What are the main similarities and differences between gravitational and electric fields?

Both obey an inverse square law for force and field strength, both define field strength as force per unit (mass or charge), and both have a potential that varies as one over r. The differences are that gravitational fields act on mass and are always attractive, while electric fields act on charge and can be attractive or repulsive. Gravity dominates for large masses because G is tiny, whereas the electric force is far stronger between charged particles.

Why is gravitational potential always negative but electric potential can be positive or negative?

Gravitational potential is defined as zero at infinity and the force is always attractive, so work is released as a mass moves inward, making the potential negative everywhere. Electric potential is also zero at infinity, but because charges can be positive or negative the potential is positive around a positive charge and negative around a negative charge.

What is the time constant of a capacitor circuit and what does it tell you?

The time constant is the product RC of the resistance and capacitance, measured in seconds. It is the time for the charge, voltage or current to fall to one over e, about 37 percent, of its initial value during discharge. A larger time constant means a slower discharge. The half-life of the decay is 0.69 times RC.

What is the difference between Faraday's law and Lenz's law?

Faraday's law gives the size of the induced emf as equal to the rate of change of flux linkage. Lenz's law gives its direction, stating that the induced current opposes the change that produced it, which is shown by the minus sign and follows from conservation of energy. Together they fully describe electromagnetic induction.

Why is electrical power transmitted at high voltage?

For a fixed power, raising the transmission voltage lowers the current because power equals voltage times current. The power wasted as heat in the cables is I squared R, so reducing the current sharply reduces these losses. Transformers step the voltage up for transmission and down again for safe use, which is why they are essential to the grid and only work with alternating current.

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AQA A-Level Physics 3.7 Fields and their consequences: a complete overview of gravitational, electric and magnetic fields

A deep-dive AQA A-Level Physics guide to module 3.7 Fields and their consequences. Covers gravitational fields and potential, orbits, electric fields and potential, capacitance and capacitor discharge, magnetic flux density, electromagnetic induction and alternating currents and transformers, with the equations and exam patterns AQA repeats.

Generated by Claude Opus 4.824 min read3.7Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What module 3.7 actually demands
Gravitational fields and potential
Orbits of planets and satellites
Electric fields and potential
Capacitance and capacitor discharge
Magnetic fields, induction and transformers
How module 3.7 is examined
Check your knowledge

What module 3.7 actually demands

Fields and their consequences is the unifying module of A-Level Physics, where gravitational, electric and magnetic fields are treated with the same mathematical language of field strength, potential and inverse square laws. The examiners reward students who can spot the deep analogy between gravity and electrostatics, handle exponential decay confidently, and apply induction to generators and transformers. This guide walks through the ten topics in specification order, then sets out the exam patterns AQA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Gravitational fields and potential

A gravitational field is a region where a mass feels a force. Newton's law of gravitation $F = \frac{Gm_1m_2}{r^2}$ is an inverse square law, and the field strength $g = \frac{GM}{r^2}$ is the force per unit mass. Gravitational potential $V = -\frac{GM}{r}$ is the work done per unit mass from infinity and is always negative. Field strength is the negative potential gradient, and equipotentials are perpendicular to field lines.

Orbits of planets and satellites

For a circular orbit, gravity provides the centripetal force, so $v = \sqrt{\frac{GM}{r}}$ and $T^2 \propto r^3$ (Kepler's third law). The total energy of an orbiting body is the sum of its kinetic and negative potential energy. A geostationary satellite has a 24 hour equatorial orbit and stays above one fixed point, which is why it is used for communications.

Electric fields and potential

Coulomb's law $F = \frac{1}{4\pi\varepsilon_0}\frac{Q_1Q_2}{r^2}$ mirrors Newton's law but can be attractive or repulsive. Electric field strength is the force per unit positive charge, radial for a point charge and uniform between parallel plates where $E = \frac{V}{d}$ . Electric potential $V = \frac{1}{4\pi\varepsilon_0}\frac{Q}{r}$ can be positive or negative, and a charged particle in a uniform field follows a parabolic path.

Capacitance and capacitor discharge

Capacitance is the charge stored per unit voltage, $C = \frac{Q}{V}$ , and the energy stored is $\frac{1}{2}CV^2$ . A dielectric increases capacitance by polarising. When a capacitor discharges through a resistor, charge, voltage and current decay exponentially as $e^{-t/RC}$ , with time constant $RC$ and half-life $0.69RC$ ; a log-linear plot gives a straight line.

Magnetic fields, induction and transformers

Magnetic flux density is defined through $F = BIl$ , and a moving charge feels $F = Bqv$ , which produces circular motion. Magnetic flux is $\Phi = BA$ and flux linkage is $N\Phi$ . Faraday's law gives the induced emf as the rate of change of flux linkage, and Lenz's law gives its direction. A rotating coil produces a sinusoidal alternating emf. Alternating quantities are described by rms values, and transformers change voltage according to the turns ratio, enabling efficient high-voltage power transmission.

How module 3.7 is examined

A typical AQA profile for this module:

Definitions and analogies. Defining field strength and potential, and comparing gravitational and electric fields.
Calculations. Field strength, potential, orbital speed and period, capacitor energy and discharge times, induced emf, and rms and transformer calculations.
Graphical questions. Field and potential against distance, log-linear discharge plots, and flux against time.
Extended answers. Explaining geostationary orbits, capacitor discharge, induction with Lenz's law, and why the grid uses high voltage.

Check your knowledge

A mix of recall and calculation questions covering module 3.7. Attempt them under timed conditions, then check against the solutions.

State Newton's law of gravitation in words. (2 marks)
Calculate the gravitational field strength at $2.0 \times 10^7 \text{ m}$ from the Earth's centre ( $GM = 3.99 \times 10^{14}$ ). (2 marks)
State Kepler's third law. (1 mark)
A $50 \text{ }\mu\text{F}$ capacitor is charged to $20 \text{ V}$ . Calculate the energy stored. (2 marks)
A capacitor discharges through a $10 \text{ k}\Omega$ resistor with $C = 470 \text{ }\mu\text{F}$ . Find the time constant. (2 marks)
State Faraday's law of electromagnetic induction. (1 mark)
A wire of length $0.40 \text{ m}$ carries $5.0 \text{ A}$ perpendicular to a $0.30 \text{ T}$ field. Calculate the force. (2 marks)
A sinusoidal supply has a peak voltage of $170 \text{ V}$ . Calculate the rms voltage. (2 marks)

Solutions

Q1: The gravitational force between two point masses is proportional to the product of their masses and inversely proportional to the square of their separation.
Q2: $g = \frac{GM}{r^2} = \frac{3.99 \times 10^{14}}{(2.0 \times 10^7)^2} = 1.0 \text{ N kg}^{-1}$ .
Q3: The square of the orbital period is proportional to the cube of the orbital radius, $T^2 \propto r^3$ .
Q4: $E = \frac{1}{2}CV^2 = \frac{1}{2} \times 50 \times 10^{-6} \times 20^2 = 1.0 \times 10^{-2} \text{ J}$ .
Q5: $\tau = RC = 10000 \times 470 \times 10^{-6} = 4.7 \text{ s}$ .
Q6: The induced emf is equal to the rate of change of magnetic flux linkage.
Q7: $F = BIl = 0.30 \times 5.0 \times 0.40 = 0.60 \text{ N}$ .
Q8: $V_{rms} = \frac{V_0}{\sqrt{2}} = \frac{170}{\sqrt{2}} = 120 \text{ V}$ .

Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)