Concept of a force field as a region in which a body experiences a…

The idea

A force field is a region of space in which a body experiences a non-contact force, meaning the force acts without the bodies touching. The field is a property of the space itself, created by a source such as a mass, a charge or a magnet, and any suitable test body placed in that region feels a force. For example, a small mass placed near a planet experiences gravitational attraction without contact; a positive test charge placed near a positive point charge experiences repulsion; a magnetic compass needle near a magnet aligns without touching it. The force may be attractive or repulsive and its size and direction generally vary from point to point, so the field is described as a vector quantity at each point. Fields allow action at a distance to be modelled, and they are central to explaining gravitational, electric and magnetic interactions throughout A-level physics.

What earns the marks

• Defines a force field as a region in which a non-contact force acts on a body placed there.
• Identifies that the field is produced by a source such as a mass, charge or magnet and exists in the surrounding space.
• States that the force is non-contact, so the interacting bodies need not touch.
• Gives a correct example such as gravitational attraction between a mass and a planet, electric force between charges, or magnetic force on a compass needle.
• Recognises that the force can be attractive or repulsive and may vary in magnitude and direction across the region.
• Explains that the field is a vector quantity at each point, with both magnitude and direction.
• Distinguishes a field from a contact force such as friction or normal contact force.

Common mistakes and how to put them right

• Thinking a force field requires the two bodies to touch.

  Put it right by stressing that the force acts at a distance through the field.

• Believing the field only exists where a test body is present.

  Put it right by explaining the field is a property of the space around the source and exists independently.

• Confusing the field with the force itself.

  Put it right by stating the field is defined at a point and the force depends on the body placed there.

Exam tips

• Define the term precisely, including the words region and non-contact force, before giving an example.
• Use a named example such as the gravitational field of a planet or the electric field of a point charge to show understanding.
• When asked to explain, link the source, the field in the surrounding space and the force on a test body in a clear chain.