Electric Fields and Magnetic Phenomena: Understanding the Basics

Posted on Jan 13, 2025 in Physics

Lines of Force

Lines of force are imaginary lines that mark the path a positive charge would follow if left free in an electric field. These lines must meet the following conditions:

  1. Lines emerge from positive charges (sources) and enter negative charges (sinks).
  2. The number of lines entering or leaving a point charge is proportional to the value of the charge.
  3. At each point in the field, the number of lines per unit area perpendicular to them is proportional to the field strength.
  4. Two lines of force can never intersect. At each point, the field has only one direction, so two lines cannot cross because the area would have two directions.

The field strength is the number of lines of force passing through a unit area placed perpendicular to the lines. In the case of a uniform field, lines are parallel, and the field at two points has the same intensity (Ea = Ep).

Electric Potential

A charge Q creates an electric field around it, which is a vector quantity. We can also assume that the presence of Q creates a scalar property called electric potential. Therefore, when a charge q is located at a point next to Q, it acquires potential energy (U = q · V). Electric potential at a point can be defined as the potential energy of a positive unit charge placed at that point. V = K • (Q / r2)

The origin of the potential is at infinity, where the potential is equal to zero. Thus, the potential at a point is the work needed to move the unit positive charge from infinity to that point. From here, we can define the electron volt: the work required to carry the charge of an electron between two points of an electric field whose potential difference is 1 V.

Equipotential Surfaces

Equipotential surfaces are those where all points are at the same potential. The work performed to move a charge along an equipotential surface is zero. The electric field vector is perpendicular at every point to an equipotential surface.

Relationship Between Electric Field and Potential

This implies that:

  1. If there is no potential variation in a particular direction, the field component in that direction is zero.
  2. Knowing the potential value at each point allows us to determine the absolute value of the electric field. The field points towards decreasing potential, as indicated by the minus sign in the equation.
  3. If we know the value of the field at each point, we can obtain the potential by integrating the field.
Oersted’s Experiment

Oersted was the first to demonstrate the relationship between electric and magnetic phenomena. He placed a compass near a wire through which current was flowing. He noted that when current flowed through the wire, the compass needle oriented itself perpendicular to the wire. This orientation ceased when the current stopped. Reversing the current’s direction changed the needle’s orientation. This experiment showed that moving electric charges produce magnetic fields.

He also conducted an experiment with Ampere: when two parallel conductors carry currents, forces arise between them. These forces are attractive if the currents have the same direction and repulsive if they have opposite directions. These experiments prove that magnetic fields generated by electric currents or magnets can be traced by moving charges.