Electromagnetism Resource

Electric fields exert a force on charged particles.

The electric field is a force field with magnitude and direction.

The electric field is a vector field, and its magnitude is usually denoted by the symbol \( E \).

The electric field is important because voltage is the net effect of electric fields.

The electric field has two main physical interpretations:

• the voltage gradient (or change of voltage) with respect to change in distance; or
• the force per unit charge.

In light of the above, the electric field has two main equivalent units:

• volts per metre or V.m^-1; or
• newtons per coulomb or N.C^-1.

Electric fields are the force that moves electric current around a circuit.

Electric fields are frequently shown using lines or arrows, these arrows show the force on an imaginary positive "test charge".

The test charge will be pushed in the direction of the electric field lines.

The electric field forces might be considered the "true" or "real" electromotive force.

Electric fields have three main sources and behaviours:

1. Charged particles: All charged particles create an electric field.
2. Time-varying magnetic fields: A time-varying magnetic field will create an electric field that goes in loops.
3. Moving a charged particle in a magnetic field: Moving charged particles in a magnetic field creates a force on that charged particle. The ratio of the force to the charge is the strength of the electric field.

Electric fields are a vector field. Electric fields originate at positively charged particles, and terminate at negatively charged particles. See image below.

Unlike gravity, which is always attractive, electric field forces can attractive or repulsive.

Like charges repel, and unlike charges attract.

The magnitude of the force from an electric field on a non-moving charge is given by:

\( F = q E \)

where \( F \) is the force (N), \( q \) is the charge (C), and \( E \) is magnitude of the electric field (V.m^-1or N.C^-1).

The direction of the force is the same as the direction of the electric field.

If a positive charge and a negative charge are brought near each other, the field will look like the one below. It is the sum of the two individual fields. Note that field lines never cross over each other.

Electric Fields and Voltages

Electric fields are not normally measured directly. Normally we measure voltage, which is the sum of electric field strength between two points over a distance.

Voltage has several possible interpretations:

• The total electromotive force that is exerted. This is the origin of calling a voltage an electromotive force (EMF).
• The amount of energy or work that is transferred by each charged particle as it moves through an electric field. In this sense, the unit V is also interpreted as J.C^-1 (joule per coulomb). The J unit comes from multiplying force by distance, i.e. 1 J = 1 Nm.

These two units, V and J.C^-1, are physically identical.