SQA Advanced Higher Physics Area 3 Electromagnetism: a complete overview of electric and magnetic fields, capacitors, inductors and electromagnetic radiation

What Area 3 actually demands

Electromagnetism brings together fields, circuits and the unification that revealed light to be an electromagnetic wave. It runs from the inverse-square electric force, through the magnetic forces on currents and charges, into the storage of energy in capacitors and inductors, and finally to Maxwell's great synthesis. The examiners reward confident field and circuit calculations, correct handling of the factor of one half in stored energy, and clear conceptual statements of Lenz's law and Maxwell's unification. This guide walks through all the key areas, then sets out the patterns the SQA repeats. Each key area has a matching dot-point page with practice questions; this overview ties them together.

Electric fields

The area opens with electric fields: Coulomb's law $F = \frac{Q_1 Q_2}{4\pi\varepsilon_0 r^2}$, the field strength $E = \frac{F}{Q}$ (with $E = \frac{V}{d}$ in a uniform field), the electric potential $V = \frac{Q}{4\pi\varepsilon_0 r}$, and the motion of charged particles accelerated through a potential difference ($E_k = QV$) or deflected like projectiles.

Magnetic fields

Magnetic fields cover the field around a current, the force on a conductor $F = BIL$ and on a moving charge $F = qvB$, magnetic flux $\Phi = BA$, and Millikan's experiment, which showed charge is quantised in units of $e$.

Capacitors

Capacitors store charge with capacitance $C = \frac{Q}{V}$ and energy $E = \tfrac{1}{2}CV^2$, charge and discharge exponentially through a resistor with time constant $\tau = RC$, and behave differently in d.c. (eventually blocking current) and a.c. (passing high frequencies more readily).

Inductors

Inductors oppose changing current through a back emf $\varepsilon = -L\frac{dI}{dt}$ (Lenz's law), store energy in a magnetic field $E = \tfrac{1}{2}LI^2$, give gradual current growth and decay in RL circuits, and have an inductive reactance that rises with frequency, the opposite of a capacitor.

Electromagnetic radiation

Electromagnetic radiation is orthogonal oscillating electric and magnetic fields. Maxwell unified electricity and magnetism and predicted the speed of light, $c = \frac{1}{\sqrt{\varepsilon_0 \mu_0}}$, showing light is an electromagnetic wave.

How Area 3 is examined

A typical SQA profile for Electromagnetism:

• Calculations. Coulomb's law, field strength and potential, charged-particle acceleration and deflection, $F = BIL$ and $F = qvB$, capacitor charge and energy, the time constant, back emf and inductor energy, and the speed of light.
• Explanation. The similarities between electric and gravitational fields, Lenz's law, the frequency behaviour of capacitors and inductors, and Maxwell's unification.
• Experiment. Millikan's oil-drop experiment and the exponential charge and discharge of a capacitor.

Check your knowledge

A mix of recall and calculation questions covering Area 3. Attempt them, then check against the solutions.

1. Write Coulomb's law for the force between two point charges. (1 mark)
2. Write the relationship for the force on a current-carrying conductor in a magnetic field. (1 mark)
3. Write the relationship for the energy stored on a capacitor. (1 mark)
4. Write the relationship for the back emf of an inductor. (1 mark)
5. Write the relationship for the speed of light in terms of the constants of free space. (1 mark)
6. State what Millikan's experiment showed. (1 mark)