Unit 1 - Fields
5.1 Force Fields
5.1.1 Students should be able to explain the concept of a field of force, using field lines to
describe the field, indicate its direction and show the field strength.
Fields of Force
A region of space within which objects with a particular property experience a force.
A GRAVITATIONAL FIELD is a region of space within which a mass will experience a
force.
An ELECTRICAL FIELD is a region of space within which a charge will experience a force.
A MAGNETIC FIELD is a region of space within which a moving charge will experience a
force.
Around every charge there exists an electrical field, around every mass there exists a
gravitational field and around moving charges, or an electric current, there exists a magnetic
field.
Field Lines
Field lines give us a visual picture of a field. The direction of the field at any point is given by
the tangent to the field line in the direction of the arrow at that point. At any given point, the
field has only one direction, and therefore field lines cannot cross. The closer together lines
are drawn, the stronger the field.
Field lines that are equally spaced and parallel are uniform fields. All the fields below are
non-uniform.
5.2 Gravitational Fields
Students should be able to:
5.2.1 define gravitational field strength;
5.2.2 recall and use the equation g = F
m
;
Unit 1 - Fields 1
, 5.2.3 state Newton’s law of universal gravitation;
5.2.4 recall and use the equation for the gravitational force between point masses, F =
G mr1 m
2
2
;
5.2.5 recall and apply the equation for gravitational field strength, g = Gm
r2
, and use this
equation to calculate mass, m;
5.2.6 apply knowledge of circular motion to planetary and satellite motion;
5.2.7 show that the mathematical form of Kepler’s third law, T 2 proportional to r 3 , is
consistent with Newton’s law of universal gravitation;
5.2.8 demonstrate an understanding of the unique conditions of period, position and
direction of rotation required of a geostationary satellite.
Definition
💡 In a gravitational field, the gravitational field strength, g, at a point is equal to the
force which would be produced on a test mass of 1kg at that point:
F −1
g= m where g = gravitational field strength (Nkg )
F = Force (N)
m = mass (kg)
Field strength is a vector, as it has both magnitude and direction.
Newton’s Law of Universal Gravitation
💡 Between every two point masses there exists an attractive gravitational force which
is directly proportional to the mass of each and inversely proportional to the square
of their separation.
F = G mr1 m
2
2
where m1 and m2 are the point masses (kg)
r = the separation of the masses (m)
G = the universal gravitational constant, 6.67 × 10−11 Nm2 kg−2
Gravitational Field Strength
FIELD LINES
The direction of a gravitational field line at a point shows the direction of the gravitational
force on a mass of 1kg at that point.
Unit 1 - Fields 2
5.1 Force Fields
5.1.1 Students should be able to explain the concept of a field of force, using field lines to
describe the field, indicate its direction and show the field strength.
Fields of Force
A region of space within which objects with a particular property experience a force.
A GRAVITATIONAL FIELD is a region of space within which a mass will experience a
force.
An ELECTRICAL FIELD is a region of space within which a charge will experience a force.
A MAGNETIC FIELD is a region of space within which a moving charge will experience a
force.
Around every charge there exists an electrical field, around every mass there exists a
gravitational field and around moving charges, or an electric current, there exists a magnetic
field.
Field Lines
Field lines give us a visual picture of a field. The direction of the field at any point is given by
the tangent to the field line in the direction of the arrow at that point. At any given point, the
field has only one direction, and therefore field lines cannot cross. The closer together lines
are drawn, the stronger the field.
Field lines that are equally spaced and parallel are uniform fields. All the fields below are
non-uniform.
5.2 Gravitational Fields
Students should be able to:
5.2.1 define gravitational field strength;
5.2.2 recall and use the equation g = F
m
;
Unit 1 - Fields 1
, 5.2.3 state Newton’s law of universal gravitation;
5.2.4 recall and use the equation for the gravitational force between point masses, F =
G mr1 m
2
2
;
5.2.5 recall and apply the equation for gravitational field strength, g = Gm
r2
, and use this
equation to calculate mass, m;
5.2.6 apply knowledge of circular motion to planetary and satellite motion;
5.2.7 show that the mathematical form of Kepler’s third law, T 2 proportional to r 3 , is
consistent with Newton’s law of universal gravitation;
5.2.8 demonstrate an understanding of the unique conditions of period, position and
direction of rotation required of a geostationary satellite.
Definition
💡 In a gravitational field, the gravitational field strength, g, at a point is equal to the
force which would be produced on a test mass of 1kg at that point:
F −1
g= m where g = gravitational field strength (Nkg )
F = Force (N)
m = mass (kg)
Field strength is a vector, as it has both magnitude and direction.
Newton’s Law of Universal Gravitation
💡 Between every two point masses there exists an attractive gravitational force which
is directly proportional to the mass of each and inversely proportional to the square
of their separation.
F = G mr1 m
2
2
where m1 and m2 are the point masses (kg)
r = the separation of the masses (m)
G = the universal gravitational constant, 6.67 × 10−11 Nm2 kg−2
Gravitational Field Strength
FIELD LINES
The direction of a gravitational field line at a point shows the direction of the gravitational
force on a mass of 1kg at that point.
Unit 1 - Fields 2
