Physics › Gravitational fields › The field concept
The field concept
Physics explains action at a distance with one great unifying idea: the field, a region where a body feels a force without being touched. Gravity and electrostatics turn out to speak almost the same language, with one telling difference.
Builds on Newton's laws and the resultant force and Scalars and vectors.
IN THIS TOPIC
- Define a force field as a region in which a body experiences a non-contact force.
- State the origins of force fields: mass, static charge, and moving charge.
- Compare gravitational and electrostatic forces: shared inverse-square machinery, one crucial difference.
WHAT YOU PROBABLY THINK
Forces need contact.
A region that pushes
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 pushing. A field is represented as a vector: at every point it has a strength and a direction, the direction found by inspection, asking which way the force on a suitable test body would point.
AQA 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 machinery you build here for gravity transfers 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 seen from outside, has a radial field. Near a planet's surface, over laboratory distances, the lines are effectively parallel and evenly spaced: a uniform field, which is why g was a constant for the whole of Year 12.
Two forces, one grammar
The spec asks directly for the comparison. Similarities: gravitational and electrostatic forces both obey inverse-square laws, and the shared mathematics brings shared tools, field lines, the idea of potential, equipotential surfaces, all of which you will meet twice. The difference: masses always attract, while charges may attract or repel. One sign of charge exists for mass, and the universe's large-scale structure, built by an unshieldable, always-attractive force, follows from that single fact.
THE EXAM BIT
- The definition earns its mark verbatim: a region in which a body experiences a non-contact force. The word non-contact is the point.
- Field origins, all three when asked: mass, static charge, and moving charges (for magnetic fields).
- The comparison answer has a fixed shape: similarities are the inverse-square law and the shared apparatus (field lines, potential, equipotentials); the difference is attract-only versus attract-or-repel.
- A field line's arrow shows the force direction on the appropriate test body; the line spacing shows the strength. Both halves are markable.
- Uniform means parallel, equally spaced lines, and it is an approximation valid near a surface, worth one line of justification 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
Think of the shared mathematics first, then who is allowed to repel.
Show the answer
Similarities: both obey an inverse-square force law, and both are described with the same apparatus of field lines, potentials and equipotential surfaces.
Difference: masses always attract; charges may attract or repel.
Uniform cases: the gravitational field near the Earth's surface over small distances, and the electric field between parallel charged plates.
A field is a region that exerts force without touch.
Gravity and electrostatics share grammar; only charge can repel.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.