. Thus we could say that the 100 m point above the lowest level in a waterfall has 980 Joules gravitational potential energy per kg of water above the lowest point.

m * g * h = E (Energy)

1 kg * 9.8 m/sec2 * 56 m = 549 J

Figure 12: Energy Levels Diagram (gravitational).

These formulas also demonstrate that potential energy is a representation of the position of a system in a field of force. The 1 kg of water in our example has higher potential energy when it is further away from the center of force (center of the Earth). At point A, the water is more "bound" (to the Earth) than at point C. We will use this idea later to draw the analogous levels to represent chemical potential energy.

 

PREV | NEXT