QCE Physics - Unit 3 - Electromagnetism

Electrostatics | QCE Physics

Learn QCE Physics electrostatics with Coulomb's law, electric fields, field strength, electric potential energy and potential difference.

Updated 2026-06-15 - 4 min read

Electrostatics is the study of electric charges when the charge distribution is not changing rapidly. In QCE Physics, this topic connects force, field and energy: charges exert forces on each other, fields describe those forces at a point, and potential difference describes energy transferred per unit charge.

Coulomb's law

Coulomb's law describes the magnitude of the electrostatic force between two point charges:

$ F = \frac{1}{4\pi\varepsilon_0}\frac{Qq}{r^2} $

This is often written as:

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

where $k = \frac{1}{4\pi\varepsilon_0}$. The force follows an inverse-square relationship. If the distance between charges doubles, the force becomes one quarter as large. If one charge doubles, the force doubles.

The sign of the charges determines direction. Like charges repel. Opposite charges attract. The formula above gives magnitude when you substitute charge magnitudes, but the physics answer still needs direction if the question asks for the force on a particular charge.

Electric fields

An electric field describes how a positive test charge would be forced at a point. Field lines point in the direction a positive charge would move. Around a positive source charge, field lines point outward. Around a negative source charge, field lines point inward.

Electric field strength is:

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

For a point source charge, the field strength at distance $r$ is:

$ E = \frac{1}{4\pi\varepsilon_0}\frac{q}{r^2} $

or:

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

The unit is $\mathrm{N\ C^{-1}}$. It is also equivalent to $\mathrm{V\ m^{-1}}$ in electric potential contexts, but QCE electrostatics questions often start from force per charge.

Electric potential energy and potential difference

Electric potential energy changes when a charge moves in an electric field. Potential difference is energy change per unit charge:

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

Rearranging gives:

$ \Delta U = qV $

This means a $2.0\ \mathrm{C}$ charge moving through a $5.0\ \mathrm{V}$ potential difference changes energy by $10\ \mathrm{J}$. The sign depends on charge sign and movement direction, but the magnitude comes from the product of charge and potential difference.

Potential difference is not the same as field strength. A field strength describes force per charge at a point; a potential difference describes energy transferred per charge between two points.

Worked example

Data interpretation

Electrostatics data often checks whether you recognise inverse-square behaviour. A graph of $F$ against $1/r^2$ should be approximately linear if the charges remain constant. A graph of $F$ against $r$ is curved because the relationship is not linear in $r$.

If experimental data has a non-zero intercept, that may indicate systematic error, background forces, calibration issues or that the charges were not constant during the experiment. A good analysis links the deviation to the physical setup rather than just saying "human error".

Connecting force and field

If a charge is placed in a known electric field, use:

$ F = QE $

This is the same relationship as $E=F/Q$, just rearranged. A positive charge experiences force in the direction of the field. A negative charge experiences force opposite to the field direction.

Quick check

Sources

• QCAA Physics subject page
• QCAA Physics 2025 syllabus
• QCAA Physics 2025 formula and data book
• OpenStax University Physics Volume 2: Coulomb's Law
• OpenStax College Physics 2e: Electric Charge and Electric Field