9Ja – FORCE FIELDS
Force Field: A field is the area around an object where it can exert a force on another object, usually
without any contact between the objects. Field can be represented as lines showing the direction of the force
experienced by another object in the field.
Magnetic fields
A magnet creates a magnetic field around it. You cannot see a magnetic field, but you can observe its
effects. A force is exerted on a magnetic material brought into a magnetic field. The force is a non-contact
force because the magnet and the material do not have to touch each other.
Finding magnetic fields:
You can use a plotting compass or iron filings to detect a magnetic field.
Drawing magnetic field diagrams
The Earth’s magnetism:
The Earth behaves as if it contains a giant magnet.
It produces a magnetic field in which the field lines are most concentrated at the poles.
This magnetic field can be detected using magnetic materials or magnets.
The Earth’s magnetic field helps to protect it from charged particles emitted by the sun.
Gravitational fields
All objects with mass have a gravitational field around them. A gravitational field is where a mass
experiences a force.
All matter has a gravitational field that attracts other objects.
The more mass an object has, the greater its gravitational field will be.
For example, the Earth has a greater gravitational field than the Moon because it has a much greater
mass than the Moon.
The Moon is attracted to the Earth because it is within the Earth’s gravitational field.
The Earth’s gravitational field strength (g) is 10 N/kg. This means that the force of attraction between
the Earth and a 1 kg mass will experience a force of 10 N. We call this force the weight of the object.
Weight(N) = mass(kg) × gravitational field strength (N/Kg)
This is when:
gravity force (weight) is measured in newtons (N)
mass is measured in kilograms (kg)
gravitational field strength (g) is measured in newtons per kilogram (N/kg)
Example
Calculate the gravity force (weight) of a skydiver with a mass 70 kg falling towards the Earth?
(Remember that gravitational field strength = 10 N/kg on Earth).
weight = mass × gravitational field strength
weight = 70 × 10
weight = 700 N
Gravitational potential energy (GPE)
On Earth we always have the force of gravity acting on us.
If an object is lifted, work is done against gravitational force. The object gains energy.
The energy needed to move the object is stored in its mass as gravitational potential energy.
The amount of GPE an object on Earth has depends on its:
mass
height above the ground
The relationship between gravitational potential energy, mass, gravitational field strength and height is as
follows:
Gravitational potential energy = mass x gravitational field strength x height
GPE = m x g x h (Joules)
