Introduction to Force Fields Revision Notes | A Level AQA Physics

1. The Invisible Architecture of Forces

Drop a ball and it falls — but nothing touches it. Hold two magnets close and you feel them push or pull — again, nothing connects them. How does a force act across empty space? The answer lies in the concept of a force field.

At A Level, you move beyond measuring forces to asking a deeper question: what is the physical structure of the space around a massive or charged object that allows it to exert influence at a distance? Force fields are the answer, and they underpin all of gravitational, electric, and magnetic physics.

The phrase non-contact force is doing important work in that definition. It distinguishes field forces — gravity, electric force, magnetic force — from contact forces like friction or a normal reaction, which require physical touching. Fields allow objects to exert forces across a vacuum.

2. Fields as Vectors — Direction Matters

Knowing that a field exists is only half the story. To use a field quantitatively, you need to know the direction of the force it would exert at every point in space. This is why force fields are represented as vectors.

Picture the gravitational field around the Earth. At any point above the surface, the field points towards the centre of the Earth — that is the direction a small test mass would be pulled. You determine this direction by inspection: look at the source, identify the nature of the interaction, and draw the arrow accordingly.

The direction of a field vector is never arbitrary — it must be determined by inspection of the physical situation. For a gravitational field, arrows always point inward towards the mass. For an electric field around a positive charge, arrows point outward (away from the source). For a negative charge, they point inward. These conventions are fixed by the AQA specification and must be applied consistently in exam answers.

• Gravitational field lines around a spherical mass: radially inward, towards the centre of mass
• Electric field lines around a positive point charge: radially outward, away from the charge
• Electric field lines around a negative point charge: radially inward, towards the charge
• Uniform fields (e.g. between parallel plates): parallel, equally spaced lines all pointing in the same direction
• Closer field lines = stronger field; wider spacing = weaker field

One subtlety worth noting: the vector nature of a field means that where two fields overlap, you must add the field vectors at each point. The resultant field direction at any location is found by vector addition — a skill that connects directly to your earlier work on resolving vectors.

This section establishes the core representational tool for all field topics: a force field is a vector field, and its direction at every point is determined by inspection of the source and the type of interaction involved.

3. The Three Origins of Force Fields

Force fields do not arise from nothing. The AQA specification identifies three distinct physical origins, each producing a different type of field. Understanding what causes each field is as important as knowing how to represent it.

The three sources are: the interaction of mass (producing gravitational fields), the interaction of static charge (producing electric fields), and the interaction between moving charges (producing magnetic fields). Each source creates a field with its own characteristics, and the differences between them are a rich source of exam questions.

Notice the key contrast in that table: gravitational fields are always attractive, because mass is always positive. Electric and magnetic fields can be either attractive or repulsive, depending on the signs of the charges or the directions of the currents involved. This distinction is frequently tested in AQA questions asking you to compare field types.

4. Gravitational Fields — Mass as the Source

Every object with mass creates a gravitational field in the space around it. Place a second mass anywhere in that region and it will experience a force directed towards the first mass. The field exists continuously — it does not switch on only when a second object arrives.

Visualise the Earth's gravitational field by imagining a grid of arrows filling all the space around the planet. Every arrow points towards the Earth's centre. Arrows near the surface are long and closely packed; arrows far away are shorter and more spread out. This picture captures both the direction (always inward) and the way field strength decreases with distance.

At A Level, you will quantify this field using gravitational field strength $g$, defined as the force per unit mass experienced by a small test mass placed at that point. For now, the key conceptual point is that mass is the source of the gravitational field, and mass is also what the field acts upon.

5. Electric Fields — Static Charge as the Source

Static charge — charge that is not flowing — creates an electric field in the surrounding space. Any other charged body placed in that region will experience a force. The direction of the force depends on the signs of the charges involved, which is why electric fields can be either attractive or repulsive.

By convention, electric field direction is defined as the direction of the force on a positive test charge. Around a positive source charge, the field points outward — a positive test charge would be repelled. Around a negative source charge, the field points inward — a positive test charge would be attracted. This convention is fixed and must be applied consistently when drawing field line diagrams.

A uniform electric field — such as the field between two oppositely charged parallel plates — is represented by equally spaced, parallel field lines all pointing in the same direction (from positive plate to negative plate). This is a common diagram in AQA exam papers, and the uniform spacing is the key feature that distinguishes it from the radial field around a point charge.

6. Magnetic Fields — Moving Charges as the Source

The third origin of a force field is the most surprising: a magnetic field arises from moving charges. A stationary charge creates only an electric field. The moment that charge begins to move — forming an electric current — it also creates a magnetic field in the surrounding space.

This is why a wire carrying a current has a magnetic field around it, and why a permanent magnet (whose field arises from the aligned spin of electrons within its atoms) can exert forces on other magnets or on current-carrying conductors. The source is always moving charge, whether that motion is a conventional current in a wire or the quantum-mechanical spin of electrons in a magnetic material.

Magnetic fields act on other moving charges — which is why a current-carrying wire experiences a force when placed in a magnetic field. A stationary charge in a magnetic field experiences no magnetic force at all. This is a critical distinction from gravitational and electric fields, which act on their respective sources regardless of motion.

7. Comparing the Three Field Types

AQA exam questions frequently ask you to compare gravitational and electric fields. The structural similarities are striking — both follow an inverse-square law, both can be represented by radial field lines around a point source, and both are vector fields whose direction is determined by inspection. The differences, however, are equally important to know precisely.

The most fundamental difference is that gravitational fields are always attractive, whereas electric fields can attract or repel. A second key difference is the relative strength: the electric force between two protons is approximately $10^{36}$ times stronger than the gravitational force between them (this is an order-of-magnitude estimate; the precise value is around $1.2 \times 10^{36}$). Gravity dominates at astronomical scales only because large objects are electrically neutral overall, so their electric fields cancel out.

Magnetic fields share the vector nature of gravitational and electric fields but differ in one crucial respect: the force they exert depends on the velocity of the charged body experiencing the field. This velocity-dependence makes magnetic fields conceptually distinct from the other two, and it is the reason magnetic fields are introduced as a separate category arising from moving charges rather than from a static property like mass or charge.

8. Exam Technique and Mark-Winning Strategies

Questions on force fields at A Level tend to cluster around three command words: state, describe, and explain. Each demands a different level of response, and confusing them is one of the most common ways students lose marks.

When asked to state what a force field is, give the definition precisely: a region in which a body experiences a non-contact force. Do not add unnecessary explanation — the mark is for the definition itself. When asked to describe a field, focus on what the field lines look like: their direction, spacing, and shape. When asked to explain, you must give a reason — connect the observation to the underlying physics using the word 'because' or an equivalent causal link.

For questions asking you to compare gravitational and electric fields, structure your answer to address both similarities and differences explicitly. A common mark-winning move is to state that both fields follow an inverse-square law and both are vector fields, before contrasting the fact that gravity is always attractive while electric fields can attract or repel.

A final point on vector direction: the instruction to determine field direction 'by inspection' means you must reason from the physical setup rather than memorise a single rule. Ask yourself: what is the source? What is the nature of the interaction (attractive or repulsive)? What would happen to a positive test charge or a small test mass placed at this point? The answer to those three questions gives you the field direction at any location.