Fundamental Forces: Gravity, Electricity, and Magnetism

Posted on Apr 16, 2025 in Physics

Fundamental Forces in Nature

Gravitational force and electromagnetic force are two fundamental forces found in the natural world.

Both are action-at-a-distance forces because their effects can be felt without direct contact.

Other types of fundamental forces include the strong nuclear force and the weak nuclear force.

Kepler’s Laws of Planetary Motion

Johannes Kepler described the motion of planets with three laws:

  • First Law: Planets move around the Sun in elliptical, almost circular, orbits. The aphelion is the farthest point from the Sun, and the perihelion is the closest point to the Sun.
  • Second Law: An imaginary line that connects a planet to the Sun covers identical areas in the same amount of time.
  • Third Law: Planets that are farther away from the Sun move slower than the planets that are closer to the Sun.

Newton’s Law of Universal Gravitation

Isaac Newton proposed the Law of Universal Gravitation.

All bodies are attracted to each other by gravitational forces. The direction of the force is along the line connecting the centers of the bodies.

Factors Affecting Gravitational Force

  • Depending on Mass: If the distance remains unchanged, the greater the mass of the objects, the greater the gravitational force between them.
  • Depending on Distance: Gravitational force decreases as the distance between the bodies increases.

Difference Between Mass and Weight

Mass is the amount of matter (‘stuff’) in an object. Weight is the force of gravity acting on that object.

The weight of a body is the gravitational force acting on it.

Calculating Weight on Earth and the Moon

Example 1: Weight of a 10 kg Body

  • On Earth: Weight = mass × gEarth = 10 kg × 9.8 N/kg = 98 N
  • On the Moon: Weight = mass × gMoon = 10 kg × 1.6 N/kg = 16 N

Example 2: Mass and Moon Weight of a Body Weighing 100 N on Earth

  • On Earth: Weight = mass × gEarth → mass = Weight / gEarth = 100 N / 9.8 N/kg ≈ 10.2 kg
  • On the Moon: Weight = mass × gMoon = 10.2 kg × 1.6 N/kg ≈ 16.3 N

Astronomical Distance Units

  • Astronomical Unit (AU): This is the average distance from the Earth to the Sun, approximately 150 million kilometers.
  • Light Year: This is the distance that light travels in one year.


Historical Discoveries in Electricity

  • 600 BC: Thales of Miletus described how rubbing amber with cat fur attracts lightweight objects.
  • 1600: William Gilbert discovered that other materials, such as glass, had similar properties to amber when rubbed. He called this phenomenon ‘electricity’.
  • 1733: Charles du Fay discovered two types of electric charge:
    1. Some materials behave like glass rubbed with silk.
    2. Others behave like amber rubbed with fur.
    He also proved that:
    • Bodies charged with the same type of electricity repel each other.
    • Bodies charged with different types of electricity attract each other.
  • 1747: Benjamin Franklin suggested that electricity was a fluid that moved from one body to another after being rubbed.
  • 19th and 20th Century: Electrically charged particles were discovered inside the atom.
    • 1897: J.J. Thomson discovered the electron (negatively charged).
    • 1918: Ernest Rutherford discovered the proton (positively charged).

How Objects Become Electrically Charged

Bodies can become electrically charged in three ways:

  • Friction: If we rub a balloon with a cloth, electric charges move from one object to another.
  • Contact: If we touch some neutral sheets of aluminum foil with a charged rod, the charge from the rod moves to the aluminum.
  • Induction: If we put a charged rod near a neutrally charged ball, without touching it, charges within the ball redistribute.

Coulomb’s Law

Coulomb’s Law states that electrically charged bodies attract or repel each other with a force that is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

Introduction to Magnetism

A magnet is an object capable of attracting certain metallic materials, such as any object made of iron.

Magnets have a North Pole and a South Pole. In a bar magnet, the poles are at the ends. In a horseshoe magnet, the poles are also at the ends.

There are two types of forces between magnets:

  1. Repulsive force: Occurs when two like poles (North-North or South-South) are brought together.
  2. Attractive force: Occurs when two opposite poles (North-South) are brought together.


Static Electricity and Lightning

Static electricity occurs if a body acquires an electric charge and maintains it. For this to happen, the body usually needs to be made of an insulating material, like plastic or glass.

  • Lightning: A large electrical discharge between clouds or between a cloud and the ground.
  • Lightning Strike: A discharge from a cloud to the ground. They can be dangerous, potentially killing people and animals.

We can use a lightning rod to help discharge lightning strikes in a controlled and safer way.

The Earth as a Magnet and Compasses

A compass contains a magnetic needle that rotates freely.

The North Pole of a compass needle is the end that points towards the Earth’s geographic North Pole. This happens because the Earth itself acts like a giant magnet, with its own magnetic poles.

Electric Current and Electromagnetism

An electric current is the flow or movement of electric charges. This movement can make different devices work, such as light bulbs.

1819: Hans Christian Oersted discovered that an electric current creates a magnetic field, behaving like a magnet. This demonstrated the link between electricity and magnetism.