AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Students should recognise that a force field can be represented as a…
Full specification wording
Fields (A-level only): Students should recognise that a force field can be represented as a vector, the direction of which must be determined by inspection.
At any point in a force field, the field can be represented by a vector whose magnitude gives the strength of the field at that point and whose direction gives the direction of the force on a…
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
At any point in a force field, the field can be represented by a vector whose magnitude gives the strength of the field at that point and whose direction gives the direction of the force on a suitable test body. For a gravitational field, the field vector points towards the attracting mass, so near a planet it points towards the planet's centre. For an electric field, the direction is the direction of the force on a positive test charge: away from a positive source charge and towards a negative source charge. For a magnetic field, the direction is the direction in which the north pole of a compass needle points. Field-line diagrams show these vectors, with arrows indicating direction and spacing indicating relative magnitude.
What earns the marks
- States that a force field can be represented by a vector at each point in the region.
- Explains that the magnitude of the vector represents the strength of the field at that point.
- Explains that the direction of the vector represents the direction of the force on a suitable test body.
- Determines direction by inspection, for example gravitational field towards the centre of a planet.
- Determines electric field direction as the force on a positive test charge, away from a positive source and towards a negative source.
- Determines magnetic field direction using the direction in which the north pole of a compass needle points.
- Interprets field-line diagrams, using arrows for direction and spacing for relative magnitude.
Common mistakes and how to put them right
Assuming the field vector always points away from the source.
Put it right by checking the type of field and the sign of the source, since gravitational and electric fields can point towards the source.
Using the force on a negative test charge to define electric field direction.
Put it right by defining the direction using a positive test charge.
Treating field lines as paths that a body follows.
Put it right by explaining that field lines show the direction of the field vector at each point, not the trajectory of a moving body.
Exam tips
- State the test body you are using, such as a positive test charge or a compass needle, when justifying a direction.
- Sketch a simple field-line diagram with arrows to support your written direction.
- Check the sign of the source before deciding whether the field points towards or away from it.
Try the lesson
The start of the Fields (A-level only): Students should recognise that a force field can be represented as a vector, the direction of which must be determined by inspection. lesson, made from checked teaching notes for this specification point.
Have a go first
A gravitational field diagram shows arrows pointing towards a planet. What does the direction of each arrow represent?
Listen
At any point in a force field, the field can be represented by a vector whose magnitude gives the strength of the field at that point and whose direction gives the direction of the force on a suitable test body. For a gravitational field, the field vector points towards the attracting mass, so near a planet it points towards the planet's centre. For an electric field, the direction is the direction of the force on a positive test charge: away from a positive source charge and towards a negative source charge. For a magnetic field, the direction is the direction in which the north pole of a compass needle points. Field-line diagrams show these vectors, with arrows indicating direction and spacing indicating relative magnitude. A strong answer earns marks for these points. States that a force field can be represented by a vector at each point in the region. Explains that the magnitude of the vector represents the strength of the field at that point. Explains that the direction of the vector represents the direction of the force on a suitable test body. Determines direction by inspection, for example gravitational field towards the centre of a planet. A common slip: Assuming the field vector always points away from the source. Instead, checking the type of field and the sign of the source, since gravitational and electric fields can point towards the source. Exam tip: State the test body you are using, such as a positive test charge or a compass needle, when justifying a direction.
See it
Key words
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Vector representation of fields
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Direction of gravitational, electric and magnetic fields
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Field-line diagrams
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Test bodies and field direction
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See every definition
- Vector representation of fields
- A force field can be represented as a vector whose magnitude gives the strength of the field at that point and whose direction gives the direction of the force on a suitable test body.
- Direction of gravitational, electric and magnetic fields
- Determined by inspection: towards the attracting mass for gravitational, the force on a positive test charge for electric, and the way a compass needle's north pole points for magnetic.
- Field-line diagrams
- Show these vectors, with arrows indicating direction and spacing indicating relative magnitude.
- Test bodies and field direction
- The direction of the vector represents the direction of the force on a suitable test body, such as a positive test charge or a compass needle.
Label the diagram
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1Box 1
2Box 2
3Box 3
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