Physics › Gravitational fields › The field concept
The field concept
A force field is a region in which a body experiences a non-contact force, and the three origins are mass, static charge, and moving charge. Field lines map the vector at every point, radial around a sphere and uniform close to a surface. Gravity and electrostatics are described in the same way, and differ in that gravity only attracts.
Choose your exam board to show the notes and questions relevant to your specification. You can change this setting at any time, and content for other boards remains available.
Builds on Newton's laws and the resultant force and Scalars and vectors and Mass and weight.
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.
Gravitational, electric and magnetic forces act without contact. Each is described by a field: a region in which a body experiences a force without touching anything.
A region that exerts a force
A force field is a region in which a body experiences a non-contact force. A falling apple is not touched by anything; the Earth's gravitational field, occupying the space through which the apple falls, exerts the force. A field is represented as a vector, so at every point it has both a magnitude and a direction. The direction at a point is the direction of the force on a suitable test body placed there.
The specification names three origins. Fields arise from mass (gravitational), from static charge (electric), and between moving charges (magnetic). This unit and the two that follow it cover all three, and the treatment of gravity here transfers with little change.
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 value of g, so the same 2.0 kg mass experiences a larger force at A than at B. The mass is unchanged; the field strength differs between the two points.
Gravity beside electrostatics
The specification asks for this comparison directly. Gravitational and electric fields have analogous mathematical descriptions: both forces obey inverse-square laws, so field lines, potential and equipotential surfaces apply to both. An equipotential surface is a surface of constant potential, and no work is done moving along one.
The difference is that gravity is always attractive, whereas electric forces may attract or repel, because charge has two signs and mass has one.
Gravitational fields cannot be screened. The fields of different masses can cancel at a particular point, but the contributions of the matter in a large body add, so gravity is the dominant force on astronomical scales.
ASSESSMENT FOCUS
- Learn the definition word for word: a region in which a body experiences a non-contact force. The word non-contact is the one that carries the mark.
- Give all three origins when asked: mass, static charge, and moving charges for the magnetic case.
- The comparison answer has a fixed structure: the similarities are the inverse-square law and the shared apparatus of field lines, potential and equipotentials; the difference is that gravity only attracts while electric forces may attract or repel.
- A field line's arrow shows the direction of the force on the appropriate test body, and the line spacing shows the field strength. Both statements carry credit. Uniform means parallel and equally spaced field lines, an approximation valid only close to a surface, so state that condition when using 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 consider which force can be repulsive.
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 force field is a region in which a body experiences a non-contact force.
Gravitational and electric fields have analogous mathematical descriptions, but gravity is always attractive whereas electric forces may attract or 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 step by step.
CHECK YOUR PROGRESS
Rate how confident you feel with each objective for this lesson. Ratings are saved in this browser, on this device, unless you sign in.
- 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.