# Electric vs Gravitational Fields(OCR A Level Physics)

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Katie M

Expertise

Physics

## Electric Fields vs Gravitational Fields

• A field can be defined as:

A region in which an object will experience a force, such as gravitational or electrostatic, at a distance

• A gravitational field can be defined as:

The gravitational force per unit mass exerted on a point mass

• An electrostatic field can be defined as:

The electric force per unit charge exerted on a small positive test charge

• Fields can be described in terms of field strength, which is defined as:

Field strength =

• Electric field strength, E, and gravitational field strength, g, therefore, have very similar equations
• Despite a few differences, they are analogous to one another in many ways
• In both cases, the nature of the test object is as follows:
• Gravitational fields: small mass, m
• Electrostatic fields: small positive charge, q

#### Uniform Fields

• A gravitational field is a region of space in which objects with mass will experience a force
• The gravitational field strength can be calculated using the equation:
• Where:
• g = gravitational field strength (N kg−1)
• F = gravitational force on the charge (N)
• m = mass (kg)

• The direction of the gravitational field is always directed towards the centre of the mass
• Gravitational forces are always attractive and cannot be repulsive
• An electric field is a region of space in which an electric charge will experience a force
• The electric field strength can be calculated using the equation:
• Where:
• E = electric field strength (N C−1)
• F = electrostatic force on the charge (N)
• Q = Charge (C)
• It is important to use a positive test charge in this definition, as this determines the direction of the electric field
• The electric field strength is a vector quantity, it is always directed:
• Away from a positive charge
• Towards a negative charge
• Opposite charges (positive and negative) attract each other
• Conversely, like charges (positive-positive or negative-negative) repel each other

• A point charge or mass produces a radial field
• A charged sphere also acts as a point charge
• A spherical mass also acts as a point mass
• Radial fields always have an inverse square law relationship with distance
• This means the field strength decreases by a factor of four when the distance r is doubled
• The gravitational force FG between two masses is defined by:

• Where:
• FG = gravitational force between two masses (N)
• G = Newton’s gravitational constant
• m1m2 = two points masses (kg)
• r = distance between the centre of the two masses (m)
• The electric field strength E at a distance r due to a point charge Q in free space is defined by:

• Where:
• Q = the point charge producing the radial electric field (C)
• r = distance from the centre of the charge (m)
• ε0 = permittivity of free space (F m1) = ()
• This equation shows:
• The electric field strength in a radial field is not constant
• As the distance, r, from the charge increases, E decreases by a factor of 1/r2

#### Gravitational vs Electrostatic Forces

• The similarities and differences between gravitational and electrostatic forces are listed in the table below:

Comparing G and E Fields

• The key similarities are:
• The magnitude of the gravitational and electrostatic force between two point masses or charges are inverse square law relationships
• The field lines around a point mass and negative point charge are identical
• The field lines in a uniform gravitational and electric field are identical
• The gravitational field strength and electric field strength both have a 1 / r relationship in a radial field
• The gravitational potential and electric potential both have a 1 / r relationship
• Equipotential surfaces for both gravitational and electric fields are spherical around a point mass or charge and equally spaced parallel lines in uniform fields
• The work done in each field is either the product of the mass and change in potential or charge and change in potential

• The key differences are:
• The gravitational force acts on particles with mass whilst the electrostatic force acts on particles with charge
• The gravitational force is always attractive whilst the electrostatic force can be attractive or repulsive
• The gravitational potential is always negative whilst the electric potential can be either negative or positive

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