Gravitational field lines always point towards the mass (gravity is attractive). They show the
Gravitational Fields - OCR A-Level Physics
- Gravitational field linesLines indicating the direction a test mass would accelerate; point radially inwards for a sphere, straight down near Earth's surface. always point towards the mass (gravity is attractive). They show the direction a test mass would accelerate.
- The densityMass per unit volume of a material. Measured in kg m⁻³. of field lines indicates field strength: closer $together = stronger field$.
- Equipotential lines/surfacesLines or surfaces connecting points of equal gravitational potential; always perpendicular to field lines. join points of equal gravitational potentialThe work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). times potential difference). per unit mass in bringing a small test mass from infinity to that point. Always negative. Measured in J kg⁻¹.. They are always perpendicular to field lines.
- Near Earth's surface: field lines are parallel (uniform field), equipotentials are horizontal planes.
- Around a spherical mass: field lines are radial (point to centre), equipotentials are concentric spheres.
- No work is done moving along an equipotential surface ($\Delta$V = 0 so W = $m\Delta$V = 0).
- Equipotentials are closer together where the field is stronger (steeper potential gradient).
Examiner Tips and Tricks
When sketching field lines and equipotentials: field lines must have arrows (towards the mass), equipotentials must be labelled with V values (all negative, less negative further away), and the two sets of lines must cross at right angles.