Gravitational, Electric, and Magnetic Fields: A Comparison
Conservative and Non-Conservative Fields
Gravitational, electric, and magnetic fields are three examples of vector fields. The first two are conservative, while the magnetic field is not. Let’s recall the main characteristics of conservative fields:
- A: The work required to move a particle from one point to another is independent of the trajectory followed, so the work done in a closed loop is zero.
- B: The circulation of the vector field along a closed line is zero.
- C: There is a scalar function, called potential, whose variation, with a change of sign, represents the intensity vector of the field.
Analogies Between Gravitational and Electric Fields
Electric and gravitational fields have the following analogies, which do not occur in magnetic fields:
- Lines of force of gravitational and electric fields are open, meaning they start at some point (either a field source or infinity) and end at another (either a field sink or infinity). However, magnetic field lines are closed and have no sources or sinks.
- In conservative fields, such as electric and gravitational fields, a scalar function, called potential, can be defined. The variation of this potential allows for the calculation of the field intensity. The lines of force of these fields are perpendicular to equipotential surfaces. In the magnetic field, there is no such scalar function.
- Positive and negative electric charges can be isolated. This does not happen in the magnetic field, where isolated north and south magnetic poles do not exist. If a magnet is broken into two pieces, each piece instantly becomes a magnet with a north and a south pole. This happens no matter how many times each piece is subdivided.
- Gravitational forces and the direction of the electric field are aligned. However, they are perpendicular to the magnetic field.
- Gravitational and electrical forces are directed toward a point; they are central forces. Magnetic forces are not.
Differences Between Gravitational and Electric Fields
The following differences exist between gravitational and electric fields:
- Gravitational forces are always attractive, but electric forces can be either attractive or repulsive.
- The gravitational field has no sources; its lines of force start at infinity. The electric field has sources (positive charges) and sinks (negative charges).
- Any material body creates a gravitational field. A body creates an electric field only if it is charged.
- A particle, whether at rest or in motion, creates a gravitational field. An electrically charged particle creates an electric field if at rest, and both an electric and a magnetic field if in motion.
- If we leave a material particle at rest within a gravitational field, it moves in the direction of the field. However, if we leave a charged particle in an electric field, it always moves in the direction of the field if it is positively charged, and opposite to the field if it is negatively charged.
Analogies Between Electric and Magnetic Fields
The following analogies are observed between electric and magnetic fields:
- Both fields exert forces on electric charges.
- Electric and magnetic dipoles exist. An electric dipole consists of two equal and opposite charges, +Q and -Q, separated by a small distance. Many molecules, like water, are natural electric dipoles. Magnetized bodies are always formed by a north and a south pole; they are always magnetic dipoles.
- A changing electric field creates a magnetic field, and vice versa.
- Dipoles, whether electric or magnetic, when they have freedom to move, are oriented in the direction of the field. Furthermore, if the field is not homogeneous, they are drawn to areas where the field is more intense.