QCE Physics - Unit 3 - Electromagnetism

Electric Field Energy and Potential | QCE Physics

Learn QCE Physics electric fields, electric potential energy, potential difference and work done moving charges in electric fields.

Updated 2026-06-17 - 4 min read

Electrostatics becomes much clearer when you separate force, field, energy and potential. They are related, but they answer different questions. Force asks what happens to a particular charge. Field asks what the space is like at a point. Potential asks how much energy is transferred per unit charge.

Electric force and electric field

Coulomb's law gives the force between two point charges:

$ F=\frac{kQq}{r^2} $

The force is repulsive for like charges and attractive for unlike charges. It acts along the line joining the charges.

An electric field describes the force that a positive test charge would experience per unit charge:

$ E=\frac{F}{q} $

For a point charge:

$ E=\frac{kQ}{r^2} $

The direction of the electric field is the direction of the force on a positive test charge. Field lines point away from positive charges and toward negative charges.

Electric potential energy

Electric potential energy is stored energy associated with the positions of charges in an electric field. If a charge moves in the direction that the electric force pushes it, electric potential energy decreases and kinetic energy may increase. If an external agent moves the charge against the electric force, electric potential energy increases.

This is similar to gravitational potential energy, but the sign of electric charge adds an important difference. A positive charge tends to accelerate in the direction of the electric field. A negative charge tends to accelerate opposite to the electric field.

Electric potential difference

Electric potential difference, or voltage, measures the change in electric potential energy per unit charge:

$ V=\frac{\Delta U}{q} $

Rearrange to find the energy change:

$ \Delta U=qV $

This equation is often the simplest way to connect fields to energy transfer. If a charge of $2.0\ \mathrm{C}$ moves through a potential difference of $5.0\ \mathrm{V}$, the energy transfer is $10\ \mathrm{J}$.

The volt is equivalent to a joule per coulomb:

$ 1\ \mathrm{V}=1\ \mathrm{J\ C^{-1}} $

Connecting field and potential

In a uniform electric field, the potential changes steadily with distance along the field. The field points from higher electric potential to lower electric potential for positive charge motion. In a point-charge field, both $E$ and potential change with distance, but $E$ follows an inverse-square relationship.

For a uniform field between parallel plates, the magnitude of the field can be connected to potential difference:

$ E=\frac{|\Delta V|}{d} $

where $d$ is the distance measured along the field direction. This means electric field strength can be written in $\mathrm{N\ C^{-1}}$ or $\mathrm{V\ m^{-1}}$.

Field diagrams and potential diagrams show different things:

• electric field lines show force direction on a positive test charge
• denser field lines mean stronger field
• equipotential lines connect points with the same electric potential
• no work is done moving a charge along an equipotential

QCE questions may ask you to interpret field diagrams, potential differences or energy changes. Always identify whether the unknown is force, field strength, potential difference or energy.

Worked example

Exam-ready interpretation

When a question includes a charge diagram, write the chain of reasoning:

1. Identify the source charge or source field.
2. Determine field direction using a positive test charge.
3. Determine force direction on the actual charge.
4. Use the correct formula for force, field, voltage or energy.
5. Interpret the sign and units.

Quick check

Sources

• QCAA Physics subject page
• QCAA Physics 2025 syllabus
• QCAA Physics 2025 formula and data book
• OpenStax University Physics Volume 2: Electric Field
• OpenStax University Physics Volume 2: Electric Potential Energy