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Fields and their consequences (3.7)›Fields (3.7.1)

Fields (3.7.1)

Understand force fields and how field lines and potentials model non-contact forces, comparing gravitational and electrostatic fields and inverse-square laws.
3 min

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Gravitational fields are one of several types of fields that exert a force on objects.

Gravitational fields always attract objects, exerting a force toward the center of gravity. The force depends on the object’s mass and the distance from the source.

Other fields that give rise to a force include:

  • Electric fields
    Electric fields exert forces on charged particles, with the direction of the force determined by the charge’s polarity.
  • Magnetic fields
    Magnetic fields exert a force on moving charged particles, with the direction of the force being perpendicular to both the velocity of the particle and the direction of the magnetic field.
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Gravitational fields share many similarities with electric fields:

Both fields obey the inverse square law – the force between two objects decreases with the square of the distance between them:

  • The strength of both fields are represented by the force experienced by an object within the field divided by its respective property: mass in gravitational fields and charge in electric fields. In both cases, the field strength is a measure of the force per unit mass or unit charge:
    • gravitational field:
    • electric field:
  • Both fields exert forces without physical contact, affecting objects at a distance.
  • Both have a radial field for point masses and point charges.
  • Both fields can be represented using field lines. Electric field lines point from positive to negative charges. Gravitational field lines point towards the mass creating the field.
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Gravitational and electric fields are similar in many ways, but they have their differences:

  • Electric fields arise from electric charges, whereas gravitational fields arise from mass.
  • Electric fields can be either attractive or repulsive depending on the charges (like charges repel, opposite charges attract). Gravitational fields are always attractive – masses always attract each other.
  • The direction of field lines in electric fields depends on the sign of the charge (field lines point away from positive charges and towards negative charges), whereas field lines in gravitational fields always point towards the source of the mass.
  • Forces exerted by gravitational fields are weaker compared to those exerted by electric fields for everyday charged objects, but gravitational forces dominate on large scales, such as between planets.
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There are many similarities and differences between electric fields and gravitational fields. Both point charges and point masses produce radial fields, and objects with charge and mass are acted upon by a force within the electric and gravitational field, respectively.

The table below summarises the key similarities and differences between electric fields and gravitational fields for point charges and point masses:

A comparison table showing properties of Electric fields and Gravitational fields. The properties include: Field is created by (Charge for Electric fields and Mass for Gravitational fields), Strength of the field (E = F/q = -Q/(4πε₀r²) for Electric fields and g = F/m = GM/r² for Gravitational fields), Force due to field (F = -Qq/(4πε₀r²) for Electric fields and F = GMm/r² for Gravitational fields), Nature of field (Positive point charges have a repulsive field i.e. points radially outwards; Negative point charges have an attractive field i.e. points radially inwards for Electric fields, and Point masses always have an attractive field that points radially inwards for Gravitational fields), Structure of field (Point charges produce radial field lines for Electric fields and Point masses produce radial field lines for Gravitational fields), Relation between force and point particles (Force ∝ product of charges for Electric fields and Force ∝ product of masses for Gravitational fields), Relation between force and distance (Force ∝ 1/(distance)² for both Electric and Gravitational fields).
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A field is a region in space where a force can be exerted on objects possessing certain properties (such as charge or mass) without physical contact.

Fields are used to explain how forces can act at a distance, allowing one object to exert influence on another across space.

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Electric fields are one of several forms of fields within physics that give rise to a force. There are many similarities and differences between different types of fields, such as electric fields, gravitational fields and magnetic fields.

All three follow the same principles that define fields, but have differences in the objects they act upon:

  • Electric fields Charged objects
  • Gravitational fields Objects with mass
  • Magnetic fields Charged objects in motion and objects with magnetic poles
  • Electromagnetic fields Combination of electric and magnetic fields.
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