Physics › Gravitational fields › The field concept
The field concept
A field is a region in which a body feels a force without anything touching it, and the same idea covers gravity, electricity and magnetism. Gravitational and electric fields turn out to follow almost identical equations, with one difference that matters: gravity only ever attracts.
Pick your board and the few notes written for the other boards quietly fold away, here and in the practice players. Nothing is deleted: every folded piece reopens on a tap.
Builds on Newton's laws and the resultant force and Scalars and vectors.
IN THIS TOPIC
- Define a force field, name the three origins the specification lists, and read a field-line diagram.
- Compare gravitational and electrostatic forces, the shared inverse-square structure and the one difference.
COMMON MISCONCEPTION
Forces need contact.
A region that exerts a force
A force field is a region in which a body experiences a non-contact force. Nothing touches the falling apple; the Earth's gravitational field, occupying the space the apple falls through, does the pulling. A field is represented as a vector, so at every point it carries both a strength and a direction. Find the direction by asking which way the force on a suitable test body would point.
The course names three origins. Fields arise from mass (gravitational), from static charge (electric), and between moving charges (magnetic). This unit and the two after it are one long study of that trio, and the ideas you build here for gravity transfer almost unchanged.
Reading the lines
Field lines draw the vector map. The arrow gives the force direction on a mass, and the spacing gives the strength, closer lines meaning a stronger field. A point mass, or anything spherical viewed from outside, has a radial field. Near a planet's surface, over laboratory distances, the lines come out effectively parallel and evenly spaced, and that is a uniform field. It is also the reason g could be treated as a constant throughout Year 12.
GUIDED PRACTICE
Reading density as strength
On a field-line diagram of a planet, point A sits where the lines crowd tightly and point B where they have spread far apart. Compare the field strengths and the forces on a 2.0 kg mass placed at each point.
Show the working
Line density is the diagram's code for field strength. Lines crowd at A, so A has the stronger field and the larger g.
The force is F = mg with the local g, so the same 2.0 kg mass feels a larger force at A than at B. The mass did not change; the field it stands in did.
Gravity beside electrostatics
The specification asks directly for this comparison, so learn it as a shape. Both forces obey inverse-square laws. Shared mathematics brings shared tools, so field lines, the potential idea and equipotential surfaces all carry over from one field to the other, and you will meet each of them twice. An equipotential surface is the contour line of a field, and moving along one costs no work. Now the difference. Masses always attract, while charges may attract or repel, because charge comes in two signs and mass in one. That single fact builds the large-scale universe, since an attraction nothing can screen and nothing can cancel simply accumulates.
ASSESSMENT FOCUS
- Learn the definition verbatim, because the single word non-contact is what earns the mark. A region in which a body experiences a non-contact force.
- Give all three origins when asked. Mass, static charge, and moving charges for the magnetic case.
- The comparison answer has a fixed shape. Similarities are the inverse-square law and the shared apparatus of field lines, potential and equipotentials; the difference is attract-only against attract-or-repel.
- A field line's arrow shows the force direction on the appropriate test body, and the line spacing shows the strength. Both halves are markable. Uniform means parallel and equally spaced, an approximation good only near a surface, so justify it in a line when you use it.
CHECK YOURSELF
State two similarities and one difference between gravitational and electrostatic forces, and give one situation where each field would be drawn as uniform.
Show a hint
Start with the shared mathematics, then ask who is allowed to repel.
Show the answer
Both obey an inverse-square force law, and both are described with the same apparatus of field lines, potentials and equipotential surfaces.
The difference is that masses always attract, while charges may attract or repel.
Gravity is drawn as uniform near the Earth's surface over small distances. The electric field between parallel charged plates is uniform too.
A field is a region that exerts force without touch.
Gravity and electrostatics share grammar; only charge can repel.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their mark schemes on the field concept questions page.
WHERE TO GO NEXT
- Trigonometry and resolving vectors is the maths this lesson leans on, worked through from GCSE.
CHECK YOUR PROGRESS
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- Define a force field, name the three origins the specification lists, and read a field-line diagram.
- Compare gravitational and electrostatic forces, the shared inverse-square structure and the one difference.
Open the full revision checklist to track your progress across the whole unit.